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POhow to use q.js promises to work with multiple asynchronous operations
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copied!<p>Note: This question is also cross-posted in Q.js mailing list over <a href="https://groups.google.com/forum/#!topic/q-continuum/nY0R3010GM4" rel="nofollow noreferrer">here</a>.</p> <hr> <p>i had a situation with multiple asynchronous operations and the <a href="https://stackoverflow.com/a/13585086/80353">answer I accepted</a> pointed out that using Promises using a library such as q.js would be more beneficial.</p> <p>I am convinced to refactor my code to use Promises but because the code is pretty long, i have trimmed the irrelevant portions and exported the crucial parts into a separate repo.</p> <p>The repo is <a href="https://github.com/simkimsia/learn-promises-javascript" rel="nofollow noreferrer">here</a> and the most important file is <a href="https://github.com/simkimsia/learn-promises-javascript/blob/master/testqjs.html" rel="nofollow noreferrer">this</a>.</p> <p>The requirement is that I want <strong>pageSizes</strong> to be non-empty after traversing all the dragged'n dropped files.</p> <p>The problem is that the FileAPI operations inside <strong>getSizeSettingsFromPage</strong> function causes <strong>getSizeSettingsFromPage</strong> to be async.</p> <p>So I cannot place <strong>checkWhenReady();</strong> like this.</p> <pre><code>function traverseFiles() { for (var i=0, l=pages.length; i<l; i++) { getSizeSettingsFromPage(pages[i], calculateRatio); } checkWhenReady(); // this always returns 0. } </code></pre> <p>This works, but it is not ideal. I prefer to call checkWhenReady just ONCE after all the <code>pages</code> have undergone this function calculateRatio successfully.</p> <pre><code>function calculateRatio(width, height, filename) { // .... code pageSizes.add(filename, object); checkWhenReady(); // this works but it is not ideal. I prefer to call this method AFTER all the `pages` have undergone calculateRatio // ..... more code... } </code></pre> <p><strong>How do I refactor the code to make use of Promises in Q.js?</strong></p>

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