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How to sort array with undefined elements in IE7 JavaScript

Answer 1

A:

First of all, your sort function is wrong as it is expected to return -1, 0 or +1, not a boolean value.
Use this instead

var arr = [.....]
arr.sort((function(a, b){
    if (!a || !b) {
        // Here you choose how to treat undefined/null elements
        return 0;
    }  
    return (a[index].id === b[index].id ? 0 : (a[index].id < b[index].id ? -1 : 1));
})

But just so that you know, that loop of yours is going to throw an error when trying to return the id property from the undefined element. Also, you should never use a for..in loop to iterate over an array, use either a loop with incrementing index or a reverse while like this

var l = arr.length; 
while (l--) {
    ..
}

Sean Kinsey 2010-05-21 07:30:14

Answer 2

+1 A:

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 http://stackoverflow.com/questions/500504/javascript-for-in-with-arrays

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

EDIT: Undeleted - I've tested the above and it works :-P

David Morrissey 2010-05-21 07:31:20

I was assuming that for (loop in myArray) would respect the natural array ordering after a sort, and in fact it seems to in Safari, Firefox and even IE7 (but only when there are no undefined values in the array). If I do an indexed loop as David suggests, it works across all browsers. Too bad, I was just starting to get used to the more compact loop syntax. Thanks for your help.

Gene Goykhman 2010-05-21 20:15:30

Answer 3

+1 A:

Maybe you can modify your comparator

myArray.sort(function compare(a, b) { return a.id || 0 - b.id || 0; });

Kunal 2010-05-21 07:32:07

Hi Kunal, this seems to work but I have no idea why. Can you explain the comparator function a little bit?

Gene Goykhman 2010-05-21 07:41:11

Yes. Custom comparator for sort is suppose to return integer value ( +ve, -ve or 0 only ). In the above scenario, when the value (a.id) is undefined, the sort function return NaN which is incorrect.

Kunal 2010-05-21 08:06:43

+1 - I find it surprising that this function works at all, after all `undefined.id` raises an exception (`undefined.id' is null or not an object` in IE). Maybe array sorting functions have a special case to return `undefined` if properties are accessed in `undefined` types?

David Morrissey 2010-05-21 08:18:52

@David,It will throw exception if you try to compare the value likeif( a.id == 10 ) { // do something. }

Kunal 2010-05-21 08:59:53

Are you sure this function works as expected? In my copy of IE8 it alerts `0 1 2 4 3 5`. Besides, the function should be `return (a.id || 0) - (b.id || 0);` because of the operator precedence in JavaScript. When I `alert` each value in the array, it doesn't popup with any `undefined` values - it looks like sorting functions only work on strings, numbers and objects like hash tables and arrays (and IE sorts the rest internally to the end of the array.) My answer of `for (var i in myArray)` replaced with `for (var i=0; i<myArray.length; i++)` alerts the correct answer, so I'll undelete it :-P

David Morrissey 2010-05-21 10:01:04

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How to sort array with undefined elements in IE7 JavaScript

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