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

The shorthand syntax is similar to the getter and setter syntax introduced in ECMAScript 2015.

Given the following code:

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

You are now able to shorten this to:

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

Generator methods

Generator methods can be defined using the shorthand syntax as well. When using them,

• the asterisk (*) in the shorthand syntax must be before the generator property name. That is, * g(){} will work but g *(){} will not;

• non-generator method definitions may not contain the yield keyword. This means that legacy generator functions won't work either and will throw a 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 methods

Async methods can also be defined using the shorthand syntax.

// 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

瀏覽器相容性

SpiderMonkey-specific notes

• Prior to SpiderMonkey 38 (Firefox 38 / Thunderbird 38 / SeaMonkey 2.35),  "get" and "set" were invalid names for generator methods. This has been fixed in bug 1073809.
• Prior to SpiderMonkey 41 (Firefox 41 / Thunderbird 41 / SeaMonkey 2.38), curly braces were not required in method definitions. They are required from now on to conform to the ES2015 specification and will throw a SyntaxError in this and later versions (bug 1150855).

  var o = {x() 12}; // SyntaxError

• The restriction that only generator methods are constructors has been implemented in SpiderMonkey 41 (Firefox 41 / Thunderbird 41 / SeaMonkey 2.38). See also bug 1059908 and bug 1166950.

參見

• get
• set
• Lexical grammar