JavaScript Basic Notes

TC39

• Technical Committees 39
• TC39 Process
• ES2015
• WinterCG: Web-interoperable Runtime Community Group

JavaScript = ECMAScript + DOM + BOM:

• ECMAScript: ECMA-262.
• DOM: DOM Core + DOM HTML (document).
• BOM: Browser Object Model API (HTML5) (window/navigator/location/screen/performance etc).

Primitive Values

Primitive data types:

• Undefined.
• Null.
• Boolean.
• Number.
• String.
• Symbol.
• BigInt.

Undefined

• 对象属性未定义时, 该属性值为 undefined.
• 未初始化变量的初值为 undefined (表示等待被赋值).

Null

当引用为空或引用对象不存在时, 值为 null. null 值表示一个空对象指针.

Null

typeof null -> object.

Boolean

Zero Value Expression

零值表达式:

• undefined.
• null.
• false.
• NaN.
• 0
• 0n.
• ''.

Boolean Conversion

x	Boolean(x)
undefined	false
null	false
boolean	x
number	0 → false, NaN → false
	Other numbers → true
bigint	0n → false
	Other numbers → true
string	'' → false
	Other strings → true
symbol	true
object	true

Number

• Binary: 0b10/0B10.
• Octal: 0o23/0O23.
• Hex: 0xFF.
• ** 指数运算符.
• BigInt.

const a = 2172141653
const b = 15346349309
const c1 = a * b
// => 33334444555566670000

const c2 = BigInt(a) * BigInt(b)
// => 33334444555566667777n

const inhabitantsOfLondon = 1_335_000
const distanceEarthSunInKm = 149_600_000

const fileSystemPermission = 0b111_111_000
const bytes = 0b1111_10101011_11110000_00001101
const words = 0xF3B_F00D

const massOfElectronInKg = 9.109_383_56e-31
const trillionInShortScale = 1e1_2

Number Conversion

x	Number(x)
undefined	NaN
null	0
boolean	false → 0, true → 1
number	x
bigint	-1n → -1, 1n → 1
string	'' → 0
	Other → parsed number, ignoring leading/trailing whitespace
symbol	Throws TypeError
object	Configurable ([Symbol.toPrimitive]()/valueOf())

assert.equal(Number(123.45), 123.45)
assert.equal(Number(''), 0)
assert.equal(Number('\n 123.45 \t'), 123.45)
assert.equal(Number('xyz'), Number.NaN)
assert.equal(Number(-123n), -123)
assert.equal(
  Number({
    valueOf() {
      return 123
    },
  }),
  123
)

Number Static Properties

• Number.NaN.
• Number.NEGATIVE_INFINITY.
• Number.POSITIVE_INFINITY.
• Number.MAX_SAFE_INTEGER.
• Number.MIN_SAFE_INTEGER.
• Number.EPSILON.
• Number.isNaN().
• Number.isFinite().
• Number.isInteger().
• Number.isSafeInteger().
• Number.toExponential().
• Number.toFixed().
• Number.toPrecision().
• Number.parseInt(string, radix).
• Number.parseFloat(string).

;(1234).toExponential()
// '1.234e+3'
;(1234).toExponential(5)
// '1.23400e+3'
;(1234).toExponential(1)
// '1.2e+3'
;(0.003).toExponential()
// '3e-3'
;(0.00000012).toFixed(10)
// '0.0000001200'
;(0.00000012).toFixed()
// '0'
;(10 ** 21).toFixed()
// '1e+21'
;(1234).toPrecision(3)
// '1.23e+3'
;(1234).toPrecision(4)
// '1234'
;(1234).toPrecision(5)
// '1234.0'
;(1.234).toPrecision(3)
// '1.23'

Not A Number

const numberType = typeof Number.NaN // 'number'

Number.isFinite(Number.NaN)
// false
Number.isNaN(Number.NaN)
// true
Number.isNaN(123)
// false
Number.isNaN('abc')
// false

function isNumber(value) {
  return typeof value === 'number' && Number.isFinite(value)
}

NaN

NaN === NaN -> false.

Infinity Number

Infinity represents all values greater than 1.7976931348623157e+308. Infinity will be converted to null with JSON.stringify().

const largeNumber = 1.7976931348623157e308
// eslint-disable-next-line ts/no-loss-of-precision
const largerNumber = 1.7976931348623157e309

console.log(largeNumber) // 1.7976931348623157e+308
console.log(largerNumber) // Infinity
console.log(46 / 0) // Infinity
console.log(Number.POSITIVE_INFINITY) // Infinity
console.log(Number.MAX_VALUE) // Infinity
// eslint-disable-next-line ts/no-loss-of-precision
console.log(-1.7976931348623157e309) // -Infinity
console.log(-46 / 0) // -Infinity
console.log(Number.NEGATIVE_INFINITY) // -Infinity
console.log(Number.MIN_VALUE) // -Infinity

console.log(Math.max()) // -Infinity
console.log(Math.min()) // Infinity

Number.isFinite(Number.POSITIVE_INFINITY)
// false
Number.isFinite(Number.NEGATIVE_INFINITY)
// false
Number.isFinite(Number.NaN)
// false
Number.isFinite(123)
// true

Safe Number

• Safe integers:
  • Precision: 53 bits plus sign.
  • Range: (−2^53, 2^53).
• Array indices:
  • Precision: 32 bits, unsigned
  • Range: [0, 2^32−1).
  • Typed Arrays have a larger range of 53 bits (safe and unsigned).
• Bitwise operators:
  • Precision: 32 bits.
  • Range of unsigned right shift (>>>): unsigned, [0, 2^32).
  • Range of all other bitwise operators: signed, [−2^31, 2^31).

assert.equal(Number.MAX_SAFE_INTEGER, 2 ** 53 - 1)
assert.equal(Number.MIN_SAFE_INTEGER, -Number.MAX_SAFE_INTEGER)

assert.equal(Number.isSafeInteger(5), true)
assert.equal(Number.isSafeInteger('5'), false)
assert.equal(Number.isSafeInteger(5.1), false)
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER), true)
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER + 1), false)

Number.isInteger(-17)
// true
Number.isInteger(33)
// true
Number.isInteger(33.1)
// false
Number.isInteger('33')
// false
Number.isInteger(Number.NaN)
// false
Number.isInteger(Number.POSITIVE_INFINITY)
// false

Float Number

• 计算浮点数时, 应先计算整数, 再利用移位/乘法/除法转化为浮点数.
• 浮点值的精确度最高可达 17 位小数.

const a = (1 + 2) / 10 // a = 0.1 + 0.2;

String

String Primitive Features

作为基本变量:

• delete 无法删除某位字符.

String Reference Features

• 赋值与传参: 传递 string 字符串常量的引用.
• 所有 string 字面量都是不可变量, 当对 string 进行操作后, 将先会在堆区创建副本, 再通过副本进行修改, 并返回副本的索引.
• for...in: 返回下标数字.
• for...of: 对字符串字符进行遍历.
• 没有被任何变量引用的 string: 垃圾回收.

const goodString = 'I\'ve been a good string'
console.log(typeof goodString) // string
console.log(goodString instanceof String) // false
console.log(Object.prototype.toString.call(goodString)) // [object String]

// eslint-disable-next-line no-new-wrappers
const badString = new String('I\'ve been a naughty string')
console.log(typeof badString) // object
console.log(badString instanceof String) // true
console.log(Object.prototype.toString.call(badString)) // [object String]

const isPrimitiveString = value => typeof value === 'string'
console.log(isPrimitiveString(goodString)) // true
console.log(isPrimitiveString(badString)) // false

const isObjectWrappedString = value => value instanceof String
console.log(isObjectWrappedString(goodString)) // false
console.log(isObjectWrappedString(badString)) // true

const isString = value => typeof value === 'string' || value instanceof String
console.log(isString(goodString)) // true
console.log(isString(badString)) // true

function isStringAlternative(value) {
  return Object.prototype.toString.call(badString) === '[object String]'
}
console.log(isStringAlternative(goodString)) // true
console.log(isStringAlternative(badString)) // true

String Conversion

x	String(x)
undefined	'undefined'
null	'null'
boolean	false → 'false', true → 'true'
number	123 → '123'
bigint	123n → '123'
string	x
symbol	Symbol('abc') → 'Symbol(abc)'
object	Configurable (toPrimitive/toStringTag/toString())

String Unicode

// eslint-disable-next-line no-self-compare
const truthy = 'z' === 'z' // true
const truthy = '\x7A' === 'z' // true
const truthy = '\u007A' === 'z' // true
const truthy = '\u{7A}' === 'z' // true

String Char Code

• string.charAt(index).
• string.charCodeAt(index).
• string.fromCharCode(charCode).
• string.codePointAt(index): 正确处理 4 字节存储字符.
• string.fromCodePoint(codePoint): 正确处理 4 字节存储字符.

function is32Bit(c) {
  return c.codePointAt(0) > 0xFFFF
}

const truthy = String.fromCodePoint(0x78, 0x1F680, 0x79) === 'x\uD83D\uDE80y'

const after = before.charAt(0).toUpperCase() + before.slice(1)

String Slice and Merge

• string.slice().
• string.substring().
• string.substr().
• string.split(separator): 选择割断符, 返回字符串数组.
• Array<string>.join(separator): 将字符串数组连接成字符串.

const stringValue = 'hello world'
console.log(stringValue.slice(3)) // "lo world"
console.log(stringValue.substring(3)) // "lo world"
console.log(stringValue.substr(3)) // "lo world"
console.log(stringValue.slice(3, 8)) // "lo wo"
console.log(stringValue.substring(3, 8)) // "lo wo"
console.log(stringValue.substr(3, 8)) // "lo world"
console.log(stringValue.slice(-3)) // "rld"
console.log(stringValue.substring(-3)) // "hello world"
console.log(stringValue.substr(-3)) // "rld"
console.log(stringValue.slice(3, -4)) // "lo w"
console.log(stringValue.substring(3, -4)) // "hel"
console.log(stringValue.substr(3, -4)) // "" (empty string)

String Query

• string.includes(substr).
• string.startsWith(substr).
• string.endsWith(substr).
• 使用第二个参数 n 时, endsWith 针对前 n 个字符, 其他两个方法针对从第 n 个位置直到字符串结束.

const s = 'Hello world!'

s.startsWith('world', 6) // true
s.endsWith('Hello', 5) // true
s.includes('Hello', 6) // false

// Arrays difference
;[
  [1, 2, 3, 4, 5],
  [5, 2, 10],
].reduce((a, b) => a.filter(c => !b.includes(c)))
// [1, 3, 4]

// Arrays intersection
;[
  [1, 2, 3],
  [101, 2, 1, 10],
  [2, 1],
].reduce((a, b) => a.filter(c => b.includes(c)))
// [1, 2]

• string.match(RegExp): string[] | null.
• string.matchAll(RegExp): string[] | null.

interface RegExpMatchArray extends Array<string> {
  index: number
  input: string
  groups: Record<string, string> | undefined
}

• string.search(string | RegExp): number.

'a2b'.search(/\d/)
// 1
'a2b'.search('[0-9]')
// 1

String Replace

• string.replace(string | RegExp, replaceValue | replacerFunction).
• string.replaceAll(string | RegExp, replaceValue | replacerFunction).

// eslint-disable-next-line prefer-regex-literals
const regexp = new RegExp('foo[a-z]*', 'g')
const str = 'table football, foosball'
const matches = str.matchAll(regexp)

for (const match of matches) {
  console.log(
    `Found ${match[0]} start=${match.index} end=${
      match.index + match[0].length
    }.`
  )
}
// expected output: "Found football start=6 end=14."
// expected output: "Found foosball start=16 end=24."

// matches iterator is exhausted after the for..of iteration
// Call matchAll again to create a new iterator
Array.from(str.matchAll(regexp), m => m[0])
// Array [ "football", "foosball" ]

'aabbcc'.replaceAll('b', '.')
// => 'aa..cc'

'aabbcc'.replaceAll(/b/g, '.')
// => 'aa..cc'

String Pad

• string.repeat(times).

'hello'.repeat(2) // "hellohello"
'na'.repeat(2.9) // "nana"

'na'.repeat(-0.9) // ""
'na'.repeat(-1) // RangeError

'na'.repeat(Number.NaN) // ""
'na'.repeat(Number.POSITIVE_INFINITY) // RangeError

'na'.repeat('na') // ""
'na'.repeat('3') // "nanana"

• string.padStart(len, paddingStr).
• string.padEnd(len, paddingStr).

'1'.padStart(10, '0') // "0000000001"
'12'.padStart(10, '0') // "0000000012"
'123456'.padStart(10, '0') // "0000123456"

'12'.padStart(10, 'YYYY-MM-DD') // "YYYY-MM-12"
'09-12'.padStart(10, 'YYYY-MM-DD') // "YYYY-09-12"

String Trim

• string.trimLeft()/string.trimStart(): remove start whitespace.
• string.trimRight()/string.trimEnd(): remove end whitespace.

String Template Literals

str 表示模板字符串:

// 普通字符串
;`In JavaScript '\n' is a line-feed.``\`Yo\` World!``In JavaScript this is // 多行字符串
 not legal.``${
  x // 引用变量
} + ${y * 2} = ${x + y * 2}``${obj.x + obj.y}``foo ${
  fn() // 调用函数
} bar`

Tagged Templates Literals

function boldify(parts, ...insertedParts) {
  return parts
    .map((s, i) => {
      if (i === insertedParts.length)
        return s
      return `${s}<strong>${insertedParts[i]}</strong>`
    })
    .join('')
}

const name = 'Sabertaz'
console.log(boldify`Hi, my name is ${name}!`)
// => "Hi, my name is <strong>Sabertaz</strong>!"

function template(strings, ...keys) {
  return function (...values) {
    const dict = values[values.length - 1] || {}
    const result = [strings[0]]
    keys.forEach((key, i) => {
      const value = Number.isInteger(key) ? values[key] : dict[key]
      result.push(value, strings[i + 1])
    })
    return result.join('')
  }
}

const t1Closure = template`${0}${1}${0}!`
t1Closure('Y', 'A') // "YAY!"
const t2Closure = template`${0} ${'foo'}!`
t2Closure('Hello', { foo: 'World' }) // "Hello World!"

