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Editorial Team
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Asked: 2026-05-14T23:54:43+00:00

I am having trouble sorting an array that includes undefined elements (a sparse array)

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I am having trouble sorting an array that includes undefined elements (a sparse array) in IE7. This works great in Safari and Firefox of course, and I haven’t tried other versions of IE, but here is a simple example.

<html>
<head>
<script type="text/javascript">
function runscript() {
    var myArray = [{id: 2},
                        undefined,
                        {id: 0},
                        {id: 1},
                        {id: 3},
                        {id: 4},
                        {id: 5}];
    myArray.sort(function compare(a, b) { return a.id - b.id; });
    var output = '';
    for (loop in myArray) {
        output += myArray[loop].id + ' ';
    }
    alert(output);
}
</script>
</head>
<body onLoad="runscript();">
</body>

The alert() at the end inexplicably shows 0 2 3 4 5 1. Removing the undefined element from the array correctly sorts it and the alert shows 0 1 2 3 4 5.

Is there a way to work around this in IE7 so that I can reliably sort arrays that include undefined elements? I don’t care where the undefined elements end up as long as the defined elements are sorted correctly.

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1 Answer

  1. Editorial Team

    Try changing for (loop in myArray) to for (var loop=0; loop<myArray.length; loop++):

    function runscript() {
    var myArray = [{id: 2},
                        undefined,
                        {id: 0},
                        {id: 1},
                        {id: 3},
                        {id: 4},
                        {id: 5}];
    myArray.sort(function compare(a, b) { return a.id - b.id; });
    var output = '';
    for (var loop=0; loop<myArray.length; loop++) {
        output += (myArray[loop]||{id: 'undefined'}).id + ' ';
    }
    alert(output);
}
runscript()

    when using the for (x in object) the items aren’t guaranteed to be in in order. See also Why is using "for…in" with array iteration a bad idea?

    (The above alerts 0 1 2 3 4 5 undefined)

    EDIT: Undeleted – I’ve tested the above and it works 😛

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