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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 Latest Draft (ECMA-262)
The definition of 'Method definitions' in that specification.
Draft  

瀏覽器相容性

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support39 Yes34 No26 No
Generator methods are not constructable (ES2016) ? ?43 No ? ?
Async methods ? ?52 No ? ?
Async generator methods ? ?55 No ? ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support3939 Yes3426 No4.0
Generator methods are not constructable (ES2016) ? ? ?43 ? ? ?
Async methods ? ? ?52 ? ? ?
Async generator methods ? ? ?55 ? ? ?

1. From version 7.0.0: this feature is behind the --harmony runtime flag.

2. From version 8.10.0: this feature is behind the --harmony runtime flag.

參見

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