编译模板 (小型模板引擎):

function compile(template) {
  const evalExpr = /<%=(.+?)%>/g
  const expr = /<%([\s\S]+?)%>/g

  template = template
    .replace(evalExpr, '`); \n  echo( $1 ); \n  echo(`')
    .replace(expr, '`); \n $1 \n  echo(`')

  template = `echo(\`${template}\`);`

  const script = `(function parse(data){
      let output = "";

      function echo(html){
        output += html;
      }

      ${template}

      return output;
    })`

  return script
}

const template = `
<ul>
  <% for(let i=0; i < data.supplies.length; i++) { %>
    <li><%= data.supplies[i] %></li>
  <% } %>
</ul>
`
const parse = compile(template)
div.innerHTML = parse({ supplies: ['broom', 'mop', 'cleaner'] })
// => <ul>
// =>   <li>broom</li>
// =>   <li>mop</li>
// =>   <li>cleaner</li>
// => </ul>

// 下面的hashTemplate函数
// 是一个自定义的模板处理函数
const libraryHtml = hashTemplate`
  <ul>
    #for book in ${myBooks}
      <li><i>#{book.title}</i> by #{book.author}</li>
    #end
  </ul>
`

国际化处理:

i18n`Welcome to ${siteName}, you are visitor number ${visitorNumber}!`
// "欢迎访问xxx, 您是第xxxx位访问者！"

XSS protection:

const message = SaferHTML`<p>${sender} has sent you a message.</p>`

function SaferHTML(templateString, ...expressions) {
  let s = templateString[0]

  for (let i = 0; i < expressions.length; i++) {
    const expression = String(expressions[i])

    // Escape special characters in the substitution.
    s += expression
      .replace(/&/g, '&amp;')
      .replace(/</g, '&lt;')
      .replace(/>/g, '&gt;')

    // Don't escape special characters in the template.
    s += templateString[i + 1]
  }

  return s
}

运行代码:

jsx`
  <div>
    <input
      ref='input'
      onChange='${this.handleChange}'
      defaultValue='${this.state.value}' />
      ${this.state.value}
   </div>
`

java`
class HelloWorldApp {
  public static void main(String[] args) {
    System.out.println("Hello World!"); // Display the string.
  }
}
`
HelloWorldApp.main()

Raw String

console.log(`\u00A9`) // ©
console.log(String.raw`\u00A9`) // \u00A9

console.log(`first line\nsecond line`)
// first line
// second line
console.log(String.raw`first line\nsecond line`)
// "first line\nsecond line"

function printRaw(strings) {
  console.log('Actual characters:')

  for (const string of strings)
    console.log(string)

  console.log('Escaped characters;')

  for (const rawString of strings.raw)
    console.log(rawString)
}

printRaw`\u00A9${'and'}\n`
// Actual characters:
// ©
// (换行符)
// Escaped characters:
// \u00A9
// \n

String Utils

function ucWords(string) {
  return string.toLowerCase().replace(/\b[a-z]/g, l => l.toUpperCase())
}

function ucFirst(string) {
  return string[0].toUpperCase() + string.substr(1)
}

function studlyCase(string) {
  return string
    .replace('-', ' ')
    .replace('_', ' ')
    .split(' ')
    .map(str => str[0].toUpperCase() + str.substr(1).toLowerCase())
    .join('')
}

function snakeCase(string, glue = '_') {
  return string
    .replace(/\W+/g, ' ')
    .split(/ |\B(?=[A-Z])/)
    .map(word => word.toLowerCase())
    .join(glue)
}

function kebabCase(string) {
  return snakeCase(string, '-')
}

function objectToQueryString(obj) {
  return Object.keys(obj)
    .reduce((carry, key) => {
      if (obj[key] || obj[key] === 0)
        return `${carry}${key}=${obj[key]}&`

      return carry
    }, '')
    .replace(/&+$/, '')
}

Symbol

• A Symbol is a unique and immutable primitive value and may be used as the key of an Object property.
• Symbols don't auto-convert to strings and can't convert to numbers.
• Symbol.for(key) create global Symbol registry.

// eslint-disable-next-line symbol-description
const genericSymbol = Symbol()
// eslint-disable-next-line symbol-description
const otherGenericSymbol = Symbol()
console.log(genericSymbol === otherGenericSymbol) // false

const fooSymbol = Symbol('foo')
const otherFooSymbol = Symbol('foo')
console.log(fooSymbol === otherFooSymbol) // false

const fooGlobalSymbol = Symbol.for('foobar') // 创建新符号
const otherFooGlobalSymbol = Symbol.for('foobar') // 重用已有符号
console.log(fooGlobalSymbol === otherFooGlobalSymbol) // true

Symbol Conversion

Convert To	Explicit Conversion	Coercion (Implicit Conversion)
boolean	Boolean(sym) → OK	!sym → OK
number	Number(sym) → TypeError	sym * 2 → TypeError
string	String(sym) → OK	'' + sym → TypeError
	sym.toString() → OK	${sym} → TypeError
object	Object(sym) → OK	Object.keys(sym) → OK

Built-in Symbol Methods

Symbol methods:

• [Symbol.iterator](): for of.
• [Symbol.asyncIterator](): for await of.
• [Symbol.match/replace/search/split](target): string.match/replace/search/split(classWithSymbolFunction).
• [Symbol.hasInstance](instance): instance of.
• [Symbol.species](): constructor for making derived objects.
• [Symbol.toPrimitive](hint): 强制类型转换.
• [Symbol.toStringTag](): string used by Object.prototype.toString().

iterator:

const arr = ['a', 'b', 'c']
const iter = arr[Symbol.iterator]()

iter.next() // { value: 'a', done: false }
iter.next() // { value: 'b', done: false }
iter.next() // { value: 'c', done: false }
iter.next() // { value: undefined, done: true }

hasInstance:

class Bar {}
class Baz extends Bar {
  static [Symbol.hasInstance](instance) {
    return false
  }
}

const b = new Baz()

console.log(Bar[Symbol.hasInstance](b)) // true
console.log(b instanceof Bar) // true
console.log(Baz[Symbol.hasInstance](b)) // false
console.log(b instanceof Baz) // false

const ReferenceType = {
  [Symbol.hasInstance](value) {
    return (
      value !== null
      && (typeof value === 'object' || typeof value === 'function')
    )
  },
}

const obj1 = {}
console.log(obj1 instanceof Object) // true
console.log(obj1 instanceof ReferenceType) // true

const obj2 = Object.create(null)
console.log(obj2 instanceof Object) // false
console.log(obj2 instanceof ReferenceType) // true

species:

class MyClass {
  static get [Symbol.species]() {
    return this
  }

  constructor(value) {
    this.value = value
  }

  clone() {
    return new this.constructor[Symbol.species](this.value)
  }
}

class MyDerivedClass1 extends MyClass {
  // empty
}

class MyDerivedClass2 extends MyClass {
  static get [Symbol.species]() {
    return MyClass
  }
}

const instance1 = new MyDerivedClass1('foo')
const instance2 = new MyDerivedClass2('bar')
const clone1 = instance1.clone()
const clone2 = instance2.clone()

console.log(clone1 instanceof MyClass) // true
console.log(clone1 instanceof MyDerivedClass1) // true
console.log(clone2 instanceof MyClass) // true
console.log(clone2 instanceof MyDerivedClass2) // false

toPrimitive:

class Temperature {
  constructor(degrees) {
    this.degrees = degrees
  }

  [Symbol.toPrimitive](hint) {
    switch (hint) {
      case 'string':
        return `${this.degrees}\u00B0` // degrees symbol
      case 'number':
        return this.degrees
      case 'default':
        return `${this.degrees} degrees`
    }
  }
}

const freezing = new Temperature(32)

console.log(`${freezing}!`) // "32 degrees!"
console.log(freezing / 2) // 16
console.log(String(freezing)) // "32째"

toStringTag:

class Person {
  constructor(name) {
    this.name = name
  }

  get [Symbol.toStringTag]() {
    return 'Person'
  }
}

const me = new Person('Me')

console.log(me.toString()) // "[object Person]"
console.log(Object.prototype.toString.call(me)) // "[object Person]"

Value	toString Tag
undefined	Undefined
null	Null
Array object	Array
string object	String
arguments	Arguments
callable	Function
error object	Error
boolean object	Boolean
number object	Number
date object	Date
regular expression object	RegExp
(Otherwise)	Object

Bigint

• Decimal: 123n.
• Binary: 0b1101n.
• Octal: 0o777n.
• Hexadecimal: 0xFFn.

/**
 * Takes a bigint as an argument and returns a bigint
 */
function nthPrime(nth) {
  if (typeof nth !== 'bigint')
    throw new TypeError('Not bigint')

  function isPrime(p) {
    for (let i = 2n; i < p; i++) {
      if (p % i === 0n)
        return false
    }

    return true
  }

  for (let i = 2n; ; i++) {
    if (isPrime(i)) {
      if (--nth === 0n)
        return i
    }
  }
}

assert.deepEqual(
  [1n, 2n, 3n, 4n, 5n].map(nth => nthPrime(nth)),
  [2n, 3n, 5n, 7n, 11n]
)

Bigint Conversion

x	BigInt(x)
undefined	Throws TypeError
null	Throws TypeError
boolean	false → 0n, true → 1n
number	123 → 123n
	Non-integer → throws RangeError
bigint	x
string	'123' → 123n
	Unparsable → throws SyntaxError
symbol	Throws TypeError
object	Configurable ([Symbol.toPrimitive]()/valueOf())

BigInt(undefined)
// TypeError: Cannot convert undefined to a BigInt
BigInt(null)
// TypeError: Cannot convert null to a BigInt
BigInt('abc')
// SyntaxError: Cannot convert abc to a BigInt
BigInt('123n')
// SyntaxError: Cannot convert 123n to a BigInt
BigInt('123')
// 123n
BigInt('0xFF')
// 255n
BigInt('0b1101')
// 13n
BigInt('0o777')
// 511n
BigInt(123.45)
// RangeError: The number 123.45 cannot be converted to a BigInt
BigInt(123)
// 123n
BigInt({
  valueOf() {
    return 123n
  },
})
// 123n

Convert To	Explicit Conversion	Coercion (Implicit Conversion)
boolean	Boolean(0n) → false	!0n → true
	Boolean(int) → true	!int → false
number	Number(7n) → 7	+int → TypeError
string	String(7n) → '7'	''+7n → '7'

Bigint Static Properties

• BigInt.asIntN(width, theInt): Casts theInt to width bits (signed).
• BigInt.asUintN(width, theInt): Casts theInt to width bits (unsigned).

const uint64a = BigInt.asUintN(64, 12345n)
const uint64b = BigInt.asUintN(64, 67890n)
const result = BigInt.asUintN(64, uint64a * uint64b)

Wrapper Objects for Primitives

Using the wrapper function without the new keyword is a useful way of coercing a value into a primitive type.

// Not recommended (primitive object wrapper):
// eslint-disable-next-line no-new-wrappers
const objectType = typeof new String(37) // object

// Safe (type coercion with wrapper function):
const stringType = typeof String(37) // string

// Primitive strings:
// eslint-disable-next-line no-self-compare
const truthy = '37' === '37' // true

// Object-wrapped string:
// eslint-disable-next-line no-new-wrappers
const falsy = new String(37) === '37' // false

// Type-coerced string:
const truthy = String(37) === '37' // true

// BAD!
// eslint-disable-next-line no-new-wrappers
const falseObject = new Boolean(false)
const result = falseObject && true
console.log(result) // true
console.log(typeof falseObject) // object
console.log(falseObject instanceof Boolean) // true

const prim = true
assert.equal(typeof prim, 'boolean')
assert.equal(prim instanceof Boolean, false)

const wrapped = Object(prim)
assert.equal(typeof wrapped, 'object')
assert.equal(wrapped instanceof Boolean, true)
assert.equal(wrapped.valueOf(), prim) // unwrap

Box and Unbox for primitive values:

• 自动创建的原始值包装对象可以让原始值拥有对象的行为.
• 自动创建的原始值包装对象只存在于访问它的那行代码执行期间.
• 常数值加括号可转化为对象.
• 可以对 primitive values 进行 ES6 解构语法.

const s1 = 'some text'
const s2 = s1.substring(2) // Call method on primitive string.
// let _s1 = new String(s1);
// const s2 = _s1.substring(2);
// _s1 = null;

const s3 = 'some text'
s3.color = 'red'
console.log(s3.color) // undefined

// primitive string
const greet = 'Hello there'
// primitive is converted to an object
// in order to use the split() method
const hello = greet.split(' ')[0] // "Hello"
// attempting to augment a primitive is not an error
greet.smile = true
// but it doesn't actually work
const undef = typeof greet.smile // "undefined"

不使用 new 关键字,包装类构造函数返回值为基本类型

const numberType = typeof Number(1) // "number"
const numberType = typeof Number('1') // "number"
// eslint-disable-next-line no-new-wrappers
const numberType = typeof Number(new Number()) // "number"
const stringType = typeof String(1) // "string"
const booleanType = typeof Boolean(1) // "boolean"

Reference Values

• Object e.g Date, RegExp.
• Function.
• Array.
• Map.
• Set.
• WeakMap.
• WeakSet.

Array

• 与 Object 同源.
• 关联数组: arrayName["string"] = value; 实际为 Array 对象添加属性{string:value}.
• 缓存数组长度:int l = list.length(访问length造成运算).
• []数组, {}对象.
• 数组在数值运算环境中转化为 0 (空数组)/ num (单一元素数组)/NaN (多元素数组/NaN 数组).

const array = [...Array(5).keys()] // => [0, 1, 2, 3, 4]

Array Length

• 数组下标满足 [0, 2^32-1) 即可
• 运用大于 length 的下标, length 自动增大, 不会发生数组边界错误
• length 等于 数组最后一个整数属性名+1, length 不一定等于 数组中有效元素个数

Array Literals

不使用构造函数,使用数组字面量创建数组

const arr1 = Array.from({ length: 3 }) // 数组长度
const arr2 = Array.from({ length: 3.14 }) // RangeError

if (typeof Array.isArray === 'undefined') {
  Array.isArray = function (arg) {
    // 其余对象返回值 [object Object/Number/String/Boolean]
    return Object.prototype.toString.call(arg) === '[object Array]'
  }
}

Array Of

Array.of(1) // [1]
Array.of(1, 2, 3) // [1, 2, 3]
Array.of(undefined) // [undefined]

Array From

强大的函数式方法:

• 伪数组对象 (Array-like object).
• 可枚举对象 (Iterable object).
• 浅克隆数组 (Shallow Clone).
• map 函数.

interface ArrayLike<T> {
  length: number
  [n: number]: T
}

interface Array {
  from: (<T>(iterable: Iterable<T> | ArrayLike<T>) => T[]) & (<T, U>(
    iterable: Iterable<T> | ArrayLike<T>,
    mapFunc: (v: T, i: number) => U,
    thisArg?: any
  ) => U[])
}

// Set
// Map

// NodeList 对象
const ps = document.querySelectorAll('p')
Array.from(ps).forEach((p) => {
  console.log(p)
})

// arguments 对象
function foo() {
  // eslint-disable-next-line prefer-rest-params
  const args = Array.from(arguments)
  // ...
}

Array.from('hello')
// => ['h', 'e', 'l', 'l', 'o']

const namesSet = new Set(['a', 'b'])
Array.from(namesSet) // ['a', 'b']

// 克隆数组
Array.from([1, 2, 3])
// => [1, 2, 3]

Array.from(arrayLike, x => x * x)
// =>
Array.from(arrayLike).map(x => x * x)

Array.from([1, 2, 3], x => x * x)
// [1, 4, 9]

// random array generation
Array.from(Array(5).keys())
// [0, 1, 2, 3, 4]

// Typed array initialization
Array.from<T>({ length: maxSize }).fill(initValue)

Array Fill

const numbers = [1, 2, 3, 4]
numbers.fill(1, 2)
console.log(numbers.toString()) // 1, 2, 1, 1
numbers.fill(0, 1, 3)
console.log(numbers.toString()) // 1, 0, 0, 1
numbers.fill(1)
console.log(numbers.toString()) // 1, 1, 1, 1

Array CopyWithin

copyWithin(dest, start, end), 替换数组元素, 修改原数组:

;[1, 2, 3, 4, 5].copyWithin(0, 3)
// => [4, 5, 3, 4, 5]
;[1, 2, 3, 4, 5].copyWithin(0, -2, -1)
// -2相当于3号位, -1相当于4号位
// => [4, 2, 3, 4, 5]

// 将2号位到数组结束, 复制到0号位
const i32a = new Int32Array([1, 2, 3, 4, 5])
i32a.copyWithin(0, 2)
// => Int32Array [3, 4, 5, 4, 5]

Array Stack

arr.unshift(value) // 添加数组首元素
arr.push(value) // 添加数组尾元素
arr.shift() // 删除数组首元素
arr.pop() // 删除数组尾元素

Array Slice and Merge

• slice 不改变原数组, splice 改变原数组.

;[].slice(start, end) // [start] - [end - 1]
;[].splice(startIndex, lengthToDelete, insertElements) // 功能强大的多态方法
;[].concat(otherArray)
;[].join(separator)

Array Query

;[].at(index) // ES2022
;[].includes(element) // boolean.
;[].find(callback) // element.
;[].findIndex(callback) // element index.
;[].indexOf(element) // -1 or other.
;[].lastIndexOf(element) // -1 or other.

