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    <p>Looks like you should use the <code>Q.all</code> function to create a master promise corresponding to when all the getSizeSettings promises are fufilled.</p> <p><a href="https://github.com/kriskowal/q#combination" rel="nofollow">https://github.com/kriskowal/q#combination</a></p> <pre><code>var ps = []; for (var i=0, l=pages.length; i&lt;l; i++) { ps[i] = getSizeSettingsFromPage(pages[i], calculateRatio); } Q.all(ps).then(function(){ callWhenReady() }) </code></pre> <p>Most promise libraries should provide a similar method to do this kind of synchronization. If you ever come across one that does not what you could do is hook each individual promise to a callback that increments a shared counter when its called. When your counter reaches <code>n</code> you know that you already resolved all promises so you can have the incrementor callback call the "real" callback as well.</p> <pre><code>//If you did not have Q.all available //Or had to code this without a promise library var to_go = pages.length; for (var i=0, l=pages.length; i&lt;l; i++) { getSizeSettingsFromPage(pages[i], calculateRatio) .then(function(){ to_go--; if(to_go == 0){ callWhenReady() } }); } </code></pre> <p>Note that in these cases until now the async calls are allowed to run parallel. If you need them to run sequentially then usually the only way is to rewrite the for loop as a recursive function</p> <pre><code>var go = function(i){ if(i&gt;=pages.length){ return call_next_step() }else{ return do_ith_calculation(i) .then(function(){ return go(i+1) }) } }; go(0); </code></pre>
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    1. POhow to use q.js promises to work with multiple asynchronous operations
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    1. COI tried https://github.com/simkimsia/learn-promises-javascript/commit/69db89b95bfd87437c30c76f763199dc53530fad which used your 2nd solution. The moment I drag and drop the files, the code broke. I cannot find anything wrong by looking at console.
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    2. COi also tried the Q.all https://github.com/simkimsia/learn-promises-javascript/commit/5084edc25a68b378be1496292f645812776dd7ce the code can run successfully, but the result returned was still 0.
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    3. COi did not try the recursive function, because i do not have an intuitive understanding of it.
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