Method definitions - JavaScript 编辑

Starting with ECMAScript 2015, a shorter syntax for method definitions on objects initializers is introduced. It is a shorthand for a function assigned to the method's name.

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Syntax

const obj = {
  get property() {},
  set property(value) {},
  property( parameters… ) {},
  *generator( parameters… ) {},
  async property( parameters… ) {},
  async* generator( parameters… ) {},

  //  with computed keys
  get [property]() {},
  set [property](value) {},
  [property]( parameters… ) {},
  *[generator]( parameters… ) {},
  async [property]( parameters… ) {},
  async* [generator]( parameters… ) {},
};

Description

The shorthand syntax is similar to the getter and setter syntax introduced in ES5.

Given the following code:

const obj = {
  foo: function() {
    // ...
  },
  bar: function() {
    // ...
  }
}

You are now able to shorten this to:

const obj = {
  foo() {
    // ...
  },
  bar() {
    // ...
  }
}

Generator methods

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

When doing so:

• 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 cannot 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
const obj2 = {
  g: function* () {
    let index = 0
    while (true) {
      yield index++
    }
  }
};

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

const 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
const obj3 = {
  f: async function () {
    await some_promise
  }
}

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

Async generator methods

Generator methods can also be async.

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

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

Method definitions are not constructable

Methods cannot be constructors! They will throw a TypeError if you try to instantiate them.

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

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

Examples

Simple test case

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

Computed property names

The shorthand syntax also supports computed property names.

const 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

// A global function
function foo() {
  return 1
}

let name = 'foo'
console.log(window[name]())  // 1

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• get
• set
• Lexical grammar