// console.log([NaN].indexOf(NaN));
// -1

console.log([Number.NaN].includes(Number.NaN))
// true

Array Element Filter

相当于 Haskell 中的 List Filter:

const numbers = [1, 2, 3, 4, 5, 4, 3, 2, 1]
const filterResult = numbers.filter((item, index, array) => item > 2)
console.log(filterResult) // 3,4,5,4,3

Array Boolean Filter

• Array.every(filer).
• Array.some(filer).

const numbers = [1, 2, 3, 4, 5, 4, 3, 2, 1]
const everyResult = numbers.every((item, index, array) => item > 2)
const someResult = numbers.some((item, index, array) => item > 2)
console.log(everyResult) // false
console.log(someResult) // true

Array With

Array.prototype.with:

const arr = [1, 2, 3, 4, 5]
console.log(arr.with(2, 6)) // [1, 2, 6, 4, 5]
console.log(arr) // [1, 2, 3, 4, 5]

const arr = [1, 2, 3, 4, 5]
console.log(arr.with(2, 6).map(x => x ** 2)) // [1, 4, 36, 16, 25]

const frameworks = ['Nuxt', 'Remix', 'SvelteKit', 'Ember']
console.log(frameworks.with(-1, 'React'))
// ✅ Returns a copy with the change: ['Nuxt', 'Remix', 'SvelteKit', 'React'].

Array Map

相当于 Haskell 中的 List Map:

;[].map(item => item + 1) // map over

Array Flat

[2, [2, 2]] => [2, 2, 2]

Array FlatMap

map + flat.

function flattenDeep(arr) {
  return Array.isArray(arr)
    ? arr.reduce((a, b) => a.concat(flattenDeep(b)), [])
    : [arr]
}

flattenDeep([1, [[2], [3, [4]], 5]])
// => [1, 2, 3, 4, 5]

// ES2019
;[1, [2, [3, [4]], 5]].flat(Number.POSITIVE_INFINITY)
// => [1, 2, 3, 4, 5]

function flattenDeep(arr) {
  return arr.flatMap((subArray, index) =>
    Array.isArray(subArray) ? flattenDeep(subArray) : subArray
  )
}

flattenDeep([1, [[2], [3, [4]], 5]])
// => [1, 2, 3, 4, 5]

Array Reduce

reduce/reduceRight:

• Accumulator: initial value, otherwise array[0].
• Current value: array[0], otherwise array[1].
• Implement array sets manipulation (reduce/filter/includes).
• Implement XXXBy functional methods.

;[].reduce(
  (previous, current, currentIndex, arr) => current + previous,
  initial
) // fold function

Implement groupBy:

const groupByLength = ['one', 'two', 'three'].reduce(
  (acc, current, _index, _array) => {
    const key = current.length
    ;(acc[key] || (acc[key] = [])).push(current)
    return acc
  },
  {}
)
// {3: ["one", "two"], 5: ["three"]}

const groupByFunction = [1.3, 2.1, 2.4].reduce(
  (acc, current, _index, _array) => {
    const key = Math.floor(current)
    ;(acc[key] || (acc[key] = [])).push(current)
    return acc
  },
  {}
)
// {1: [1.3], 2: [2.1, 2.4]}

Array Traversal

array.forEach((val) => {}) // 遍历数组所有元素.

Array Sort

toExchange:

• return 1: a, b 交换位置.
• return -1: a, b 不交换位置.

arr.sort(toExchange)
strings.sort((a, b) => a.localeCompare(b))
strings.sort((a, b) => new Intl.Collator('en').compare(a, b))

Array Reverse

;[].reverse()

// Tips
// 反转字符串
const reverseStr = normalizedStr.split('').reverse().join('')

Array Spread

• Shallow Clone.
• Iterable Consumer.

arr2.push(...arr1)

const obj = { x: 1, y: 2, z: 3 }

obj[Symbol.iterator] = function* () {
  yield 1
  yield 2
  yield 3
}

const array = [...obj] // print [1, 2, 3]

Array Deep Clone

const nestedArray = [1, [2], 3]
const arrayCopy = JSON.parse(JSON.stringify(nestedArray))

// Make some changes
arrayCopy[0] = '1' // change shallow element
arrayCopy[1][0] = '3' // change nested element
console.log(arrayCopy) // [ '1', [ '3' ], 3 ]

// Good: Nested array NOT affected
console.log(nestedArray) //  1, [ 2 ], 3 ]

Typed Array

Typed Array 是 ArrayBuffer (用于 Web GL 高效率内存操作) 其中一种视图:

• File.
• XMLHttpRequest.
• Fetch.
• Web Worker.
• WebSocket.
• Canvas.
• WebGL.
• Web Audio.

// 第一个参数是应该返回的数组类型
// 其余参数是应该拼接在一起的定型数组
function typedArrayConcat(TypedArrayConstructor, ...typedArrays) {
  // 计算所有数组中包含的元素总数
  const numElements = typedArrays.reduce((x, y) => (x.length || x) + y.length)
  // 按照提供的类型创建一个数组, 为所有元素留出空间
  const resultArray = new TypedArrayConstructor(numElements)
  // 依次转移数组
  let currentOffset = 0
  typedArrays.forEach((x) => {
    resultArray.set(x, currentOffset)
    currentOffset += x.length
  })
  return resultArray
}

const concatArray = typedArrayConcat(
  Int32Array,
  Int8Array.of(1, 2, 3),
  Int16Array.of(4, 5, 6),
  Float32Array.of(7, 8, 9)
)

console.log(concatArray) // [1, 2, 3, 4, 5, 6, 7, 8, 9]
console.log(concatArray instanceof Int32Array) // true

const view = new Int16Array([25, 50])
console.log(view instanceof Int16Array) // true
// eslint-disable-next-line unicorn/no-instanceof-array
console.log(view instanceof Array) // false
console.log(Array.isArray(view)) // false

Map

• size.
• has().
• get().
• set().
• delete().
• clear().
• keys().
• values().
• entries().

const map = new Map([
  // You define a map via an array of 2-element arrays. The first
  // element of each nested array is the key, and the 2nd is the value
  ['name', 'Jean-Luc Picard'],
  ['age', 59],
  ['rank', 'Captain'],
])

// To get the value associated with a given `key` in a map, you
// need to call `map.get(key)`. Using `map.key` will **not** work.
map.get('name') // 'Jean-Luc Picard'

const map = new Map([])

// eslint-disable-next-line no-new-wrappers
const n1 = new Number(5)
// eslint-disable-next-line no-new-wrappers
const n2 = new Number(5)

map.set(n1, 'One')
map.set(n2, 'Two')

// `n1` and `n2` are objects, so `n1 !== n2`. That means the map has
// separate keys for `n1` and `n2`.
map.get(n1) // 'One'
map.get(n2) // 'Two'
map.get(5) // undefined

// If you were to do this with an object, `n2` would overwrite `n1`
const obj = {}
obj[n1] = 'One'
obj[n2] = 'Two'

const two1 = obj[n1] // 'Two'
const two2 = obj[5] // 'Two'

const objectClone = new Map(Object.entries(object))
const arrayClone = new Map(Array.from(map.entries))
const map = new Map([
  ['name', 'Jean-Luc Picard'],
  ['age', 59],
  ['rank', 'Captain'],
])

// The `for/of` loop can loop through iterators
for (const key of map.keys())
  console.log(key) // 'name', 'age', 'rank'

for (const value of map.values())
  console.log(value) // 'Jean-Luc Picard', 59, 'Captain'

for (const [key, value] of map.entries()) {
  console.log(key) // 'name', 'age', 'rank'
  console.log(value) // 'Jean-Luc Picard', 59, 'Captain'
}

Set

• size.
• has().
• add().
• delete().
• clear().
• keys().
• values().
• entries().

class XSet extends Set {
  union(...sets) {
    return XSet.union(this, ...sets)
  }

  intersection(...sets) {
    return XSet.intersection(this, ...sets)
  }

  difference(set) {
    return XSet.difference(this, set)
  }

  symmetricDifference(set) {
    return XSet.symmetricDifference(this, set)
  }

  cartesianProduct(set) {
    return XSet.cartesianProduct(this, set)
  }

  powerSet() {
    return XSet.powerSet(this)
  }

  // 返回两个或更多集合的并集
  // new Set([...setA, ...setB]);
  static union(a, ...bSets) {
    const unionSet = new XSet(a)

    for (const b of bSets) {
      for (const bValue of b)
        unionSet.add(bValue)
    }

    return unionSet
  }

  // 返回两个或更多集合的交集
  // new Set([...setA].filter(x => setB.has(x)))
  static intersection(a, ...bSets) {
    const intersectionSet = new XSet(a)

    for (const aValue of intersectionSet) {
      for (const b of bSets) {
        if (!b.has(aValue))
          intersectionSet.delete(aValue)
      }
    }

    return intersectionSet
  }

  // 返回两个集合的差集
  // new Set([...setA].filter(x => !setB.has(x)))
  static difference(a, b) {
    const differenceSet = new XSet(a)

    for (const bValue of b) {
      if (a.has(bValue))
        differenceSet.delete(bValue)
    }

    return differenceSet
  }

  // 返回两个集合的对称差集
  static symmetricDifference(a, b) {
    // 按照定义, 对称差集可以表达为:
    return a.union(b).difference(a.intersection(b))
  }

  // 返回两个集合 (数组对形式) 的笛卡儿积
  // 必须返回数组集合, 因为笛卡儿积可能包含相同值的对
  static cartesianProduct(a, b) {
    const cartesianProductSet = new XSet()

    for (const aValue of a) {
      for (const bValue of b)
        cartesianProductSet.add([aValue, bValue])
    }

    return cartesianProductSet
  }

  // 返回一个集合的幂集
  static powerSet(a) {
    const powerSet = new XSet().add(new XSet())

    for (const aValue of a) {
      for (const set of new XSet(powerSet))
        powerSet.add(new XSet(set).add(aValue))
    }

    return powerSet
  }
}

WeakMap and WeakSet

WeakMap 结构与 Map 结构基本类似, 唯一的区别就是 WeakMap 只接受非 null 对象作为键名:

• 弱键: 键名构建的引用无法阻止对象执行垃圾回收.
• 不可迭代键: 键/值随时可能被垃圾回收, 无需提供迭代能力, 无 clear() 方法.

它的键所对应的对象可能会在将来消失. 一个对应 DOM 元素的 WeakMap 结构, 当某个 DOM 元素被清除, 其所对应的 WeakMap 记录就会自动被移除.

有时候我们会把对象作为一个对象的键用来存放属性值, 普通集合类型比如简单对象 (Object/Map/Set) 会阻止垃圾回收器对这些作为属性键存在的对象的回收, 有造成内存泄漏的危险, WeakMap/WeakSet 则更加内存安全:

• Caching computed results.
• Managing listeners.
• Keeping private data.

Date

• Date and Temporal API CheatSheet
• Definitive Guide

const now = new Date()
now.getFullYear() // 1-n
now.getMonth() // Warn: 0-11
now.getDate() // 1-n
now.getDay() // Warn: 0-6
now.getHours()
now.getSeconds()
now.toString()
now.toDateString()
now.toTimeString()
now.toUTCString()
now.toLocaleString()
now.toLocaleDateString()
now.toLocaleTimeString()

function daysOfMonth(year, month) {
  // `0` for last month of next month
  return new Date(year, month + 1, 0).getDate()
}

function prevYear(year) {
  return new Date(year - 1, 0).getFullYear()
}

function nextYear(year) {
  return new Date(year + 1, 0).getFullYear()
}

function prevMonth(year, month) {
  return new Date(year, month - 1).getMonth()
}

function nextMonth(year, month) {
  return new Date(year, month + 1).getMonth()
}

function getDateItemList(year, month) {
  const days = daysOfMonth(year, month)
  const currentDateItemList = [...Array(days).keys()].map((index) => {
    return DateItem(year, month, 1 + index)
  })

  const firstDayItem = DateItem(year, month, 1)
  const firstDayWeekday = firstDayItem.day
  const lastMonthDays = daysOfMonth(year, month - 1)
  const prefixDays = firstDayWeekday === 0 ? 7 : firstDayWeekday
  const prefixFirstDay = lastMonthDays - prefixDays + 1
  const prefixYear = prevYear(year)
  const prefixMonth = prevMonth(year, month)
  const prefixDateItemList = [...Array(prefixDays).keys()].map((index) => {
    return DateItem(prefixYear, prefixMonth, prefixFirstDay + index)
  })

  const lastDayItem = DateItem(year, month, days)
  const lastDayWeekday = lastDayItem.day
  const suffixDays = lastDayWeekday === 6 ? 7 : 6 - lastDayWeekday
  const suffixYear = nextYear(year)
  const suffixMonth = nextMonth(year, month)
  const suffixDateItemList = [...Array(suffixDays).keys()].map((index) => {
    return DateItem(suffixYear, suffixMonth, 1 + index)
  })

  const dateItemList = [
    ...prefixDateItemList,
    ...currentDateItemList,
    ...suffixDateItemList,
  ]

  return dateItemList
}

Temporal

Temporal Basis

Temporal.ZonedDateTime.from({
  year,
  month,
  day,
  timeZone: Temporal.Now.timeZone(),
})

Temporal.ZonedDateTime.from({
  year,
  month,
  day,
  hour,
  minute,
  timeZone: Temporal.Now.timeZone(),
})

const second = Temporal.Now.zonedDateTimeISO().second
const hour = Temporal.Now.zonedDateTimeISO().hour
const day = Temporal.Now.zonedDateTimeISO().day

Temporal.Now.zonedDateTimeISO().with({ second: 30 })
Temporal.Now.zonedDateTimeISO().with({ hour: 13 })
Temporal.Now.zonedDateTimeISO().with({ day: 1 })
Temporal.Now.zonedDateTimeISO().withPlainTime(
  new Temporal.PlainTime(23, 59, 59, 999, 999, 999)
)

Temporal Range

const dayOfWeek = Temporal.Now.zonedDateTimeISO().dayOfWeek
const dayOfYear = Temporal.Now.zonedDateTimeISO().dayOfYear
const daysInMonth = new Temporal.PlainYearMonth(2012, 2).daysInMonth
const daysInMonth = Temporal.PlainYearMonth.from('2012-02').daysInMonth
const weekOfYear = Temporal.Now.zonedDateTimeISO().weekOfYear
const weekOfYear = Temporal.PlainDate.from({
  day: 31,
  month: 12,
  year: Temporal.Now.plainDateISO(),
}).weekOfYear
const inLeapYear = Temporal.PlainDate.from('2000-01-01').inLeapYear

Temporal.Now.zonedDateTimeISO().add(Temporal.Duration.from({ days: 7 }))
Temporal.Now.zonedDateTimeISO().subtract(Temporal.Duration.from({ days: 14 }))
Temporal.Now.zonedDateTimeISO()
  .with({ month: 1, day: 1 })
  .add(Temporal.Duration.from({ days: 256 }))
Temporal.Now.zonedDateTimeISO()
  .with({ month: 1, day: 1 })
  .add(Temporal.Duration.from({ weeks: 23 }))

Temporal.Instant.fromEpochMilliseconds(Math.max.apply(null, dateArrays))
Temporal.Instant.fromEpochMilliseconds(Math.min.apply(null, dateArrays))

Temporal Display

new Intl.DateTimeFormat('en-GB', {
  dateStyle: 'full',
  timeStyle: 'medium',
}).format(Temporal.Now.zonedDateTimeISO())
new Intl.DateTimeFormat('de-DE', { weekday: 'short', hour: 'numeric' }).format(
  Temporal.Now.zonedDateTimeISO()
)

Temporal.PlainDate.from('2007-01-27').until('2007-01-29')
Temporal.PlainDate.from('2007-01-27')
  .since('2007-01-29')
  .total({ unit: 'millisecond' })
Temporal.PlainDate.from('2007-01-27').since('2007-01-29').total({ unit: 'day' })

Temporal Query

const isBefore = Temporal.PlainDate.compare('2010-10-20', '2010-10-21') === -1
const isAfter = Temporal.PlainDate.compare('2010-10-20', '2010-10-19') === 1
const isEqual = Temporal.PlainDate.from('2010-10-20').equals('2010-10-21')
const isEqual = Temporal.PlainDate.from('2010-10-20').equals('2010-10-20')
const isEqual
  = Temporal.PlainDate.from('2010-10-20').month
  === Temporal.PlainDate.from('2010-10-21').month

const isPlainTime = Temporal.Now.plainTimeISO() instanceof Temporal.PlainTime
const isPlainDate = Temporal.Now.plainDateISO() instanceof Temporal.PlainDate
const isPlainDateTime
  = Temporal.Now.plainDateTimeISO() instanceof Temporal.PlainDateTime
const isZonedDateTime
  = Temporal.Now.zonedDateTimeISO() instanceof Temporal.ZonedDateTime

Variable

Variable Hoisting

• 一方面规定, var/function 声明的全局变量, 依旧是全局对象的属性, 意味着会Hoisting.
• 另一方面规定, let/const/class 声明的全局变量, 不属于全局对象的属性, 意味着不会Hoisting.
• var 只有函数作用域, let/const 拥有块级作用域.
• var 表达式和 function 声明都将会被提升到当前作用域 (全局作用域/函数作用域) 顶部, 其余表达式顺序不变.

