Question

I noticed IE9 sort order is changing elements order when comparison function returns 0.

See:

var myarray=[
    {id:1,val:0},
    {id:2,val:0},
    {id:3,val:7},
    {id:4,val:41}
];
myarray.sort(function(a,b){return a.val - b.val});

for(var i in myarray)
{
    console.log(myarray[i].id);
}

Current stable versions of Chrome, Firefox, Opera and Safari got the following output: 1 2 3 4.

Same output for IE7 and IE8.

IE9 output is: 2 1 3 4

Why? Is that normal?

Answer 1

From MDC (emphasis mine):

If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. [Note: the ECMAscript standard does not guarantee this behaviour], and thus not all browsers (e.g. Mozilla versions dating back to at least 2003) respect this.

In my experience, only Chrome/Firefox get this right. Opera 11’s behavior for me is .. not well defined.

E.g., using sort to move all zeroes to the top of the array:

[1, 0, 3, 0, 5, 0, 2].sort(function (a, b) { return b === 0 && 1 || 0;});

• Chromium 10: [0, 0, 0, 1, 3, 5, 2]
• Firefox 4: [0, 0, 0, 1, 3, 5, 2]
• Opera 11: [0, 0, 0, 2, 1, 5, 3] <- does not maintain order of non-zeroes