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方法定義

自 ECMAScript 2015 開始，引入了一種於物件初始器（objects initializers）中定義方法的簡短語法。是一個將函式指派予方法名稱的簡便方式。

語法

var obj = {
  property( parameters… ) {},
  *generator( parameters… ) {},
  async property( parameters… ) {},
  async* generator( parameters… ) {},

  // with computed keys:
  [property]( parameters… ) {},
  *[generator]( parameters… ) {},
  async [property]( parameters… ) {},

  // compare getter/setter syntax:
  get property() {},
  set property(value) {}
};

說明

這個簡短的語法和在 ECMAScript 2015 引入 getter 以及 setter 類似。

請看以下程式碼：

var obj = {
  foo: function() {
    /* code */
  },
  bar: function() {
    /* code */
  }
};

你可以把它縮減為：

var obj = {
  foo() {
    /* code */
  },
  bar() {
    /* code */
  }
};

產生器方法

產生器方法（Generator method）也可以透過簡短語法定義之。用的時候：

  • 簡短語法的星號（*）必須放在產生器方法的屬性名前面。也就是說 * g(){} 能動但 g *(){} 不行；

  • 非產生器方法的定義可能不會有 yield 關鍵字。也就是說過往的產生器函式動不了、並拋出SyntaxError。Always use yield in conjunction with the asterisk (*).

// Using a named property
var obj2 = {
  g: function* () {
    var index = 0;
    while (true)
      yield index++;
  }
};

// The same object using shorthand syntax
var obj2 = {
  * g() {
    var index = 0;
    while (true)
      yield index++;
  }
};

var it = obj2.g();
console.log(it.next().value); // 0
console.log(it.next().value); // 1

Async 方法

Async 方法 也可以透過簡短語法定義。

// Using a named property
var obj3 = {
  f: async function () {
    await some_promise;
  }
};

// The same object using shorthand syntax
var obj3 = {
  async f() {
    await some_promise;
  }
};

Async generator methods

Generator methods can also be async.

var obj4 = {
  f: async function* () {
    yield 1;
    yield 2;
    yield 3;
  }
};

// The same object using shorthand syntax
var obj4 = {
  async* f() {
   yield 1;
   yield 2;
   yield 3;
  }
};

Method definitions are not constructable

All method definitions are not constructors and will throw a TypeError if you try to instantiate them.

var obj = {
  method() {}
};
new obj.method; // TypeError: obj.method is not a constructor

var obj = {
  * g() {}
};
new obj.g; // TypeError: obj.g is not a constructor (changed in ES2016)

範例

Simple test case

var obj = {
  a: 'foo',
  b() { return this.a; }
};
console.log(obj.b()); // "foo"

Computed property names

The shorthand syntax also supports computed property names.

var bar = {
  foo0: function() { return 0; },
  foo1() { return 1; },
  ['foo' + 2]() { return 2; }
};

console.log(bar.foo0()); // 0
console.log(bar.foo1()); // 1
console.log(bar.foo2()); // 2

規範

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Method definitions' in that specification.		 Standard Initial definition.
ECMAScript 2016 (ECMA-262)
The definition of 'Method definitions' in that specification.		 Standard Changed that generator methods should also not have a [[Construct]] trap and will throw when used with new.
ECMAScript (ECMA-262)
The definition of 'Method definitions' in that specification.		 Living Standard  

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