	Hoisting	Scope	Creates Global Properties
var	Declaration	Function	Yes
let	Temporal dead zone	Block	No
const	Temporal dead zone	Block	No
class	Temporal dead zone	Block	No
function	Complete	Block	Yes
import	Complete	Module-global	No

// 我们知道这个行不通 (假设没有未定义的全局变量)
function example() {
  console.log(notDefined) // => throws a ReferenceError
}

// 在引用变量后创建变量声明将会因变量提升而起作用.
// 注意: 真正的值 `true` 不会被提升.
function example() {
  console.log(declaredButNotAssigned) // => undefined
  var declaredButNotAssigned = true
}

// 解释器将变量提升到函数的顶部
// 这意味着我们可以将上边的例子重写为:
function example() {
  let declaredButNotAssigned
  console.log(declaredButNotAssigned) // => undefined
  declaredButNotAssigned = true
}

// 使用 const 和 let
function example() {
  console.log(declaredButNotAssigned) // => throws a ReferenceError
  console.log(typeof declaredButNotAssigned) // => throws a ReferenceError
  const declaredButNotAssigned = true
}

function example() {
  console.log(named) // => undefined

  named() // => TypeError named is not a function

  superPower() // => ReferenceError superPower is not defined

  var named = function superPower() {
    console.log('Flying')
  }
}

Let Variable

• 块级作用域内定义的变量/函数, 在块级作用域外 ReferenceError.
• 不存在变量提升, 导致暂时性死区 (Temporal Dead Zone).
• let variable in for-loop closure, every closure for each loop binds the block-scoped variable.

const a = 1

b = 3 // temporal dead zone: throw reference error

let b = 2

let 变量拥有块级作用域 (每个 setTimeout 引用的都是不同的变量实例):

// for (var i = 0; i < 5; ++i) {
//   setTimeout(() => console.log(i), 0);
// }
// Output 5, 5, 5, 5, 5.
// 所有的 i 都是同一个变量, 输出同一个最终值.

for (let i = 0; i < 5; ++i)
  setTimeout(() => console.log(i), 0)

// Output: 0, 1, 2, 3, 4.
// JavaScript 引擎会为每个迭代循环声明一个新的迭代变量.
// 每个 setTimeout 引用的都是不同的变量实例.

Const Variable

• const 一旦声明变量, 就必须立即初始化, 不能留到以后赋值.
• 引用一个Reference变量时, 只表示此变量地址不可变, 但所引用变量的值/属性可变 (xxx *const, 即const指针, 指向一个变量).
• 块级作用域.
• 不存在变量提升, 导致暂时性死区 (Temporal Dead Zone).

const f = () => g()
const g = () => 123

// We call f() after g() was declared:
assert.equal(f(), 123)

funcDecl()

const MY_STR = 'abc'
function funcDecl() {
  assert.throws(() => MY_STR, ReferenceError)
}

Type Detection

function typeOf(o) {
  const _toString = Object.prototype.toString
  const _type = {
    'undefined': 'undefined',
    'number': 'number',
    'boolean': 'boolean',
    'string': 'string',
    '[object Function]': 'function',
    '[object GeneratorFunction]': 'function',
    '[object Array]': 'array',
    '[object Date]': 'date',
    '[object RegExp]': 'regexp',
    '[object Error]': 'error',
    '[object JSON]': 'json',
  }

  return _type[typeof o] || _type[_toString.call(o)] || (o ? 'object' : 'null')
}

function type(item) {
  const reTypeOf = /^\[object\s(.*?)\]$/

  return Object.prototype.toString
    .call(item)
    .replace(reTypeOf, '$1')
    .toLowerCase()
}

Null Detection

不应使用 typeof 检测 null, 应使用 ===/!==.

/*
 * ECMAScript 标准的重大 bug
 */
const objectType = typeof null // => object

Property Detection

• 由于属性值可能为零值值表达式, 不应使用零值表达式(0/NaN/''/null/undefined) 检测属性值.
• 应使用 for in 进行属性检测.

Custom Object Detection

• object instanceof Constructor: 在原型链上查找构造器的原型对象 (Constructor.prototype).
• prop in object: 查找原型链属性名.

/**
 * L 表示左表达式, R 表示右表达式: L 为变量, R 为类型.
 */
function instanceOf(L, R) {
  const prototype = R.prototype
  let chain = L[[proto]]

  while (true) {
    if (chain === null)
      return false

    if (prototype === chain)
      return true

    chain = chain[[proto]]
  }
}

Type Conversion

Type Conversion Context

• 字符串 -> 整数: +string/Number(string)/parseInt(string, arg1).
• any -> bool: !!any.
• const -> object: (const).
• parseInt(str, base):
  • 遇到非数字字符立即停止运行, 返回当前转化值.
  • 将 0 开头字符串解析为八进制数, 0x 开头字符串解析为十六进制数.
• boolean在数值运算环境中 true => 1, false => 0.
• 数组在数值运算环境中转化为 0 (空数组)/num (单一元素数组)/NaN (多元素数组/NaN 数组).
• 对象在逻辑运算环境中转化为 true , 包括 false 的封装对象.
• 对象在数值运算环境中先利用 valueOf(object), 再利用 toString() 转化为数字, 若转化失败, 则返回 NaN.
• 对象与数值加号运算: 先数值加, (失败后)再字符串加.

// good
const totalScore = String(this.reviewScore)

// good
const val = Number(inputValue)

// good
const val = Number.parseInt(inputValue, 10)

// good
const hasAge = Boolean(age)

// best
const hasAge = !!age

Type Conversion Algorithms

function ToString(argument) {
  if (argument === undefined) {
    return 'undefined'
  } else if (argument === null) {
    return 'null'
  } else if (argument === true) {
    return 'true'
  } else if (argument === false) {
    return 'false'
  } else if (TypeOf(argument) === 'number') {
    return Number.toString(argument)
  } else if (TypeOf(argument) === 'string') {
    return argument
  } else if (TypeOf(argument) === 'symbol') {
    return Symbol.toString(argument)
  } else if (TypeOf(argument) === 'bigint') {
    return BigInt.toString(argument)
  } else {
    // argument is an object
    const primValue = ToPrimitive(argument, 'string')
    return ToString(primValue)
  }
}

function ToPropertyKey(argument) {
  const key = ToPrimitive(argument, 'string') // (A)

  if (TypeOf(key) === 'symbol')
    return key

  return ToString(key)
}

function ToNumeric(value) {
  const primValue = ToPrimitive(value, 'number')

  if (TypeOf(primValue) === 'bigint')
    return primValue

  return ToNumber(primValue)
}

function ToNumber(argument) {
  if (argument === undefined) {
    return Number.NaN
  } else if (argument === null) {
    return +0
  } else if (argument === true) {
    return 1
  } else if (argument === false) {
    return +0
  } else if (TypeOf(argument) === 'number') {
    return argument
  } else if (TypeOf(argument) === 'string') {
    return parseTheString(argument) // not shown here
  } else if (TypeOf(argument) === 'symbol') {
    throw new TypeError('Failed!')
  } else if (TypeOf(argument) === 'bigint') {
    throw new TypeError('Failed!')
  } else {
    // argument is an object
    const primValue = ToPrimitive(argument, 'number')
    return ToNumber(primValue)
  }
}

ToPrimitive:

• [Symbol.toPrimitive]().
• toString().
• valueOf().

/**
 * @param input input string
 * @param hint Which type is preferred for the result string, number etc.
 */
function ToPrimitive(
  input: any,
  hint: 'string' | 'number' | 'default' = 'default'
) {
  if (TypeOf(input) === 'object') {
    const exoticToPrim = input[Symbol.toPrimitive] // (A)

    if (exoticToPrim !== undefined) {
      const result = exoticToPrim.call(input, hint)

      if (TypeOf(result) !== 'object')
        return result

      throw new TypeError('[Symbol.toPrimitive]() failed!')
    }

    if (hint === 'default')
      hint = 'number'

    return OrdinaryToPrimitive(input, hint)
  } else {
    // input is already primitive
    return input
  }
}

function OrdinaryToPrimitive(O: object, hint: 'string' | 'number') {
  const methodNames
    = hint === 'string' ? ['toString', 'valueOf'] : ['valueOf', 'toString']

  for (const name of methodNames) {
    const method = O[name]

    if (IsCallable(method)) {
      const result = method.call(O)

      if (TypeOf(result) !== 'object')
        return result
    }
  }

  throw new TypeError('Conversion failed!')
}

Operators

Loose Comparison

== 与 != loose comparison:

• Type conversion first, then comparison.
• Return comparison between ToNumber(x) and ToPrimitive(y).

/** Loose equality (==) */
function abstractEqualityComparison(x, y) {
  if (TypeOf(x) === TypeOf(y)) {
    // Use strict equality (===)
    return strictEqualityComparison(x, y)
  }

  // Comparing null with undefined
  if (x === null && y === undefined)
    return true

  if (x === undefined && y === null)
    return true

  // Comparing a number and a string
  if (TypeOf(x) === 'number' && TypeOf(y) === 'string')
    return abstractEqualityComparison(x, Number(y))

  if (TypeOf(x) === 'string' && TypeOf(y) === 'number')
    return abstractEqualityComparison(Number(x), y)

  // Comparing a bigint and a string
  if (TypeOf(x) === 'bigint' && TypeOf(y) === 'string') {
    const n = StringToBigInt(y)

    if (Number.isNaN(n))
      return false

    return abstractEqualityComparison(x, n)
  }
  if (TypeOf(x) === 'string' && TypeOf(y) === 'bigint')
    return abstractEqualityComparison(y, x)

  // Comparing a boolean with a non-boolean
  if (TypeOf(x) === 'boolean')
    return abstractEqualityComparison(Number(x), y)

  if (TypeOf(y) === 'boolean')
    return abstractEqualityComparison(x, Number(y))

  // Comparing an object with a primitive
  // (other than undefined, null, a boolean)
  if (
    ['string', 'number', 'bigint', 'symbol'].includes(TypeOf(x))
    && TypeOf(y) === 'object'
  ) {
    return abstractEqualityComparison(x, ToPrimitive(y))
  }

  if (
    TypeOf(x) === 'object'
    && ['string', 'number', 'bigint', 'symbol'].includes(TypeOf(y))
  ) {
    return abstractEqualityComparison(ToPrimitive(x), y)
  }

  // Comparing a bigint with a number
  if (
    (TypeOf(x) === 'bigint' && TypeOf(y) === 'number')
    || (TypeOf(x) === 'number' && TypeOf(y) === 'bigint')
  ) {
    if (
      [Number.NaN, +Number.POSITIVE_INFINITY, Number.NEGATIVE_INFINITY].includes(x)
      || [Number.NaN, +Number.POSITIVE_INFINITY, Number.NEGATIVE_INFINITY].includes(y)
    ) {
      return false
    }

    if (isSameMathematicalValue(x, y))
      return true
    else
      return false
  }

  return false
}

Strict Comparison

=== 与 !==:

• Strings: same length, same characters in corresponding positions.
• Numbers: numerically equal.
• Objects: refer to same Object.
• Positive and negative 0 are equal to one another.
• NaN is not equal to anything, including NaN.
• null and undefined types are not equal with ===, but equal with ==.

const true1 = 0 == false // true
const false1 = 0 === false // false
const true2 = 1 == '1' // true
const false2 = 1 === '1' // false
const true3 = undefined == null // true
const false3 = undefined === null // false
const true4 = '0' == false // true
const false4 = '0' === false // false
// eslint-disable-next-line no-self-compare
const false5 = [] == [] // false, refer different objects in memory
// eslint-disable-next-line no-self-compare
const false6 = [] === [] // false, refer different objects in memory
// eslint-disable-next-line no-self-compare
const false7 = {} == {} // false, refer different objects in memory
// eslint-disable-next-line no-self-compare
const false8 = {} === {} // false, refer different objects in memory

Object.is:

// Case 1: Evaluation result is the same as using ===
Object.is(25, 25) // true
Object.is('foo', 'foo') // true
Object.is('foo', 'bar') // false
Object.is(null, null) // true
Object.is(undefined, undefined) // true
Object.is(window, window) // true
Object.is([], []) // false
const foo = { a: 1 }
const bar = { a: 1 }
Object.is(foo, foo) // true
Object.is(foo, bar) // false: different reference pointers.

// Case 2: Signed zero
Object.is(0, -0) // false
Object.is(+0, -0) // false
Object.is(-0, -0) // true
Object.is(0n, -0n) // true

// Case 3: NaN
Object.is(Number.NaN, 0 / 0) // true
Object.is(Number.NaN, Number.NaN) // true

if (!Object.is) {
  Object.defineProperty(Object, 'is', {
    value: (x, y) => {
      // SameValue algorithm
      if (x === y) {
        // return true if x and y are not 0, OR
        // if x and y are both 0 of the same sign.
        // This checks for cases 1 and 2 above.
        return x !== 0 || 1 / x === 1 / y
      } else {
        // return true if both x AND y evaluate to NaN.
        // The only possibility for a variable to not be strictly equal to itself
        // is when that variable evaluates to NaN (example: Number.NaN, 0/0, NaN).
        // This checks for case 3.
        // eslint-disable-next-line no-self-compare
        return x !== x && y !== y
      }
    },
  })
}

Conditional Expression

养成使用分号结束句子的习惯, 需分行显示的语句必须确保单行不会形成完整语义:

const i = a ? 1 : b ? 2 : c ? 3 : 4

Add Operator

a + b:

• 如果有一个是对象, 则遵循对象对原始值的转换过程:
  • Date 对象直接调用 toString 完成转换.
  • 其他对象通过 valueOf 转化, 如果转换不成功则调用 toString.
• 如果两个都是对象, 两个对象都遵循步骤 1 转换到字符串.
• 两个数字, 进行算数运算.
• 两个字符串, 直接拼接.
• 一个字符串一个数字, 直接拼接为字符串.

Dot Operator

. 优先级高于 =: el.data 优先求值, 引用 old, 指向 old.data. 5 => el, 5 => el.data (old.data).

let el = { data: 1 }
const old = el

el.data = el = 5
console.log(el) // 5
console.log(el.data) // undefined
console.log(old) // { data: 5 }
console.log(old.data) // 5

Logical Operator

• Optional Chaining Operator ?.: Legible property chains that don't throw an error if a requested reference is missing.
• Nullish coalescing operator ??: Binary operator. If the value of left side expression is null or undefined, right side of the operator is evaluated.
• Logical assignment operators: &&=, ||=, ??=.

