Method definitions
Starting with ECMAScript 2015 (ES6), a shorter syntax for method definitions on objects initializers is introduced. It is a shorthand for a function assigned to the method's name.
Syntax
var obj = { <var>property</var>( <var>parameters…</var> ) {}, *<var>generator</var>( <var>parameters…</var> ) {}, // also with computed keys: [property]( <var>parameters…</var> ) {}, *[generator]( <var>parameters…</var> ) {}, // compare ES5 getter/setter syntax: get <var>property</var>() {}, set <var>property</var>(<var>value</var>) {} };
Description
The shorthand syntax is similar to the getter and setter syntax introduced in ECMAScript 2015.
Given the following code:
var obj = { foo: function() {}, bar: function() {} };
You are now able to shorten this to:
var obj = {
foo() {},
bar() {}
};
Note : The shorthand syntax uses named function instead of anonymous functions (as in …foo: function() {}
…). Named functions can be called from the function body (this is impossible for anonymous function as there is no identifier to refer to). For more details, see function
.
Shorthand generator methods
Generator methods can be defined using the shorthand syntax as well. Note that the asterisk (*) in the shorthand syntax must be before the generator property name. That is, * g(){}
will work but g *(){}
will not.
// Using a named property (pre-ES6) 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
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)
Examples
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
Specifications
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 2017 Draft (ECMA-262) The definition of 'Method definitions' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Method definition shorthand | 39 | 34 (34) | No support | 26 | No support |
Generator methods are not constructable (ES2016) | ? | 43 (43) | ? | ? | ? |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Method definition shorthand | No support | No support | 34.0 (34) | No support | No support | No support |
Generator methods are not constructable (ES2016) | ? | ? | 43.0 (43) | ? | ? | ? |
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 ES6 specification and will throw a
SyntaxError
in this and later versions (bug 1150855).JavaScriptCopy Codevar 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.
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/javascript/reference/functions/method_definitions