方法的定义

从ECMAScript 2015开始,在对象初始器中引入了一种更简短定义方法的语法,这是一种把方法名直接赋给函数的简写方式。

语法

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) {}
};

描述

该简写语法与ECMAScript 2015的gettersetter语法类似。

如下代码:

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

现可被简写为:

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

注意:简写语法使用命名函数而不是匿名函数(如…foo: function() {}…)。命名函数可以从函数体调用(这对匿名函数是不可能的,因为没有标识符可以引用)。详细信息,请参阅function

生成器方法

生成器方法也可以用这种简写语法定义。使用它们时,

  • 简写语法中的星号(*)必须出现在生成器名前,也就是说* g(){}可以正常工作,而g *(){}不行。
  • 非生成器方法定义可能不包含yield关键字。这意味着遗留的生成器函数也不会工作,并且将抛出 SyntaxError。始终使用yield与星号(*)结合使用。

// 用有属性名的语法定义方法(ES6之前):
var obj2 = {
  g: function*() {
    var index = 0;
    while(true)
      yield index++;
  }
};

// 同一个方法,简写语法:
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 方法也可以使用简写语法来定义。

// 用有属性名的语法定义方法(ES6之前):
var obj3 = {
  f: async function () {
    await some_promise;
  }
};

// 同一个方法,简写语法:
var obj3 = { 
  async f() {
    await some_promise;
  }
};

Async 生成器方法

生成器方法也能成为 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;
  }
};

方法定义不是构造函数

所有方法定义不是构造函数,如果您尝试实例化它们,将抛出TypeError

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)

示例

简单示例

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

计算的属性名

该简写语法还支持计算的属性名称作为方法名。

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

规范

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

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参见