Assignment Operator	Equivalent To	Only Assigns When a
a ||= b	a || (a = b)	Falsy
a &&= b	a && (a = b)	Truthy
a ??= b	a ?? (a = b)	Nullish

Delete Operator

delete operator returns a boolean value:

• true on a successful deletion.
• false on a failed deletion: var/let/const variables cannot be deleted using delete operator.

const name = 'Lydia'
age = 21

// eslint-disable-next-line no-delete-var
console.log(delete name) // false
// eslint-disable-next-line no-delete-var
console.log(delete age) // true

Operator Reference

• Operator Lookup

Destructuring Pattern Matching

• 建议只要有可能, 就不要在模式中放置圆括号.
• 赋值语句的非模式部分, 可以使用圆括号.
• Every time access value via .: stop and think whether use destructuring instead.
• Destructure as early as possible.
• Remember to include default values, especially in nested destructuring.

Destructuring Default Value

• ES6 内部使用严格相等运算符 (===), 判断一个位置是否有值. 若此位置无值, 则使用默认值.
• 如果一个数组成员不严格等于 undefined, 默认值不会生效.

const [x = 1] = [undefined]
console.log(x) // 1

const [x = 1] = [null]
console.log(x) // null

let [x = 1, y = x] = [] // x=1; y=1
let [x = 1, y = x] = [2] // x=2; y=2
let [x = 1, y = x] = [1, 2] // x=1; y=2
let [x = y, y = 1] = [] // ReferenceError

Object Destructuring

• 解构赋值的规则: 只要等号右边的值不是对象, 就先将其转为对象.
• undefined/null 无法转化为对象:

const { prop: x } = undefined // TypeError
const { prop: y } = null // TypeError

const { bar, foo } = { foo: 'aaa', bar: 'bbb' }
console.log(foo) // "aaa"
console.log(bar) // "bbb"

const { baz } = { foo: 'aaa', bar: 'bbb' }
console.log(baz) // undefined

• 真正被赋值的是后者, 而不是前者:

const { foo: baz } = { foo: 'aaa', bar: 'bbb' }
console.log(baz) // "aaa"

const { first: f, last: l } = { first: 'hello', last: 'world' }
console.log(f) // 'hello'
console.log(l) // 'world'

• Left-hand side of a normal assignment:

const obj = {}
;[first, ...obj.prop] = ['a', 'b', 'c']
// first = 'a'; obj.prop = ['b', 'c']

const arr = []
;({ bar: arr[0] } = { bar: true })
console.log(arr) // [true]

JSON Object Destructuring

const jsonData = {
  id: 42,
  status: 'OK',
  data: [867, 5309],
}

const { id, status, data: number } = jsonData

console.log(id, status, number)
// 42, "OK", [867, 5309]

Import Destructuring

const { SourceMapConsumer, SourceNode } = require('source-map')

Number and Boolean Destructuring

number/boolean 会自动构造原始值包装对象:

let { toString: s } = 123
const truthy = s === Number.prototype.toString // true

let { toString: s } = true
const truthy = s === Boolean.prototype.toString // true

Iterator Destructuring

等号右边必须为数组等实现了 Iterator 接口的对象, 否则报错:

• Array.
• Set.
• Generator function.

const [foo, [[bar], baz]] = [1, [[2], 3]]
console.log(foo) // 1
console.log(bar) // 2
console.log(baz) // 3

const [, , third] = ['foo', 'bar', 'baz']
console.log(third) // "baz"

const [x, , y] = [1, 2, 3]
console.log(x) // 1
console.log(y) // 3

const [head, ...tail] = [1, 2, 3, 4]
console.log(head) // 1
console.log(tail) // [2, 3, 4]

const [x, y, ...z] = ['a']
console.log(x) // "a"
console.log(y) // undefined
console.log(z) // []

// Generator 函数
function* fibs() {
  let a = 0
  let b = 1

  while (true) {
    yield a
    ;[a, b] = [b, a + b]
  }
}

const [first, second, third, fourth, fifth, sixth] = fibs()
console.log(sixth) // 5

• Left-hand side of a normal assignment:

let x = 1
let y = 2
;[x, y] = [y, x]

Map and List Destructuring

• for index in Iterable<T>: key.
• for [key, value] of Iterable<T>: entry.

const map = new Map()
map.set('first', 'hello')
map.set('second', 'world')

for (const [key, value] of map)
  console.log(`${key} is ${value}`)

// first is hello
// second is world

// 获取键名
for (const [key] of map) {
  // ...
}

// 获取键值
for (const [, value] of map) {
  // ...
}

String Destructuring

const [a, b, c, d, e] = 'hello'
console.log(a) // "h"
console.log(b) // "e"
console.log(c) // "l"
console.log(d) // "l"
console.log(e) // "o"

const { length: len } = 'hello'
console.log(len) // 5

Function Parameters Destructuring

• 可用于工厂 (factory) / 设置 (options) 模式传参一般为 options 对象,
• 具有固定的属性名.
• 一次性定义多个参数.
• 一次性定义多个参数的默认值.

// 参数是一组有次序的值
function f1([x, y, z]) {}
f1([1, 2, 3])

// 参数是一组无次序的值
function f2({ x, y, z }) {}
f2({ z: 3, y: 2, x: 1 })

// 可省略 const foo = config.foo || 'default foo';
jQuery.ajax = function (
  url,
  {
    async = true,
    beforeSend = function () {},
    cache = true,
    complete = function () {},
    crossDomain = false,
    global = true,
    // ... more config
  }
) {
  // ... do stuff
}

Function Return Value Destructuring

返回多个值:

// 返回一个数组
function example1() {
  return [1, 2, 3]
}

const [a, b, c] = example1()

// 返回一个对象
function example2() {
  return {
    foo: 1,
    bar: 2,
  }
}

const { foo, bar } = example2()

Control Flow

Switch Case Statement

用 Strategy Pattern 代替 switch/case 语句:

function doAction(action) {
  const actions = {
    hack() {
      return 'hack'
    },

    slash() {
      return 'slash'
    },

    run() {
      return 'run'
    },
  }

  if (typeof actions[action] !== 'function')
    throw new TypeError('Invalid action.')

  // 闭包方法集
  return actions[action]()
}

Object

OOP Features

共用方法, 单独属性, 封装细节:

• 原型代理 (享元模式): 利用享元模式共享公有属性与通用方法.
• 实例状态 (原型克隆): 利用原型克隆拥有各自属性值.
• 封装性 (闭包式继承): 利用闭包方法实现属性私有化.

Prototype Chain

• 实例化对象仅有属性__proto__, 没有属性prototype, 函数才具有属性 prototype (指向引擎为其自动创建的原型对象): Instance.__proto__ === Constructor.prototype.
• 所有引用类型 (包括对象/数组/函数/构造函数) 都有属性__proto__(隐式原型).
• 所有函数/构造函数的 __proto__ 都指向 Function.prototype.
• 除Object.prototype.__proto__指向 null 外, 其余函数/构造函数的原型对象的__proto__ 都指向 Object.prototype.
• 除Object.create()外, 所新建对象的 __proto__ 指向构造该对象的构造函数的原型对象(prototype).
• 除typeof Function.prototype 为 'function' 外, 其余函数/构造函数的原型对象都为 '对象'(typeof 为 'object').
• 先有Object.prototype(原型链顶端), Function.prototype 继承Object.prototype而产生, 最后Object/Function/Array/其它构造函数继承Function.prototype而产生.

// True because of `Object` is `function Object()` and inherited from `Function.prototype`
// Object has its own `prototype` property refer to `Object.prototype`
const truthy = Object[[proto]] === Function.prototype

// True because of `Array` is `function Array()` and inherited from `Function.prototype`
// Array has its own `prototype` property refer to `Array.prototype`
const truthy = Array[[proto]] === Function.prototype

// True because of Function is `function Function()` and inherited from `Function.prototype`
// Function has its own `prototype` property refer to `Function.prototype`
const truthy = Function[[proto]] === Function.prototype

// True because of Object.prototype is the top of inheritance chains (null is Object.prototype.__proto__)
// all `object/function/array instance`.__proto__......__proto__ refer to Object.prototype
const truthy = Function[[proto]][[proto]] === Object.prototype

// True:
const truthy = Object instanceof Function
const truthy = Function instanceof Object

Prototype Chain

• Object.__proto__ -> Function.prototype.
• Function.prototype.__proto__ -> Object.prototype.
• Object.prototype.__proto__ -> null.

__proto__:

• [[proto]] getter is Object.getPrototypeOf(object).
• [[proto]] setter is Object.setPrototypeOf(object, prototype).

function Foo(value) {
  this.val = value
}

// Auto create FooPrototype
// Foo.prototype -> FooPrototype
// FooPrototype.constructor -> [function Foo]
// foo.__proto__ -> FooPrototype
const foo = new Foo(2)

function Person() {}
const person1 = new Person()
const person2 = new Person()

console.log(person1 !== Person) // true
console.log(person1 !== Person.prototype) // true
console.log(Person.prototype !== Person) // true

// eslint-disable-next-line no-proto, no-restricted-properties
console.log(person1.__proto__ === Person.prototype) // true
// eslint-disable-next-line no-proto, no-restricted-properties
console.log(person1.__proto__.constructor === Person) // true
// eslint-disable-next-line no-proto, no-restricted-properties
console.log(person1.__proto__ === person2.__proto__) // true

// eslint-disable-next-line no-prototype-builtins
console.log(Person.prototype.isPrototypeOf(person1)) // true
// eslint-disable-next-line no-prototype-builtins
console.log(Person.prototype.isPrototypeOf(person2)) // true
console.log(Object.getPrototypeOf(person1) === Person.prototype) // true

console.log(person1 instanceof Person) // true
console.log(person1 instanceof Object) // true
console.log(Person.prototype instanceof Object) // true

下面五种操作 (方法/属性/运算符) 可以触发 JS 引擎读取一个对象的原型, 可以触发 getPrototypeOf() 代理方法的运行:

const obj = {}
const p = new Proxy(obj, {
  getPrototypeOf(target) {
    return Array.prototype
  },
})

console.log(
  Object.getPrototypeOf(p) === Array.prototype, // true
  Reflect.getPrototypeOf(p) === Array.prototype, // true
  // eslint-disable-next-line no-prototype-builtins
  Array.prototype.isPrototypeOf(p), // true
  // eslint-disable-next-line no-proto, no-restricted-properties
  p.__proto__ === Array.prototype, // true
  // eslint-disable-next-line unicorn/no-instanceof-array
  p instanceof Array // true
)

Set the inherited property will create own property (overrides value of inherited property):

const proto = {
  protoProp: 'a',
}

const obj = {
  __proto__: proto,
  objProp: 'b',
}

// In the beginning, obj has one own property:
assert.deepEqual(Object.keys(obj), ['objProp'])

obj.protoProp = 'x'

// Created a new own property:
assert.deepEqual(Object.keys(obj), ['objProp', 'protoProp'])

// The inherited property itself is unchanged:
assert.equal(proto.protoProp, 'a')

// The own property overrides the inherited property:
assert.equal(obj.protoProp, 'x')

Object Conversion

对象转换为布尔值:

• 直接转换为 true (包装类型也一样), 不调用 valueOf 和 toString.

对象转换为数字:

• 如果对象具有 valueOf 方法 (返回原始值), 则将该原始值转换为数字 (转换失败会返回 NaN), 并返回这个数字.
• 如果对象具有 toString 方法 (返回原始值), 则将该原始值转换为数字 (转换失败会返回 NaN), 并返回这个数字.
• 转换失败, 抛出 TypeError.

对象转换为字符串:

• 如果对象具有 toString 方法 (返回原始值), 则将该原始值转换为字符串, 并返回该字符串.
• 如果对象具有 valueOf 方法 (返回原始值), 则将该原始值转换为字符串, 并返回该字符串.
• 转换失败, 抛出 TypeError.

x	Object(x)
undefined	{}
null	{}
boolean	new Boolean(x)
number	new Number(x)
bigint	An instance of BigInt (new throws TypeError)
string	new String(x)
symbol	An instance of Symbol (new throws TypeError)
object	x

// 保存原始的valueOf
const valueOf = Object.prototype.valueOf
const toString = Object.prototype.toString

// 添加valueOf日志
// eslint-disable-next-line no-extend-native
Object.prototype.valueOf = function () {
  console.log('valueOf')
  return valueOf.call(this)
}
// 添加toString日志
// eslint-disable-next-line no-extend-native
Object.prototype.toString = function () {
  console.log('toString')
  return toString.call(this)
}
const a = {}
// eslint-disable-next-line no-new-wrappers
const b = new Boolean(false)

if (a)
  console.log(1)

if (b)
  console.log(2)

// output:
// 1
// 2
// 未调用valueOf和toString, 符合 [对象到布尔值] 的转换规则

// 保存原始的valueOf
const valueOf = Object.prototype.valueOf
const toString = Object.prototype.toString

// 添加valueOf日志
// eslint-disable-next-line no-extend-native
Object.prototype.valueOf = function () {
  console.log('valueOf')
  return valueOf.call(this)
}
// 添加toString日志
// eslint-disable-next-line no-extend-native
Object.prototype.toString = function () {
  console.log('toString')
  return toString.call(this)
}

let a = {}
console.log(++a)

// output:
// valueOf
// toString
// NaN
// 1. valueOf方法返回的是对象本身, 不是原始值, 继续执行
// 2. toString方法返回的是”[object Object]”, 是原始值(字符串), 将字符串转换为数字NaN

// 保存原始的valueOf
const valueOf = Object.prototype.valueOf
const toString = Object.prototype.toString

// 添加valueOf日志
// eslint-disable-next-line no-extend-native
Object.prototype.valueOf = function () {
  console.log('valueOf')
  return '1' // 强制返回原始值
}
// 添加toString日志
// eslint-disable-next-line no-extend-native
Object.prototype.toString = function () {
  console.log('toString')
  return toString.call(this)
}

let a = {}
console.log(++a)

// output:
// valueOf
// 2
// valueOf 返回原始值(字符串), 直接将该字符串转换为数字, 得到 1

// 保存原始的valueOf
const valueOf = Object.prototype.valueOf
const toString = Object.prototype.toString

// 添加valueOf日志
// eslint-disable-next-line no-extend-native
Object.prototype.valueOf = function () {
  console.log('valueOf')
  return valueOf.call(this)
}
// 添加toString日志
// eslint-disable-next-line no-extend-native
Object.prototype.toString = function () {
  console.log('toString')
  return toString.call(this)
}

const a = {}
alert(a)

// output:
// toString
// 弹出 "[object Object]"
// 调用toString方法, 返回了字符串”[object Object]”, 对象最终转换为该字符串

// 保存原始的valueOf
const valueOf = Object.prototype.valueOf
const toString = Object.prototype.toString

// 添加valueOf日志
// eslint-disable-next-line no-extend-native
Object.prototype.valueOf = function () {
  console.log('valueOf')
  return valueOf.call(this)
}
// 添加toString日志
// eslint-disable-next-line no-extend-native
Object.prototype.toString = function () {
  console.log('toString')
  return this
}

const a = {}
alert(a)

// output:
// toString
// valueOf
// Uncaught TypeError: Cannot convert object to primitive value
// 调用toString方法, 返回的不是 primitive value, 继续执行
// 调用valueOf方法, 返回的不是 primitive value, 继续执行
// 抛出 TypeError

Object Constructor

• 首字母大写.
• 所有函数 (包括构造函数) 有 prototype 属性.

