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 -- empty symbol description
const genericSymbol = Symbol()
// eslint-disable-next-line symbol-description -- empty 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