Adam Rackis Adam Rackis - 3 months ago 8
Javascript Question

Trailing commas in JavaScript

Are trailing commas standard in JavaScript, or do most browsers like Chrome and FF just tolerate them?

I thought they were standard, but IE8 puked after encountering one—of course IE not supporting something hardly means it's not standard.

Here's an example of what I mean: (after the last element of the books array)

var viewModel = {
books: ko.observableArray([
{ title: "..", display: function() { return ".."; } },
{ title: "..", display: function() { return ".."; } },
{ title: "..", display: function() { return ".."; } }, // <--right there
]),
currentTemplate: ko.observable("bookTemplate1"),
displayTemplate: function() { return viewModel.currentTemplate(); }
};

Answer

Specs: ECMAScript 5 and ECMAScript 3


Section 11.1.5 in the ECMAScript 5 specification:

ObjectLiteral :
    { }
    { PropertyNameAndValueList }
    { PropertyNameAndValueList , }

So yes, it is part of the specification.

Update: Apparently this is new in ES5. In ES3 (page 41), the definition was just:

ObjectLiteral :
    { }
    { PropertyNameAndValueList }

For arrays literals (Section 11.1.4) it is even more interesting (Update: this already existed in ES3):

ArrayLiteral :
    [ Elisionopt ]
    [ ElementList ]
    [ ElementList , Elision_opt ]

(where Elision_opt is Elisionopt, meaning the Elision is optional)

Elision is defined as

Elision :
    ,
    Elision ,

So, an array literal like

var arr = [1,2,,,,];

is perfectly legal. This creates an array with two elements but sets the array length to 2 + 3 = 5.

Don't expect too much from IE (before IE9)...