IE9 JavaScript array initialization bug

Question

Apparently JS implementation in IE9 contains (IMO, critical) bug in handling array literals.

In IE9 in some cases this code:

var a = [1,2,3,4,];


        

Answer 1

A single trailing comma in an array literal should be ignored. Two trailing commas is an elision and should add one to the array's length. So:

alert( [1,2,3,4,].length );   // 4

alert( [1,2,3,4,,].length );  // 5

Some versions of IE (< 9?) treat the single trainling comma as an elison and incorrectly add one to length, so the results above are 5 and 6 respsectively. That is inconsistent with ECMA-262 §11.1.3 and therefore is a bug.

The purpose of an elision is to increase array length without creating a extra property or assigning directly to length, so:

var x = [,1,,];

is equivalent to:

var x = new Array(3);
x[1] = 1;

The result in both cases should be an array with length 3 and one property named '1' with value 1. The leading comma and trailing comma pair are elisions, they only affect the length, they do not create properties. IE interprets the leading comma correctly but incorrectly interprets both trailing commas as elisions, incrementing the length by 1 too many.

var x = [,1,,3,,];
var s = 'length: ' + x.length;

for (var p in x) {
  s += '\nindex ' + p + ' has value ' +  x[p]; 
}
alert(s);

The result should be:

length: 5
index 1 has value 1
index 3 has value 3

Incidentally, this bug has probably been around since IE allowed array literals, version 4 at least (1997?).

Answer 2

That's not a bug. That's exactly how it should behave. Microsoft did that on purpose. If you want an array with only 4 items, get rid of the last comma. Simple as that.

If the results you're after is to have an extra, undefined value at the end, you're in luck. Even without the comma, it'll be undefined. It, and every single number after 3.