深入理解ES6之《Symbol》

摘要：创建及辨别方法是原始值，因此调用会导致程序抛出错误使用来辨别其类型可用于计算对象字面量属性名如果想创建一个共享的，可使用方法方法在全局注册表中检索与有关的键全局注册表是一个类似全局作用域的共享环境，也就是说你不能假设目前环境中存在哪些键与类

Symbol是原始值，因此调用new Symbol会导致程序抛出错误

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

使用typeof来辨别其类型

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

Symbol可用于计算对象字面量属性名、Object.defineProperty、Object.defineProperties

如果想创建一个共享的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全局注册表中检索与Symbol有关的键

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全局注册表是一个类似全局作用域的共享环境，也就是说你不能假设目前环境中存在哪些键

不能将Symbol强制转换成数字类型，Symbol也不可以被转换成字符串

let uid = Symbol.for("uid")
let desc = uid + ""//报错 Uncaught TypeError: Cannot convert a Symbol value to a string

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

但是却可以像下面这样使用

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只接受一个参数，即要检查的值
每个函数都有Symbol.hasInstance方法，用于确定对象是否为函数实例，该方法在Function.prototype中定义，该方法被定义不为可写、不可配置、不可枚举

let obj = []
//obj instanceof Array //下面一句代码与此句功能等价
Array[Symbol.hasInstance](obj)//true 

我们可以通过Object.defineProperty方法改写一个不可写属性
所以其实可以重写所有内建函数（如Date和Error）默认的Symbol.hasInstance属性

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属性值为true表明属性值应当作为独立元素添加到数组中

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

为什么上面一段代码结果是["red", "green", "blue", "black", "yellow"]而不是["red", "green", ["blue", "black"], "yellow"]？

如果把上面的代码稍微改一下，结果就完全不一样了

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))//["",""]

在执行特定操作时，经常会尝试将对象转换到相应的原始值
Symbol.toPrimitive被定义在每一个标准类型的原型上，并且规定了当对象被转换为原始值时应该执行的操作
每当执行类型转换总会调用Symbol.toPrimitive方法并传入一个值作为参数，这个值在规范中被称作类型提示hint，类型提示参数只有三种选择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°

以前判断一个对象的类型一般都是这样

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

ES6中我们可以自定义对象字符串标签

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]

如果我们不重写toString方法，则me.toString返回的值就是[object Person]
我们可以给Person.prototype[Symbol.toStringTag]赋任意值，当然也可以修改原生对象的字符串标签