Object Literal Creation

对象字面量由 Object 构造函数隐式构造;

const obj = {
  name: 'sabertazimi',
}

console.log(obj[[proto]] === Object.prototype) // true

Object New Constructor

new 构造函数作用原理如下:

• 形成原型链: 隐式原型指向构造函数的原型对象 obj.__proto__ = constructor.prototype
• 构造函数对象 (Constructor) 与原型对象 (Prototype) 之间形成闭环:
  • Constructor.prototype = Prototype.
  • Prototype.constructor = Constructor.

function newInstance(constructor, ...args) {
  // var this = Object.create(Person.prototype);
  // this.__proto__ = F.prototype
  // F.prototype = Person.prototype
  // 即 this.__proto__ = Person.prototype;
  const obj = {}
  obj[[proto]] = constructor.prototype
  constructor.apply(obj, args)
  return obj
}

// =>
const instance = new Constructor(arguments)

function Employee(name) {
  this.name = name
  this.getName = function () {
    return this.name
  }
}

const employee = newInstance(Employee, 'Jack')
// =>
const employee = new Employee('Jack')

new.target:

function Foo() {
  if (!new.target)
    throw new Error('Foo() must be called with new')
}

function Waffle() {
  // 当未使用 `new` 关键字时, `this` 指向全局对象
  if (!(this instanceof Waffle))
    return new Waffle()

  // 正常构造函数
  this.tastes = 'yummy'
}

Object Create Constructor

• 原型式继承非常适合不需要单独创建构造函数, 但仍然需要在对象间共享信息的场合.
• 属性中包含的引用值始终会在相关对象间共享.

Object.create = function (o) {
  if (arguments.length > 1) {
    throw new Error(
      'Object.create implementation only accepts the first parameter.'
    )
  }

  function F() {}
  F.prototype = o
  return new F()
}
// 1. `F.prototype === o`.
// 2. `new F()` lead to `f.__proto__ === F.prototype`.
// Finally: `f.__proto__ === o`.

Constructor Return Value

• 返回 this 或 user-defined literal object.
• 当返回值为基本类型时, 仍然可得到 this 指针指向的原有对象.

const ObjectMaker = function () {
  this.name = 'This is it'
  // user-defined literal object
  // 直接忽略 this.name.
  const that = {}
  that.name = 'And that\'s that'
  return that
}

const MyClass = function () {
  this.name = 'sven'
  return 'anne' // 返回 string.
}
const obj = new MyClass()
console.log(obj.name) // 输出: sven .

Constructor Instance Detection

若在实例对象的原型链 (__proto__) 中能找到构造函数的 prototype 属性 (Prototype 对象), 则返回true, 否则返回false:

// true only if
// 1. Foo.__proto__ === Bar.prototype
// 2. Foo.__proto__......__proto__ === Bar.prototype
console.log(Foo instanceof Bar)

Object Property Descriptor

Property Descriptor Definition

对象的属性描述符:

• Object.defineProperty(O, Prop, descriptor).
• Object.defineProperties(O, descriptors).

数据描述符:

• value: 属性值, 默认 undefined.
• writable: 是否是只读 property, 默认 false.
• enumerable: 是否可以被枚举 (for in), 默认 false.
• configurable: 是否可以被删除与修改 descriptor, 默认 false.

存取描述符:

• get: 返回 property 值的方法, 默认 undefined.
• set: 为 property 设置值的方法, 默认 undefined.
• enumerable: 是否可以被枚举 (for in), 默认 false.
• configurable: 是否可以被删除与修改 descriptor, 默认 false.

interface DataPropertyDescriptor {
  value?: any
  writable?: boolean
  enumerable?: boolean
  configurable?: boolean
}

interface AccessorPropertyDescriptor {
  get?: (this: any) => any
  set?: (this: any, v: any) => void
  enumerable?: boolean
  configurable?: boolean
}

type PropertyDescriptor = DataPropertyDescriptor | AccessorPropertyDescriptor

Object.defineProperty(o, 'age', {
  value: 24,
  writable: true,
  enumerable: true,
  configurable: true,
})

Object.defineProperty(o, 'sex', {
  value: 'male',
  writable: false, //  不可赋值
  enumerable: false, //  不可遍历/枚举
  configurable: false,
})

Object.defineProperties(o, {
  age: {
    value: 24,
    writable: true,
    enumerable: true,
    configurable: true,
  },
  sex: {
    value: 'male',
    writable: false,
    enumerable: false,
    configurable: false,
  },
})

Object.defineProperties(o, {
  kind: {
    value: 'Plate 1x3',
    writable: true,
    enumerable: true,
    configurable: true,
  },
  color: {
    value: 'yellow',
    writable: true,
    enumerable: true,
    configurable: true,
  },
  description: {
    get() {
      return `${this.kind} (${this.color})`
    },
    enumerable: true,
    configurable: true,
  },
})

Object keys (excluding Symbols) are strings under the hood, object keys are automatically converted into strings:

const obj = { 1: 'a', 2: 'b', 3: 'c' }
const set = new Set([1, 2, 3, 4, 5])
Object.hasOwn(obj, '1') // true
Object.hasOwn(obj, 1) // true
set.has('1') // false
set.has(1) // true

const a = {}
const b = { key: 'b' }
const c = { key: 'c' }
a[b] = 123
a[c] = 456
console.log(a[b]) // a["[object Object]"] = 456

Object keys 遍历顺序:

• 首先遍历所有数值键, 按照数值升序排列.
• 其次遍历所有字符串键, 按照加入时间升序排列.
• 最后遍历所有 Symbol 键, 按照加入时间升序排列.

Property Descriptor Functions

• Object.create(prototype[, descriptors]).

const o = Object.create({
  say() {
    alert(this.name)
  },
  name: 'Byron',
})

const obj = Object.create(Object.prototype, {
  kind: {
    value: 'Plate 1x3',
    writable: true,
    enumerable: true,
    configurable: true,
  },
  color: {
    value: 'yellow',
    writable: true,
    enumerable: true,
    configurable: true,
  },
  description: {
    get() {
      return `${this.kind} (${this.color})`
    },
    enumerable: true,
    configurable: true,
  },
})

• Object.getOwnPropertyDescriptor(object, property).
• Object.getOwnPropertyDescriptors(object).

Object.defineProperties(target, Object.getOwnPropertyDescriptors(source))

const clone = Object.create(
  Object.getPrototypeOf(obj),
  Object.getOwnPropertyDescriptors(obj)
)

• Object.hasOwn(object, property): Boolean.
• in operator: 检测实例及其原型链上所有属性名.

// Arrays
const trees = ['redwood', 'bay', 'cedar', 'oak', 'maple']
const truthy = 0 in trees
const truthy = 3 in trees
const falsy = 6 in trees
const falsy = 'bay' in trees
const truthy = 'length' in trees
const truthy = Symbol.iterator in trees

// Predefined objects
const truthy = 'PI' in Math

// Custom objects
const car = { make: 'Honda', model: 'Accord', year: 1998 }
const truthy = 'make' in car
const truthy = 'model' in car

• for...in: 获取实例及其原型链上所有可枚举属性名.
• Object.getOwnPropertySymbols(object): 获取实例上 Symbol 属性名.
• Object.getOwnPropertyNames(object): 获取实例上非 Symbol 属性名 (包括不可枚举属性名).
• Object.keys(object): 获取实例上可枚举属性名.

const k1 = Symbol('k1')
const k2 = Symbol('k2')
const o = {
  1: 1,
  first: 'first',
  [k1]: 'sym2',
  second: 'second',
  0: 0,
}
o[k2] = 'sym2'
o[3] = 3
o.third = 'third'
o[2] = 2

console.log(Object.getOwnPropertyNames(o))
// ['0', '1', '2', '3', 'first', 'second', 'third']
console.log(Object.getOwnPropertySymbols(o))
// [Symbol(k1), Symbol(k2)]

function Person() {}
Person.prototype.name = 'Nicholas'
Person.prototype.age = 29
Person.prototype.job = 'Software Engineer'
Person.prototype.sayName = function () {
  console.log(this.name)
}

const keys = Object.keys(Person.prototype)
console.log(keys) // 'name, age, job, sayName'

const p1 = new Person()
p1.name = 'Rob'
p1.age = 31
const p1keys = Object.keys(p1)
console.log(p1keys) // '[name, age]'

• Object.values(O): object own enumerable property values.
• Object.entries(O): object own enumerable string-keyed property [key, value] pairs.
• Object.fromEntries().

const score = {
  saber: 42,
  todd: 19,
  ken: 4,
  gan: 41,
}

Object.keys(score).map(k => score[k])
// => [ 42, 19, 4, 41 ]

Object.values(score)
// => [ 42, 19, 4, 41 ]

Object.entries(score)
/**
 * =>
 * [
 * [ 'saber', 42 ],
 * [ 'todd', 19 ],
 * [ 'ken', 4 ],
 * [ 'gan', 41 ],
 * ]
 */

function findKey(object, callback, thisValue) {
  for (const [key, value] of Object.entries(object)) {
    if (callback.call(thisValue, value, key, object))
      return key
  }

  return undefined
}

const object = { x: 42, y: 50, abc: 9001 }
const result = Object.fromEntries(
  Object.entries(object)
    .filter(([key, value]) => key.length === 1)
    .map(([key, value]) => [key, value * 2])
)

const map = new Map(Object.entries(object))
const objectCopy = Object.fromEntries(map)

function pick(object, ...keys) {
  const filteredEntries = Object.entries(object).filter(([key, _value]) =>
    keys.includes(key)
  )

  return Object.fromEntries(filteredEntries)
}

function invert(object) {
  const mappedEntries = Object.entries(object).map(([key, value]) => [
    value,
    key,
  ])

  return Object.fromEntries(mappedEntries)
}

function mapObject(object, callback, thisValue) {
  const mappedEntries = Object.entries(object).map(([key, value]) => {
    const mappedValue = callback.call(thisValue, value, key, object)
    return [key, mappedValue]
  })

  return Object.fromEntries(mappedEntries)
}

Operation (Only Enumerable)	String Key	Symbol Key	Inherited
Object.keys()	✔	✘	✘
Object.values()	✔	✘	✘
Object.entries()	✔	✘	✘
Object.assign()	✔	✔	✘
Spreading {...x}	✔	✔	✘
JSON.stringify()	✔	✘	✘
for...in	✔	✘	✔

Operation (Include Non-enumerable)	String Key	Symbol Key	Inherited
Object.getOwnPropertyNames()	✔	✘	✘
Object.getOwnPropertySymbols()	✘	✔	✘
Object.getOwnPropertyDescriptors()	✔	✔	✘
Reflect.ownKeys()	✔	✔	✘

• Object.preventExtensions(O)/Object.isExtensible(O): 不可新增属性, 可删除/修改属性.
• Object.seal(O)/Object.isSealed(O): 设置 configurable 为 false (不可删除, 不可修改 descriptor), 可修改属性.
• Object.freeze(O)/Object.isFrozen(O): 不可新增/删除/修改属性 (Shallow).

const obj = Object.freeze({ foo: {} })

obj.bar = 123
// TypeError: Can't add property bar, object is not extensible

obj.foo = {}
// TypeError: Cannot assign to read only property 'foo' of #<Object>

obj.foo.qux = 'abc'
console.log(obj.foo.qux)
// 'abc'

Private Property and Method

Private Property

实现方式: 闭包

function Gadget() {
  // private member
  const name = 'iPod'
  // public function
  this.getName = function () {
    return name
  }
}

Private Method

getter: 返回基本类型值/引用类型深拷贝(POLA 最低授权原则).

function Gadget() {
  // private member
  const pref = {}
  // public function
  this.getPref = function () {
    return pref.clone()
  }
}

即使函数模式 + 揭示模式:

• 实现私有属性与私有方法.
• 提供私有方法的公共(读/执行 not 写)接口,公共接口发生意外,私有方法仍安全.

// 匿名即时函数模式.
const obj = (function () {
  // private member
  const name = 'tazimi'
  // private method
  const getName = function getName() {
    return name
  }
  // 闭包
  return {
    // 公共接口 - 私有方法
    getName,
  }
})()

Static Property and Method

Static Property

实现方式: 闭包/原型代理

Static Method

直接向构造函数添加方法

Object.isArray = function () {}

Object Property

编写函数时,一般用[]访问对象属性

Object Method

为 prototype 添加方法,可以通过实现语法糖 method()简化代码(链模式)

if (typeof Function.prototype.method !== 'function') {
  // eslint-disable-next-line no-extend-native
  Function.prototype.method = function (name, implementation) {
    this.prototype[name] = implementation
    return this
  }
}

const Person = function (name) {
  this.name = name
}
  .method('getName', function () {
    return this.name
  })
  .method('setName', function (name) {
    this.name = name
    return this
  })

Object Clone

Object Shallow Clone

Object.assign:

• Enumerable: 可枚举属性扩展.
• Own: 自有属性扩展.
• Shallow: 浅拷贝扩展.
• Trigger sourceObj.set and change sourceObj.

const dest = {}
const src = { a: {} }
Object.assign(dest, src)

// 浅复制意味着只会复制对象的引用
console.log(dest) // { a :{} }
console.log(dest.a === src.a) // true

... object spread syntax:

• Enumerable: 可枚举属性扩展.
• Own: 自有属性扩展.
• Shallow: 浅拷贝扩展.
• Not copy prototype (__proto__).
• Not copy getter and setter.
• Not trigger sourceObj.set and not change sourceObj (create new properties).

// Shallow copy
const foo = { a: 1 }
const bar = { b: 2, c: { d: 3 } }
const foobar = { ...foo, ...bar }
console.log(foobar.c === bar.c) // true

// Not copy prototype (`__proto__`)
class MyClass {}
const original = new MyClass()
assert.equal(original instanceof MyClass, true)
const copy = { ...original }
assert.equal(copy instanceof MyClass, false)

Object Deep Clone

Recursively copy all properties of an object:

function deepClone(original) {
  if (Array.isArray(original)) {
    return original.map(elem => deepClone(elem))
  } else if (typeof original === 'object' && original !== null) {
    return Object.fromEntries(
      Object.entries(original).map(([key, value]) => [key, deepClone(value)])
    )
  } else {
    // Primitive value: atomic, no need to copy
    return original
  }
}

function deepUpdate(original, keys, value) {
  if (keys.length === 0)
    return value

  const currentKey = keys[0]

  if (Array.isArray(original)) {
    return original.map((v, index) =>
      index === currentKey ? deepUpdate(v, keys.slice(1), value) : v
    )
  } else if (typeof original === 'object' && original !== null) {
    return Object.fromEntries(
      Object.entries(original).map(([k, v]) =>
        k === currentKey ? [k, deepUpdate(v, keys.slice(1), value)] : [k, v]
      )
    )
  } else {
    // Primitive value
    return original
  }
}

Using JSON.parse(JSON.stringify(obj)):

• Not copy prototype (__proto__).
• Not copy getter and setter.
• Not copy non-enumerable properties.
• Not copy Symbol properties.
• Not copy circular references.
• Not copy undefined, function, symbol.

const obj = { a: 1, b: { c: 2 } }
const clone = JSON.parse(JSON.stringify(obj))
console.log(clone) // { a: 1, b: { c: 2 } }

Using window.structuredClone():

// Create an object with a value and a circular reference to itself.
const original = { name: 'MDN' }
original.itself = original

// Clone it
const clone = structuredClone(original)

console.assert(clone !== original) // the objects are not the same (not same identity)
console.assert(clone.name === 'MDN') // they do have the same values
console.assert(clone.itself === clone) // and the circular reference is preserved

const room1 = {
  people: ['Alan', 'Bob'],
}

const room2 = structuredClone(room1)

room2.people.push('Charlie')
room1.people.pop()

console.log(room2.people) // ["Alan", "Bob", "Charlie"]
console.log(room1.people) // ["Alan"]

