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POjquery: what happens when you pass a deferred object into "then"?
 

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  1. POjquery: what happens when you pass a deferred object into "then"?
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    <p>I have been researching jquery deferred objects for a while now and I'm stumped on one thing.</p> <p>The "done," "always," "then," etc methods take, as their arguments, functions which should be called when the deferred object is resolved.</p> <p>However I tried chaining requests by passing a deferred object into the "always" method, and that <em>seems</em> to work too:</p> <pre><code>// A is an array of data var req = ajax(url + "?d=" + A[0]).done(processResults).fail(reportFailure); for (var i = 1 ; i &lt; A.length ; i++) { var tmp = ajax(url + "?d=" + A[i]).done(processResults).fail(reportFailure); req.always(tmp); req = tmp; } </code></pre> <p>However, it doesn't really work. Because when I follow the above code with this:</p> <pre><code>req.always(foobar); </code></pre> <p>foobar seems to get called at some random time, not necessarily after the last element in the array is processed.</p> <p>Would it be better to use when?</p> <pre><code>// A is an array of data var req = $.when(ajax(url + "?d=" + A[0]).done(processResults).fail(reportFailure)); for (var i = 1 ; i &lt; A.length ; i++) { req = $.when(req, ajax(url + "?d=" + A[i]).done(processResults).fail(reportFailure)); } req.always(foobar); </code></pre> <p>Would the above code (using "when") result in the ajax requests happening one after the other, or would they occur simultaneously?</p> <p>I looked into chaining with "pipe" by the way, but because of scoping issues I think it would be harder to use "pipe" with a for loop as above.</p> <p>Also, why does the original code "almost work"? What is happening there? Is it executing the deferred object as if it were a function, and what happens when it does that?</p>
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    1. COp.s.: I tried using $.when and it does appear to work. However I still don't get why doing it the other way "sort of" worked. This seems like a better way to create chains of requests than by using pipe -- but I'm not sure if that is how "when" was intended to be used.
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    2. CO.pipe() is useful when the result of an ajax call are used to build the input of another ajax call. In your case it looks like parallel ajax calls, merged using $.when(), are more appropriated. In my answer I show you how to register a single successcallback using .done() on the merged Deferred
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    3. COp.p.s. using $.wait doesn't really work. But the solution with "pipe", below, does appear to work, though I do have to resolve the scoping issue with "i"
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