创建Symbol及辨别方法
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])//12345console.log(uid)//Symbol(uid)let uid2 = Symbol.for('uid')console.log(uid === uid2)//trueconsole.log(object[uid2])//12345console.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))//uidlet uid2 = Symbol.for('uid')console.log(Symbol.keyFor(uid2))//uidlet 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
Symbol与类型强制转换
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) Symbol属性检索
Object.getOwnPropertySymbols
let uid = Symbol.for('uid')let object = { [uid]: '12345'}let symbols=Object.getOwnPropertySymbols(object) Symbol.hasInstance
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)//trueconsole.log(zero instanceof SpecialNumber)//false Symbol.isConcatSpreadable
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"] Symbol.match,Symbol.replace,Symbol.search,Symbol.split
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))//nullconsole.log(msg2.match(hasLenOf10))//[Hello John]console.log(msg1.replace(hasLenOf10))//Hello worldconsole.log(msg2.replace(hasLenOf10))//undefinedconsole.log(msg1.search(hasLenOf10))//-1console.log(msg2.search(hasLenOf10))//0console.log(msg1.split(hasLenOf10))//["Hello world"]console.log(msg2.split(hasLenOf10))//["",""] Symbol.toPrimitive
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、defaultfunction 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)//16console.log(String(freezing))//32° Symbol.toStringTag
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())//angelaconsole.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