Does Internet Explorer 9 choke on extra commas at the end of array and object literals?

Question

Modern browsers and environments like Node.js allow you to say {a:1, b:2,} or [1,2,3,]. This has historically been problematic with Internet Explorer. Is this fixed in Internet Explorer 9?

Answer 1

There are two different answers to this, one for dangling commas in object initializers and one for dangling commas in array initializers:

For object initializers, e.g.:

var obj = {
    a: 1,
    b: 2,
    c: 3,
};

It's fixed in IE8 and above. Test it here: http://jsbin.com/UXuHopeC/1 (source). IE7 and earlier will throw a syntax error on the } after the dangling comma.

For array initializers, e.g.:

var arr = [
    1,
    2,
    3,
];

It was "fixed" in IE9 and above. Test it here: http://jsbin.com/UXuHopeC/2 (source). IE8 and earlier will give that array four entries, the last one having the value undefined. IE9 and above give it three entries.

I put "fixed" in quotes because the spec was originally unclear about whether the array should have a final undefined entry or not, so neither behavior was incorrect. It's just that IE went one way and everyone else went the other. :-)