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. :-)

Answer 2

This document claims it is/will be corrected: http://blogs.msdn.com/b/ie/archive/2010/06/25/enhanced-scripting-in-ie9-ecmascript-5-support-and-more.aspx

Corrected Issues

Trailing commas in array literals added to the array’s length

Example

var len = [1,2,3,].length;

alert(len); //should be 3, IE8 says 4

It makes no specific mention of Objects. Just Arrays.

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EDIT: More info. From this PDF document:

http://download.microsoft.com/download/8/4/2/8427CF1B-08B3-4557-952D-102E7A8FA64C/[MS-ES3].pdf

...dowloaded from this page: http://msdn.microsoft.com/en-us/library/ff520996(VS.85).aspx

JScript 5.8 supports the occurrence of a single trailing comma as the last item within an ObjectLiteral. JScript 5.7 does not support this extension.