Tuesday, 21 September 2010

A literal improvement

JavaScript literals are getting better. Until recently, the grammar for object literals didn't explicitly allow a trailing comma, like this:

var obj = {
foo: 42,
bar: 27, // <== This is the problem
};
SpiderMonkey (Firefox), V8 (Chrome), and whatever Safari and Opera use don't care, but JScript (IE) prior to JScript 6 (IE8) throws a parsing exception on the comma and your script dies. (JScript 6 / IE8 fix this.)

A trailing comma in an array literal has a different issue:
var a = [1, 2, 3, ];
All versions of JScript so far (including JScript 6 / IE8) create an array with four (yes, four) entries, the last of which is undefined. This isn't unreasonable, because the spec wasn't explicit about it and we were always allowed to have blank entries (e.g., var a = [1, , 3];) and those entries defaulted to undefined — but everyone else went the other way and created an array with three entries instead.

Fortunately, ECMAScript 5 clears this up. The trailing comma is explicitly allowed in object literals (Section 11.1.5), and in array literals (Section 11.1.4). In the case of array literals, the trailing comma doesn't add to the length of the array (a.length above is 3).

The team behind IE9 are very engaged with standards bodies now, so hopefully that includes the JScript folks and they'll change the array behavior, though you know it must be a much harder sell for them than the object literal was — it involves changing the behavior of something that did work. Still, here's hoping.

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