Object Inheritance

Prototype Proxy Inheritance

可用于所有继承模式中, 减少内存消耗:

const inherit = (function () {
  // 减少继承过程中父类的实例化,减少资源消耗
  // 实例化一个空类所需资源更少
  const F = function () {}
  return function (C, P) {
    // c.__proto__ = C.prototype = f
    // f.__proto__ = F.prototype
    // F.prototype = P.prototype
    // c.__proto__.__proto__ = f.__proto__ = P.prototype
    F.prototype = P.prototype // f.__proto__ = F.prototype = P.prototype
    C.prototype = new F() // C.prototype = f
    C.prototype.constructor = C
    C.super = P.prototype // 此句可提高代码的重用性
  }
})()

Child.prototype.add = function () {
  return Child.super.add.call(this)
}

Class Simulation Inheritance

复制式地继承, 将会消耗大量内存单元:

const classSim = function (Parent, props) {
  // 新的构造函数
  const Child = function (...args) {
    if (
      Child.uber
      && Object.prototype.hasOwnProperty.call(Child.uber, '_construct')
    ) {
      Child.uber._construct.apply(this, args)
    }

    if (Object.prototype.hasOwnProperty.call(Child.prototype, '_construct'))
      Child.prototype._construct.apply(this, args)
  }

  // 类式继承
  Parent = Parent || Object

  // 代理构造函数F
  const F = function () {}
  F.prototype = Parent.prototype
  Child.prototype = new F()
  Child.prototype.constructor = Child

  // 添加属性与方法
  for (const i in props) {
    if (Object.prototype.hasOwnProperty.call(props, i))
      Child.prototype[i] = props[i]
  }

  // return the "class"
  return Child
}

const SuperMan = classSim(Man, {
  _construct(what) {
    console.log('SuperMan\'s constructor')
  },
  getName() {
    const name = SuperMan.uber.getName.call(this)
    return `I am ${name}`
  },
})

Composite Inheritance

原型继承 (设置原型) 与类式继承 (借用构造函数) 组合继承模式:

• child.prototype = new Parent(); Child.prototype.constructor = Child.
• Parent.apply(this, arguments): 借用构造函数可以防止引用类型被迫共享.
• 此模式会调用两次父类构造函数, 使得子类属性继承两次, 存在一定的效率问题.

function Parent(name) {
  this.name = name || 'Adam'
}
// Adding functionality to the prototype
Parent.prototype.say = function () {
  return this.name
}

// Child constructor
function Child(...args) {
  // 解决引用类型共享问题
  Parent.apply(this, args)
  this.childName = 'Child Name'
}

// Child.prototype = Object.create(Parent.prototype);
Child.prototype = new Parent() // 设置原型链,建立继承关系
Child.prototype.constructor = Child // 使得 Prototype 对象与 Constructor 对象形成闭环

Class

Class Prototype

• Class 定义不能提升.
• Class 具有块作用域.
• Class 内部为严格模式.
• Class 内部定义方法不可枚举.
• Class 所有方法 (包括静态方法和实例方法) 都没有原型对象 .prototype, 没有 [[construct]], 不能使用 new 调用.
• Class 必须使用 new 调用, 否则会报错.
• typeof Class: function.

class A {
  constructor(value) {
    this.val = value
  }
}

class B extends A {
  constructor(value) {
    super(value)
    console.log('New')
  }
}

const b = new B(6)

console.log(B[[proto]] === A) // true
console.log(B.prototype.constructor === B) // true
console.log(B.prototype[[proto]] === A.prototype) // true
console.log(b[[proto]] === B.prototype) // true

function AA(value) {
  this.val = value
}

function BB(value) {
  AA.call(this, value)
}

BB.prototype = Object.create(AA.prototype)
BB.prototype.constructor = BB

const bb = new BB(6)

console.log(BB[[proto]] === Function.prototype) // true (not consistence to Class)
console.log(BB.prototype.constructor === BB) // true
console.log(BB.prototype[[proto]] === AA.prototype) // true
console.log(bb[[proto]] === BB.prototype) // true

	Writable	Enumerable	Configurable
Foo.prototype	false	false	false
Foo.prototype.constructor	false	false	true
Static properties Foo.*	true	false	true
Prototype properties Foo.prototype.*	true	false	true

Class Inheritance

• 隔离: 添加到 this 的所有属性都会存在于不同的实例上.
• 共享:
  • 在类块中定义的所有方法都会定义在类的原型上.
  • 静态属性定义在类本身上.

class Person {
  constructor() {
    // 添加到 this 的所有内容都会存在于不同的实例上
    this.locate = () => console.log('instance', this)
  }

  // 在类块中定义的所有内容都会定义在类的原型上
  locate() {
    console.log('prototype', this)
  }

  // 定义在类本身上
  static locate() {
    console.log('class', this)
  }
}

const p = new Person()
p.locate() // instance, Person {}
Person.prototype.locate() // prototype, {constructor: ... }
Person.locate() // class, class Person {}

Class Definition	Class Prototype	Class.prototype Prototype
C	Function.prototype	Object.prototype
C extends null	Function.prototype	null
C extends Object	Object	Object.prototype
C extends B	B	B.prototype

ES5 vs ES6 Inheritance

• ES5 继承先创造子类实例对象 this, 然后再将父类的属性与方法添加到 this 上 (Parent.apply(this)), 实例在前继承在后.
• ES6 继承先将父类的属性与方法添加到 this 上 (必须先调用 super() 方法), 然后再用子类的构造函数修改 this, 继承在前实例在后.
• ES5 继承 Child.__proto__ === Function.prototype.
• ES6 继承 Child.__proto__ === Parent, 子类可以直接通过 [[proto]] 寻址到父类.

Super Class

super:

• super 只能在派生类构造函数和静态方法中使用.
• 不能单独引用 super 关键字, 要么用它调用构造函数, 要么用它引用静态方法.
• 调用 super() 会调用父类构造函数, 并将返回的实例赋值给 this.
• 在类构造函数中, 不能在调用 super() 之前引用 this.
• super() 等价于调用构造函数, 若需给父类构造函数传参, 则需手动传入.
• 若没有显式定义派生类构造函数, 在实例化派生类时会自动调用 super(), 且会自动传入所有传给派生类的参数.
• 若显式定义了派生类构造函数, 则必须在其中调用 super() , 或返回一个对象.
• 实例化时检测 new.target 是不是抽象基类, 可以阻止对抽象基类的实例化.

Abstract Base Class

class Shape {
  constructor() {
    if (new.target === Shape)
      throw new TypeError('This class cannot be instantiated directly.')
  }
}

class Rectangle extends Shape {
  constructor(length, width) {
    super()
    this.length = length
    this.width = width
  }
}

const x = new Shape() // throws error
const y = new Rectangle(3, 4) // no error
console.log(y instanceof Shape) // true

Class Expression

// Anonymous class expression
const Person = class {}

// Named class expression
const Person = class PersonName {
  identify() {
    console.log(Person.name, PersonName.name)
  }
}

const p = new Person()
p.identify() // PersonName PersonName
console.log(Person.name) // PersonName
console.log(PersonName) // ReferenceError: PersonName is not defined

Class Constructor

class A {
  constructor() {
    console.log(new.target.name)
  }
}

class B extends A {
  constructor() {
    super()
    console.log('New')
  }
}

const a = new A() // logs "A"
const b = new B() // logs "B"

class C {
  constructor() {
    console.log(new.target)
  }
}

class D extends C {
  constructor() {
    super()
    console.log('New')
  }
}

const c = new C() // logs class C{constructor(){console.log(new.target);}}
const d = new D() // logs class D extends C{constructor(){super();}}

Class Private Member

• Private access.
• Aren't stored in .prototype.
• Aren't create instance own properties.

class Dong {
  constructor() {
    this.#name = 'dog'
    this.#age = 20
    this.friend = 'cat'
  }

  hello() {
    return `I'm ${this.#name} ${this.#age} years old`
  }
}

Private member can only be accessed inside body of its class, can’t even access it from a subclass:

class SuperClass {
  #superProp = 'superProp'
}

class SubClass extends SuperClass {
  getSuperProp() {
    return this.#superProp
  }
}
// SyntaxError: Private field '#superProp'
// must be declared in an enclosing class

class Color {
  #name

  constructor(name) {
    this.#name = name
  }

  static getName(obj) {
    return obj.#name
  }
}

class Person {
  #name

  constructor(name) {
    this.#name = name
  }
}

// Can’t access the private slot #name of a Person:
assert.equal(Color.getName(new Color('green')), 'green')
assert.throws(() => Color.getName(new Person('Jane')), {
  name: 'TypeError',
  message:
    'Cannot read private member #name from'
    + ' an object whose class did not declare it',
})

Private member never clash, they aren't stored in .prototype objects and aren't inherited:

class SuperClass {
  #privateField = 'super'

  getSuperPrivateField() {
    return this.#privateField
  }
}

class SubClass extends SuperClass {
  #privateField = 'sub'

  getSubPrivateField() {
    return this.#privateField
  }
}

const inst = new SubClass()
assert.equal(inst.getSuperPrivateField(), 'super')
assert.equal(inst.getSubPrivateField(), 'sub')

Private member can't be accessed by Reflect.ownKeys():

class InstPrivateClass {
  #privateField1 = 'private field 1'
  #privateField2
  constructor(value) {
    this.#privateField2 = value
  }
}

// No instance properties were created
const inst = new InstPrivateClass('constructor argument')
assert.deepEqual(Reflect.ownKeys(inst), [])

Private member WeakMap polyfill:

function classPrivateFieldGet(receiver, state) {
  return state.get(receiver)
}

function classPrivateFieldSet(receiver, state, value) {
  state.set(receiver, value)
}

const dongName = new WeakMap()
const dongAge = new WeakMap()

class Dong {
  constructor() {
    classPrivateFieldSet(this, dongName, 'dong')
    classPrivateFieldSet(this, dongAge, 20)
  }

  hello() {
    return `I'm ${classPrivateFieldGet(this, dongName)}, ${classPrivateFieldGet(
      this,
      dongAge
    )} years old`
  }
}

Class Public Fields

class SuperClass {
  superProp = console.log('superProp')

  constructor() {
    console.log('super-constructor')
  }
}

class SubClass extends SuperClass {
  subProp = console.log('subProp')

  constructor() {
    console.log('BEFORE super()')
    super()
    console.log('AFTER super()')
  }
}

const sub = new SubClass()
// Output:
// 'BEFORE super()'
// 'superProp'
// 'super-constructor'
// 'subProp'
// 'AFTER super()'

Class Static Blocks

Static blocks have access to class private member. Its mainly useful whenever set up multiple static fields.

class Foo {
  static #count = 0

  get count() {
    return Foo.#count
  }

  static {
    try {
      const lastInstances = loadLastInstances()
      Foo.#count += lastInstances.length
    } catch {}
  }
}

class Translator {
  static translations = {
    yes: 'ja',
  }

  static englishWords = []
  static germanWords = []
  static {
    for (const [english, german] of Object.entries(translations)) {
      this.englishWords.push(english)
      this.germanWords.push(german)
    }
  }
}

class SuperClass {
  static superField1 = console.log('superField1')
  static {
    assert.equal(this, SuperClass)
    console.log('static block 1 SuperClass')
  }

  static superField2 = console.log('superField2')
  static {
    console.log('static block 2 SuperClass')
  }
}

class SubClass extends SuperClass {
  static subField1 = console.log('subField1')
  static {
    assert.equal(this, SubClass)
    console.log('static block 1 SubClass')
  }

  static subField2 = console.log('subField2')
  static {
    console.log('static block 2 SubClass')
  }
}

// Output:
// 'superField1'
// 'static block 1 SuperClass'
// 'superField2'
// 'static block 2 SuperClass'
// 'subField1'
// 'static block 1 SubClass'
// 'subField2'
// 'static block 2 SubClass'

Class Best Practice

Avoid using class when:

• Singleton:
  • Only instantiate your class once in a given runtime.
• Stateless:
  • Data structure no need for local state.
  • Data structure no need for extending.
• Redundant:
  • Minimal public methods.
  • Constructors are only used for dependency injection.
  • Constructors are always called with same arguments.
• Want to avoid using this.

Global Object

// 立即函数模式:
// 此时返回值不是函数本身, 而是函数执行后的 return 语句返回值.
const global = (function () {
  // 返回全局对象
  return this
})()

Global Object 属性:

• undefined.
• NaN.
• Infinity.
• Object.
• Array.
• Function.
• Boolean.
• String.
• Number.
• Date.
• RegExp.
• Symbol.
• Error.
• EvalError.
• RangeError.
• ReferenceError.
• SyntaxError.
• TypeError.
• URIError.
• encodeURI.
• encodeURIComponent.
• decodeURI.
• decodeURIComponent.
• eval.

;(function () {
  // Grab browser's default global variables.
  const iframe = window.document.createElement('iframe')
  iframe.src = 'about:blank'
  window.document.body.appendChild(iframe)
  const browserGlobals = Object.keys(iframe.contentWindow)
  window.document.body.removeChild(iframe)

  // Get the global variables added at runtime by filtering out the browser's
  // default global variables from the current window object.
  const runtimeGlobals = Object.keys(window).filter((key) => {
    const isFromBrowser = browserGlobals.includes(key)
    return !isFromBrowser
  })

  console.log('Runtime globals', runtimeGlobals)
})()

Function

• 函数是对象.
• 函数提供局部作用域.
• Object 是 Function 的实例对象, Function.prototype是 Object 的实例对象.

const truthy = Object[[proto]] === Function.prototype // true
const truthy = Function[[proto]] === Function.prototype // true
const truthy = Function[[proto]][[proto]] === Object.prototype // true

Implicit Invocation

• Function Invocation 普通调用模式: this 绑定至全局对象/undefined (strict mode)
  • setTimeout 和 setInterval 中传入的 Callbacks 会自动转变为 Function Invocation, this bind to global/undefined object.
  • React Class Component 中传入的 Event Handlers 会自动转变为 Function Invocation, 需要显式地 this.handleClick = this.handleClick.bind(this);
• Method Invocation 方法调用模式: this 绑定至此方法所属的对象.

// Non-strict mode:
window.identity = 'The Window'

const object = {
  identity: 'My Object',
  getIdentityFunc() {
    return function () {
      return this.identity
    }
  },
}

// Function invocation:
// Anonymous closure function `this` bind to `window`.
console.log(object.getIdentityFunc()()) // 'The Window'

add(1, 2) // `this` -> `global`

const obj = {
  value: 1,
  foo() {
    // 若不将 `this` 赋值给 `that`, 而在内部函数中直接使用 `this.value`,
    // 则会发生错误: 内部函数的 `this` 指向全局对象而不是 `obj`.
    // eslint-disable-next-line ts/no-this-alias
    const that = this

    function inner() {
      return that.value
    }

    return inner()
  },
}

obj.foo() // 1

class Hero {
  constructor(heroName) {
    this.heroName = heroName
  }

  logName() {
    console.log(this.heroName)
  }
}

const batman = new Hero('Batman')
setTimeout(batman.logName, 1000)
// after 1 second logs "undefined"

Explicit Invocation

Apply/Bind/Call Invocation: 函数引用不可以改变函数定义作用域 (scope), 但可以改变函数执行作用域 (context).

this.construct = Foo
this.construct(options)
// =>
Foo.call(this, options)

• Function.call(contextObj, arg1, arg2, ...)
• Function.apply(contextArray, [arg1, arg2, ...]/arguments)
• call 效率高于 apply.

window.function.call()
window.function.apply()
// js解释器临时将数组/字符串包装成对象原型.
;[].arrayStaticFunction.call()
;[].arrayStaticFunction.apply()
Array.prototype.arrayStaticFunction.call()
Array.prototype.arrayStaticFunction.apply()
''.stringStaticFunction.call()
''.stringStaticFunction.apply()
String.prototype.stringStaticFunction.call()
String.prototype.stringStaticFunction.apply()

相当于:

context.function(arguments)

Explicit Bind Invocation

• Change function runtime context (ignore innovation pattern function/method/new/call/apply).
• Curry function.
• Can't change this in arrow function.

const boundFunc = func.bind(context, arg1, arg2, argN)

Explicit Call and Apply Invocation

function bind(o, m) {
  return function (...args) {
    return m.apply(o, args)
  }
}

const one = {
  name: 'object',
  say(greet) {
    return `${greet}, ${this.name}`
  },
}
const two = { name: 'another object' }
const twoSay = bind(two, one.say)

twoSay('yo') // "yo, another object"

Constructor Invocation

Constructor invocation (new call):

• this 绑定至传入的空对象.
• new.target 引用构造函数.

function newConstructor(Func, ...args) {
  // 1. 创建一个新对象
  const obj = {}
  // 2. 新对象原型指向构造函数原型对象
  obj[[proto]] = Func.prototype
  // 3. 将构建函数的 `this` 指向新对象
  const result = Func.apply(obj, args)
  // 4. 返回对象
  return result instanceof Object ? result : obj
}

Function Invocation Priority

new constructor invocation > explicit invocation > implicit invocation > default invocation.

