Over a million developers have joined DZone.
Platinum Partner

Quick JavaScript tip: trailing commas inside an object literal

· Web Dev Zone

The Web Dev Zone is brought to you in partnership with Mendix.  Discover how IT departments looking for ways to keep up with demand for business apps has caused a new breed of developers to surface - the Rapid Application Developer.

It used to be that some JavaScript engines weren’t picky about trailing commas inside an object literals, while others threw a syntax error. The ECMAScript 5 language specification [1] has made trailing commas legal, via the following syntax rule (in Sect. 11.1.5):

    ObjectLiteral :
        { PropertyNameAndValueList }
        { PropertyNameAndValueList , }
You can use this syntax in all modern browsers [3]. It makes it easier to maintain the content inside object literals, especially long ones: You don’t have to keep track which property is last when rearranging things. Take the following example.
    var obj = {
        foo: 123,
        bar: function () { ... },
Thanks to the trailing comma, you save two steps when swapping foo and bar, as there is no need to add a comma after bar or to remove the comma after foo.

Related reading:

  1. Standard ECMA-262: ECMAScript Language Specification
  2. What’s new in ECMAScript 5
  3. ECMAScript 5 compatibility table


From http://www.2ality.com/2011/06/object-literal-comma.html

The Web Dev Zone is brought to you in partnership with Mendix.  Learn more about The Essentials of Digital Innovation and how it needs to be at the heart of every organization.


{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}