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    <p>If you'd like a version on npm, <a href="https://www.npmjs.com/package/array-move" rel="noreferrer">array-move</a> is the closest to this answer, although it's not the same implementation. See its usage section for more details. The previous version of this answer (that modified Array.prototype.move) can be found on npm at <a href="https://www.npmjs.com/package/array.prototype.move" rel="noreferrer">array.prototype.move</a>.</p> <hr> <p>I had fairly good success with this function:</p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>function array_move(arr, old_index, new_index) { if (new_index &gt;= arr.length) { var k = new_index - arr.length + 1; while (k--) { arr.push(undefined); } } arr.splice(new_index, 0, arr.splice(old_index, 1)[0]); return arr; // for testing }; // returns [2, 1, 3] console.log(array_move([1, 2, 3], 0, 1)); </code></pre> </div> </div> </p> <p>Note that the last <code>return</code> is simply for testing purposes: <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/splice" rel="noreferrer"><code>splice</code></a> performs operations on the array in-place, so a return is not necessary. By extension, this <code>move</code> is an in-place operation. If you want to avoid that and return a copy, use <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/slice" rel="noreferrer"><code>slice</code></a>.</p> <p>Stepping through the code:</p> <ol> <li>If <code>new_index</code> is greater than the length of the array, we want (I presume) to pad the array properly with new <code>undefined</code>s. This little snippet handles this by pushing <code>undefined</code> on the array until we have the proper length.</li> <li>Then, in <code>arr.splice(old_index, 1)[0]</code>, we splice out the old element. <code>splice</code> returns the element that was spliced out, but it's in an array. In our above example, this was <code>[1]</code>. So we take the first index of that array to get the raw <code>1</code> there.</li> <li>Then we use <code>splice</code> to insert this element in the new_index's place. Since we padded the array above if <code>new_index &gt; arr.length</code>, it will probably appear in the right place, unless they've done something strange like pass in a negative number. </li> </ol> <p>A fancier version to account for negative indices: </p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>function array_move(arr, old_index, new_index) { while (old_index &lt; 0) { old_index += arr.length; } while (new_index &lt; 0) { new_index += arr.length; } if (new_index &gt;= arr.length) { var k = new_index - arr.length + 1; while (k--) { arr.push(undefined); } } arr.splice(new_index, 0, arr.splice(old_index, 1)[0]); return arr; // for testing purposes }; // returns [1, 3, 2] console.log(array_move([1, 2, 3], -1, -2));</code></pre> </div> </div> </p> <p>Which should account for things like <code>array_move([1, 2, 3], -1, -2)</code> properly (move the last element to the second to last place). Result for that should be <code>[1, 3, 2]</code>. </p> <p>Either way, in your original question, you would do <code>array_move(arr, 0, 2)</code> for <code>a</code> after <code>c</code>. For <code>d</code> before <code>b</code>, you would do <code>array_move(arr, 3, 1)</code>.</p>
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    1. COThis works perfectly! And your explanation is very clear. Thanks for taking the time to write this up.
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    2. COYou shouldn't manipulate Object and Array prototypes, it causes problems when iterating elements.
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      1. USburak emre
    3. CO@burakemre: I think that conclusion is not so clearly reached. Most good JS programmers (and most popular libraries) will use a `.hasOwnProperty` check when iterating with things like for..in, especially with libraries like Prototype and MooTools which modify prototypes. Anyway, I didn't feel it was a particularly important issue in a relatively limited example like this, and there is a nice split in the community over whether or not prototype modification is a good idea. Normally, iteration problems are the least concern, though.
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