Arrow Function Invocation

• No this binding (lexical scope):
  • this defined where arrow function defined (not called).
  • Not suited as method: this in arrow function bound to lexical scope, not bound to method receiver.
  • apply/call/bind can't change this in arrow function.
• No arguments binding (lexical scope).
• Not suited as new constructor:
  • No super binding (lexical scope).
  • No new.target binding (lexical scope).
• No function.prototype: arrow function prototype property is undefined.
• No yield binding: 箭头函数不能用作 Generator 函数.

const obj = {
  foo() {
    const inner = () => {
      return this.value
    }

    return inner()
  },
}

const func = obj.foo

obj.foo() // `this` in `inner` function refer to `obj`
func() // `this` in `inner` function refer to `window`

This Binding Invocation

	function Call	Method Call	new Call
Traditional function (sloppy)	window	receiver	instance
Traditional function (strict)	undefined	receiver	instance
Method (sloppy)	window	receiver	TypeError
Method (strict)	undefined	receiver	TypeError
Generator function (sloppy)	window	receiver	TypeError
Generator function (strict)	undefined	receiver	TypeError
Generator method (sloppy)	window	receiver	TypeError
Generator method (strict)	undefined	receiver	TypeError
Arrow function	lexical	lexical	TypeError
Class	TypeError	TypeError	SC protocol

Context and Scope

执行上下文:

• 每个上下文都有一个关联的变量对象 (Variable Object), 这个上下文中定义的所有变量和函数都存在于这个对象上.
• 每个执行环境拥有独立的作用域链, 例如独立活动对象 -> 独立全局对象:
  • scope chain -> (list) [0]活动对象 -> [1]全局对象.
  • ES6 Block Scope -> Function Scope -> Global Scope.
• 全局上下文中关联的变量对象 (Variable Object), 会在代码执行期间始终存在.
• 函数上下文将其活动对象 (Activation Object) 用作变量对象 (Variable Object):
  • 函数每次运行时, 都会新建执行环境上下文与作用域链, 执行完后销毁上下文与作用域链.
  • 存在闭包时, 函数上下文关联的作用域链中被引用的活动对象不会被销毁.
• 可动态改变作用域链的语句:
  • with.
  • try catch: 异常对象入列, 位于作用域链链首.

function createComparisonFunction(propertyName) {
  return function (object1, object2) {
    const value1 = object1[propertyName]
    const value2 = object2[propertyName]

    if (value1 < value2)
      return -1
    else if (value1 > value2)
      return 1
    else
      return 0
  }
}

const compare = createComparisonFunction('name')
const result = compare({ name: 'Nicholas' }, { name: 'Matt' })

执行上述代码后的上下文栈与作用域链如下图所示:

createComparisonFunction() 的活动对象并不能在它执行完毕后销毁, 因为匿名函数的作用域链中仍然存在对它的引用. 在 createComparisonFunction() 执行完毕后, 其执行上下文的作用域链会销毁, 但它的活动对象仍然会保留在内存中, 直到匿名函数被销毁后才会被销毁.

Function Name

• 所有函数对象都会暴露一个只读的 name 属性, 其中包含关于函数的信息.
• The spec operation SetFunctionName(F, name, [, prefix]) sets up function name:
  • Getters and setters get prefixes get and set.
  • Function.prototype.bind() get prefix bound.
  • Function declaration name are set up when entering a scope (hoisted).
  • Named function expression name are set up via SetFunctionName().
  • Arrow function and anonymous function expression name aren't set up (SetFunctionName() is not invoked).

function foo() {}
const bar = function () {}
function baz() {}

console.log(foo.name) // foo
console.log(bar.name) // bar
console.log(baz.name) // baz
console.log((() => {}).name) // (空字符串)
// eslint-disable-next-line no-new-func
console.log(new Function().name) // anonymous
console.log(foo.bind(null).name) // bound foo

const dog = {
  years: 1,

  get age() {
    return this.years
  },

  set age(newAge) {
    this.years = newAge
  },
}

const propertyDescriptor = Object.getOwnPropertyDescriptor(dog, 'age')
console.log(propertyDescriptor.get.name) // get age
console.log(propertyDescriptor.set.name) // set age

Function Prototype

• 实例化对象没有 prototype 属性.
• 每个函数都有 prototype 属性.
• prototype 属性指向函数的原型对象 (由 JS 引擎自动创建).
• 每个函数的 __proto__ 都指向 Function.prototype.

Function Arguments

• 函数的参数在内部表现为一个数组: 函数不关心参数个数与参数类型, 不存在验证命名参数的机制.
• arguments 不是真正的数组, 但有 length 属性 (实参个数).

Arguments Callee

• 引用 arguments 所属 function, 可以利用 callee 实现匿名递归函数.
• arguments.callee.length: 即 function.length, 形参个数.
• 严格模式下禁止使用 arguments.callee.

try {
  // eslint-disable-next-line no-caller
  if (arguments.length !== arguments.callee.length)
    throw new Error('传递的参数个数不匹配')
} catch (err) {
  console.log(err)
  return this
}

Function Parameters

• 所有函数参数都是按值传递 (复制原子值/引用值).
• function.length: number of parameters before the first one with a default value.
• 无副作用的函数: 注意是否需要拷贝传入对象, 使原有对象不受函数影响, 并返回新对象.

const obj = {
  value: 2,
}

function setValueEffect(obj, val) {
  obj.value = val
  return obj
}

function setValuePure(obj, val) {
  const instance = extend({}, obj, { value: val })
  return instance
}

Default Parameter

默认参数:

• 默认参数可以使用原子值/对象值/函数返回值.
• 函数调用且未传参时, 默认参数才会进行求值.
• 默认参数按照从左往右的顺序进行求值.

// Mark required parameters via a function that throws an exception
function foo(required = throwException()) {}

Rest Parameter

• Rest parameter is array containing all remaining arguments.
• Rest parameter can only be last parameter.

// Enforcing a maximum parameters
function f(x, y, ...empty) {
  if (empty.length > 0)
    throw new Error('Redundant parameters!')
}

Function Declaration

• 函数声明提升: 函数声明会在代码执行之前获得定义.

console.log(foo.name)

// Function Declaration 函数声明:
function foo() {}

const obj = {
  say: function say() {},
}

• 函数声明对于函数内部而言无法修改:

const b = 10
;(function b() {
  b = 20
  console.log(b)
})()

// print out function b { ... }

Function Expression

任何时候, 只要函数被当作值来使用, 它就是一个函数表达式:

// 函数表达式:
const foo = function foo() {}

// `function f() {}` 是命名函数表达式:
const factorial = function f(num) {
  if (num <= 1)
    return 1
  else
    return num * f(num - 1)
}

The name funcExpr only exists inside the function body:

const func = function funcExpr() {
  return funcExpr
}

assert.equal(func(), func)
assert.throws(() => funcExpr(), ReferenceError)

Immediately Invoked Function Expression

IIFE Pattern

立即函数模式, 通过调用立即匿名函数, 返回一个对象, 暴露公共接口 (Exposed to Public):

• IIFE Syntax:
  • 函数表达式.
  • 末尾添加括号(传参), 使函数立即执行.
  • 将整个函数置于括号内.
• 闭包: 定义私有变量与特权方法.
• 返回对象: 即使通过外部代码改变返回对象的接口, 也不会影响原接口.

;(function () {
  console.log('watch out')
})()

IIFE Return Value

Left hand side 不被赋予 function 值, 而被赋予函数执行后的返回值: 此返回值可设为函数, 可产生闭包.

const getResult = (function () {
  const res = 2 + 2
  return function () {
    return res
  }
})()

IIFE Usage

• 使得匿名函数内部的代码能够立即执行.
• 不泄漏只使用一次的局部变量与方法.
• 创建命名空间, 防止变量命名冲突.

const obj = (function () {
  // private member
  let name = 'tazimi'

  // private method
  // excluded in return object

  // privileged method
  function getName() {
    return name
  }

  function setName(n) {
    if (typeof n === 'string')
      name = n

    return this
  }

  // public method
  function logName() {
    console.log(name)
  }

  // 闭包
  // 公共接口: 特权/公共方法
  return {
    // 特权方法
    getName,
    setName,

    // 公共方法
    log: logName,
  }
})()

const App = App || {}
App.utils = {}
;(function () {
  let val = 5

  this.getValue = function () {
    return val
  }

  this.setValue = function (newVal) {
    val = newVal
  }

  // also introduce a new sub-namespace
  this.tools = {}
}).apply(App.utils)

// inject new behavior into the tools namespace
// which we defined via the utilities module
;(function () {
  this.diagnose = function () {
    return 'diagnosis'
  }
}).apply(App.utils.tools)

Tail Call Optimization

• Tail call optimization only work in strict mode due to func.arguments and func.caller (forbidden in strict mode): func.arguments and func.caller will reference outer function stack frame, so can't reduce outer function stack frame.
• Tail call optimization reduce function stack frame.

// Following function is not tail recursive:
function factorial(x) {
  if (x <= 0)
    return 1
  else
    return x * factorial(x - 1) // (A): Not tail position.
}

function factorial(n) {
  return facRec(n, 1)
}

// Following function is tail recursive:
function facRec(x, acc) {
  if (x <= 1)
    return acc
  else
    return facRec(x - 1, x * acc) // (A): Tail position.
}

Closure

Closure is a function that remembers the variables from the place where it is defined (lexical scope), regardless of where it is executed later:

• 函数外部不可对函数内部进行赋值或引用.
• 函数中的闭包函数可对函数进行赋值或引用 (函数对于闭包来说是外部, 即内部引用外部).
• 特权性质 (Private Getter): 从外部通过闭包方法访问内部 (函数作用域) 局部变量.
• Local Scope -> Outer Functions Scope -> Global Scope.
• 闭包中的变量全部保存在堆内存中, 防止函数结束后变量内存被自动回收.

// global scope
const e = 10

function sum(a) {
  return function (b) {
    return function (c) {
      // outer functions scope
      return function (d) {
        // local scope
        return a + b + c + d + e
      }
    }
  }
}

console.log(sum(1)(2)(3)(4)) // log 20

// BAD
function MyObject(name, message) {
  this.name = name.toString()
  this.message = message.toString()
  this.getName = function () {
    return this.name
  }

  this.getMessage = function () {
    return this.message
  }
}

// GOOD: avoid unnecessary
function MyObject(name, message) {
  this.name = name.toString()
  this.message = message.toString()
}
MyObject.prototype.getName = function () {
  return this.name
}
MyObject.prototype.getMessage = function () {
  return this.message
}

Closure Structure

• 优先级: this > 局部变量 > 形参 > arguments > 函数名.
• innerFunc() has access to outerVar from its lexical scope, even being executed outside of its lexical scope.

function outerFunc() {
  const outerVar = 'I am outside!'

  function innerFunc() {
    console.log(outerVar) // => logs "I am outside!"
  }

  return innerFunc
}

const myInnerFunc = outerFunc()
myInnerFunc()

Closure Usage

• 闭包实现封装.
• 闭包实现私有属性与方法.
• 闭包实现工厂方法.
• 闭包实现对象缓存.

const createLoginLayer = (function (creator) {
  let singleton

  return function () {
    if (!singleton)
      singleton = creator()
    return singleton
  }
})(loginCreator)

const { called } = new (class {
  count = 0
  called = () => {
    this.count++
    console.log(`Called : ${this.count}`)
  }
})()

called() // Called : 1
called() // Called : 2

Callback Function

// check if callback is callable
if (typeof callback !== 'function')
  callback = false

// now callback:
if (callback)
  callback()

const findNodes = function (callback) {
  let i = 100000
  const nodes = []
  let found

  // check if callback is callable
  if (typeof callback !== 'function')
    callback = false

  while (i) {
    i -= 1

    // now callback:
    if (callback)
      callback(found)

    nodes.push(found)
  }

  return nodes
}

当回调函数为对象方法时 (特别时方法中使用 this 指针), 需同时传入对象参数, 并利用 apply/call 改变执行环境.

const findNodes = function (callbackObj, callback) {
  if (typeof callback === 'function')
    callback.call(callbackObj, found)
}

const findNodes = function (callbackObj, callback) {
  if (typeof callback === 'string')
    callback = callbackObj[callback]

  if (typeof callback === 'function')
    callback.call(callbackObj, found)
}

Lazy Function Definition

Lazy Function Definition (Self-Defining Function):

• 第一次执行时,进行初始化并重新定义函数变量.
• 第二次执行时,不再进行初始化(函数被重定义至真正函数).
• 第一次执行为 promise, 将重复使用的部分进行初始化, 之后的调用不再浪费新空间, 提升性能.

// definition
let foo = function () {
  // initialize code;
  const t = new Date()

  foo = function () {
    return t
  }

  // 使得第一次调用可以产生预期值,保证每次调用的行为保持一致
  return foo()
}

// first run: same behavior as second run
console.log(foo()) // t
// second run
console.log(foo()) // t

let addEvent = function (el, type, handle) {
  addEvent = el.addEventListener
    ? function (el, type, handle) {
      el.addEventListener(type, handle, false)
    }
    : function (el, type, handle) {
      el.attachEvent(`on${type}`, handle)
    }

  // 保持每次调用对外表现行为一致
  addEvent(el, type, handle)
}

Polymorphism Function

const greet = function greet(options, ...rest) {
  // 运用 if/switch 方法分情况调用函数, 实现多态方法.
  if (typeof options === 'string' && typeof methods[options] === 'function') {
    // 方法集中含有此方法:
    return methods[options](...rest)
  }
}

多态与面向对象

多态最根本的作用: 通过把过程化的条件分支语句转化为对象的多态性, 从而消除条件分支语句.

每个对象的职责, 成为该对象的属性与方法, 被安装在对象内部, 每个对象负责它们自己的行为. 这些对象可以根据同一个消息, 有条不紊地分别进行各自的工作.

Eval Function

• 不要使用eval()函数
• 不要使用字符串作参数 new Function();(会调用eval函数)
• 不要使用字符串作setTimeOut/setInterval的第一个参数(会调用eval函数)

// Anti-pattern:
const property = 'name'
// eslint-disable-next-line no-eval
alert(eval(`obj.${property}`))
// Preferred:
const property = 'name'
alert(obj[property])

// Anti-pattern:
// eslint-disable-next-line no-implied-eval
setTimeout('myFunc()', 1000)
// eslint-disable-next-line no-implied-eval
setTimeout('myFunc(1, 2, 3)', 1000)

// Preferred:
setTimeout(myFunc, 1000)
setTimeout(() => {
  myFunc(1, 2, 3)
}, 1000)