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copied!<h3>Update</h3> <p>Since writing this answer, there is now a better solution. Please see <a href="https://stackoverflow.com/a/31821896/156755">xuanji's answer</a> for details</p> <h3>Original</h3> <p><strong>Thanks to @genexp</strong> for a simple and concise example in the comments below...</p> <pre><code>async.each(someArray, function(item, done){ console.log(someArray.indexOf(item)); }); </code></pre> <hr> <p>Having read the docs, I suspected that there wasn't any way to access an integer representing position in the list...</p> <blockquote> <p>Applies an iterator function to each item in an array, <strong>in parallel</strong>. The iterator is called with an item from the list and a callback for when it has finished. If the iterator passes an error to this callback, the main callback for the each function is immediately called with the error.</p> <p>Note, that since this function applies the iterator to each item in parallel there is no guarantee that the iterator functions will complete in order.</p> </blockquote> <p>So I dug a little deeper (<a href="https://github.com/caolan/async/blob/master/lib/async.js#L101-L121" rel="nofollow noreferrer">Source code link</a>) </p> <pre><code>async.each = function (arr, iterator, callback) { callback = callback || function () {}; if (!arr.length) { return callback(); } var completed = 0; _each(arr, function (x) { iterator(x, only_once(function (err) { if (err) { callback(err); callback = function () {}; } else { completed += 1; if (completed >= arr.length) { callback(null); } } })); }); }; </code></pre> <p>As you can see there's a <code>completed</code> count which is updated as each callback completes but no actual index position.</p> <p>Incidentally, there's no issue with race conditions on updating the <code>completed</code> counter as JavaScript is <a href="http://ejohn.org/blog/how-javascript-timers-work/" rel="nofollow noreferrer">purely single-threaded</a> under the covers.</p> <p><strong>Edit:</strong> After digging further into the iterator, it looks like you <em>might</em> be able to reference an <code>index</code> variable thanks to closures...</p> <pre><code>async.iterator = function (tasks) { var makeCallback = function (index) { var fn = function () { if (tasks.length) { tasks[index].apply(null, arguments); } return fn.next(); }; fn.next = function () { return (index < tasks.length - 1) ? makeCallback(index + 1): null; }; return fn; }; return makeCallback(0); }; </code></pre>

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