ES6：Symbol

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Creating Symbols

  1. 声明Symbol

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    let symbolVal = Symbol()

  2. Symbol的描述信息会被存储在内部属性[[Description]]上，当调用toString()时，会读取这个属性

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    let symbolWithDescription = Symbol('first name')
    symbolWithDescription.toString() // 'Symbol(first name)'

  3. 使用typeof判断类型

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    let symbol = Symbol('test symbol')
    console.log(typeof symbol)  // 'symbol'

Sharing Symbols

  1. Symbol.for(key)：创建被共享的Symbol，key相同的Symbol相等

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    let uid1 = Symbol.for('uid')
    let uid2 = Symbol.for('uid')
    uid1 === uid2 // true

  2. Symbol.for(key)的创建步骤

    • 先搜索the global symbol registry
    • 如果注册表中存在标识为key的Symbol，则返回这个Symbol。
    • 如果不存在，则创建一个新的Symbol，并将这个Symbol注册到注册表中，其标识为key

  3. Symbol.keyFor(Symbol)：访问shared Symbol的key

获取一个对象的Symbol属性

  • Object.getOwnPropertySymbols()：只获取Symbol属性
  • Reflect.ownKeys()：获取所有的属性，包括Symbol属性

Exposing Internal Operations with Well-known Symbol

  1. Symbol.hasInstance：可以通过修改Symbol.hasInstance改变instanceof的默认行为

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    function MyObject() {
        // empty
    }
    // 必须通过Object.defineProperty去修改Symbol.hasInstance
    Object.defineProperty(MyObject, Symbol.hasInstance, {
      value: function(v) {
        return false;
      }
    });
    let obj = new MyObject();
    console.log(obj instanceof MyObject);  // false

  2. Symbol.isConcatSpreadable：当使用concat(param)时，用于确定param是否应该展开

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    let collection = {
      0: "Hello",
      1: "world",
      length: 2,
      [Symbol.isConcatSpreadable]: true
    };
    let messages = [ "Hi" ].concat(collection);
    console.log(messages.length);    // 3
    console.log(messages);  // ["hi","Hello","world"]

  3. Symbol.iterator：返回一个iterator

  4. Symbol.species
  5. Symbol.match、Symbol.replace、Symbol.search、Symbol.split：创建类似regular expressions的对象

  6. Symbol.toPrimitive：重写类型转化时的默认行为

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

  7. Symbol.toStringTag

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    Array.prototype[Symbol.toStringTag] = "Magic";
    var values = [];
    console.log(Object.prototype.toString.call(values));    // "[object Magic]"

  8. Symbol.unscopables：定义了在with声明的作用域里会被排除的属性