摘要：創(chuàng)建及辨別方法是原始值，因此調(diào)用會(huì)導(dǎo)致程序拋出錯(cuò)誤使用來辨別其類型可用于計(jì)算對象字面量屬性名如果想創(chuàng)建一個(gè)共享的，可使用方法方法在全局注冊表中檢索與有關(guān)的鍵全局注冊表是一個(gè)類似全局作用域的共享環(huán)境，也就是說你不能假設(shè)目前環(huán)境中存在哪些鍵與類

Symbol是原始值，因此調(diào)用new Symbol會(huì)導(dǎo)致程序拋出錯(cuò)誤

let firstName = Symbol()
let person = {}
person[firstName] = "angela"
console.log(person[firstName])

使用typeof來辨別其類型

let symbol=Symbol("this is test description")
console.log(typeof symbol)//symbol

Symbol可用于計(jì)算對象字面量屬性名、Object.defineProperty、Object.defineProperties

如果想創(chuàng)建一個(gè)共享的Symbol，可使用Symbol.for方法

let uid = Symbol.for("uid")
let object = {
  [uid]: "12345"
}
console.log(object[uid])//12345
console.log(uid)//Symbol(uid)
let uid2 = Symbol.for("uid")
console.log(uid === uid2)//true
console.log(object[uid2])//12345
console.log(uid2)//Symbol(uid)

Symbol.keyFor方法在Symbol全局注冊表中檢索與Symbol有關(guān)的鍵

let uid = Symbol.for("uid")
console.log(Symbol.keyFor(uid))//uid
let uid2 = Symbol.for("uid")
console.log(Symbol.keyFor(uid2))//uid
let uid3=Symbol("uid")
console.log(Symbol.keyFor(uid3))//undefined

Symbol全局注冊表是一個(gè)類似全局作用域的共享環(huán)境，也就是說你不能假設(shè)目前環(huán)境中存在哪些鍵

不能將Symbol強(qiáng)制轉(zhuǎn)換成數(shù)字類型，Symbol也不可以被轉(zhuǎn)換成字符串

let uid = Symbol.for("uid")
let desc = uid + ""http://報(bào)錯(cuò) Uncaught TypeError: Cannot convert a Symbol value to a string

let uid = Symbol.for("uid")
let desc = uid / 1//報(bào)錯(cuò)  Uncaught TypeError: Cannot convert a Symbol value to a number

但是卻可以像下面這樣使用

let uid = Symbol.for("uid")
let desc = String(uid)
console.log(desc)//Symbol(uid)

Object.getOwnPropertySymbols

let uid = Symbol.for("uid")
let object = {
  [uid]: "12345"
}
let symbols=Object.getOwnPropertySymbols(object)

Symbol.hasInstance只接受一個(gè)參數(shù)，即要檢查的值
每個(gè)函數(shù)都有Symbol.hasInstance方法，用于確定對象是否為函數(shù)實(shí)例，該方法在Function.prototype中定義，該方法被定義不為可寫、不可配置、不可枚舉

let obj = []
//obj instanceof Array //下面一句代碼與此句功能等價(jià)
Array[Symbol.hasInstance](obj)//true 

我們可以通過Object.defineProperty方法改寫一個(gè)不可寫屬性
所以其實(shí)可以重寫所有內(nèi)建函數(shù)（如Date和Error）默認(rèn)的Symbol.hasInstance屬性

function SpecialNumber() { }
Object.defineProperty(SpecialNumber, Symbol.hasInstance, {
  value: function (v) {
    return (v instanceof Number) && (v >= 1 && v <= 100)
  }
})
var two = new Number(2),
  zero = new Number(0);
console.log(two instanceof SpecialNumber)//true
console.log(zero instanceof SpecialNumber)//false

Symbol.isConcatSpreadable屬性值為true表明屬性值應(yīng)當(dāng)作為獨(dú)立元素添加到數(shù)組中

let c1 = ["red", "green"],
  c2 = c1.concat(["blue", "black"], "yellow")
console.log(c2)//["red", "green", "blue", "black", "yellow"]

為什么上面一段代碼結(jié)果是["red", "green", "blue", "black", "yellow"]而不是["red", "green", ["blue", "black"], "yellow"]？

如果把上面的代碼稍微改一下，結(jié)果就完全不一樣了

let collection = {
  0: "hello",
  1: "world",
  2: "demo",
  3: "test",
  [Symbol.isConcatSpreadable]: true,
  length: 4
}
let msg = ["Hi"].concat(collection)
console.log(msg)//["Hi", "hello", "world", "demo", "test"]

let hasLenOf10 = {
  [Symbol.match](v) {
    return v.length === 10 ? [v.substring(0, 10)] : null
  },
  [Symbol.replace](v, replacement) {
    return v.length === 10 ? replacement + v.substring(10) : v;
  },
  [Symbol.search](v) {
    return v.length === 10 ? 0 : -1
  },
  [Symbol.split](v) {
    return v.length === 10 ? ["", ""] : [v]
  }
}
let msg1 = "Hello world",
  msg2 = "Hello John";
console.log(msg1.match(hasLenOf10))//null
console.log(msg2.match(hasLenOf10))//[Hello John]
console.log(msg1.replace(hasLenOf10))//Hello world
console.log(msg2.replace(hasLenOf10))//undefined
console.log(msg1.search(hasLenOf10))//-1
console.log(msg2.search(hasLenOf10))//0
console.log(msg1.split(hasLenOf10))//["Hello world"]
console.log(msg2.split(hasLenOf10))//["",""]

在執(zhí)行特定操作時(shí)，經(jīng)常會(huì)嘗試將對象轉(zhuǎn)換到相應(yīng)的原始值
Symbol.toPrimitive被定義在每一個(gè)標(biāo)準(zhǔn)類型的原型上，并且規(guī)定了當(dāng)對象被轉(zhuǎn)換為原始值時(shí)應(yīng)該執(zhí)行的操作
每當(dāng)執(zhí)行類型轉(zhuǎn)換總會(huì)調(diào)用Symbol.toPrimitive方法并傳入一個(gè)值作為參數(shù)，這個(gè)值在規(guī)范中被稱作類型提示hint，類型提示參數(shù)只有三種選擇number、string、default

function Temperature(degree) {
  this.degree = degree
}
Temperature.prototype[Symbol.toPrimitive] = function (hint) {
  switch (hint) {
    case "string":
      return this.degree + "u00b0";
    case "number":
      return this.degree;
    case "default":
      return this.degree + " degrees"
  }
}
var freezing = new Temperature(32)
console.log(freezing + "!")//32 degrees!
console.log(freezing / 2)//16
console.log(String(freezing))//32°

以前判斷一個(gè)對象的類型一般都是這樣

function isArray(value) {
  return Object.prototype.toString.call(value) === "[object Array]"
}

ES6中我們可以自定義對象字符串標(biāo)簽

function Person(name) {
  this.name = name
}
Person.prototype[Symbol.toStringTag] = "Person"
Person.prototype.toString = function () {
  return this.name
}
var me = new Person("angela")
console.log(me.toString())//angela
console.log(Object.prototype.toString.call(me))//[object Person]

如果我們不重寫toString方法，則me.toString返回的值就是[object Person]
我們可以給Person.prototype[Symbol.toStringTag]賦任意值，當(dāng)然也可以修改原生對象的字符串標(biāo)簽