Loop through associative array in reverse

Question

I\'m using a javascript associative array (arr) and am using this method to loop through it.

for(var i in arr) {
    var value = arr[i];
    alert(i =\") \"+         


        

Answer 1

Four things:

1. JavaScript has arrays (integer-indexed [see comments below]) and objects (string-indexed). What you would call an associative array in another language is called an object in JS.

2. You shouldn't use for in to loop through a JS array.

3. If you're looping through an object, use: hasOwnProperty.

4. JavaScript doesn't guarantee the order of keys in an object. If you care about order, use an array instead.

If you're using a normal array, do this:

for (var i = arr.length - 1; i >= 0; i--) {
    //do something with arr[i]
}

Answer 2

Warning: this answer is ancient.

If you're here for a quick fix, kindly refer to the much better answer below.

^{Original answer retained, because reasons. See comments.}

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Using a temporary array holding the keys in reverse order:

var keys = new Array();

for (var k in arr) {
    keys.unshift(k);
}

for (var c = keys.length, n = 0; n < c; n++) {
   alert(arr[keys[n]]);
}

Answer 3

For a normal array, I would have done this:

var i = arr.length;
while (i--) {
    var value = arr[i];
    alert(i =") "+ value);
}

This is faster than a "for" loop.

http://blogs.oracle.com/greimer/entry/best_way_to_code_a

Answer 4

In modern browsers you can now use Object.keys to get your array of properties and step through it in reverse order, allowing you to skip the preliminary key collection loop.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys

var keys = Object.keys(subject);
for (var i = keys.length-1; i >= 0; i--) {
    var k = keys[i],
        v = subject[k];
    console.log(k+":",v);
}