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POAccess server response from same javascript function that initiated the request
 

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  1. POAccess server response from same javascript function that initiated the request
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    <p>With complete control over both the client and server side code, I'd like to accomplish the following:</p> <ul> <li>Initiate a server request in a javascript function</li> <li>Be able to abandon the request (from the user experience perspective) after a specified time </li> <li>Access information about the response (e.g. either a redirect URL or part of the response body) <em>before</em> exiting the original function (this part is non-negotiable; setting a window interval, for example, will not cut it)</li> </ul> <p>This sounds a lot like multithreading to me, which of course javascript doesn't do. Perhaps there's no solution, but I'm exhausting my options before admitting to that. In the non-working example below, function <code>foo()</code> sets an iframe's <code>src</code> to the url of a page -- redirect.aspx here -- which after a short delay redirects to another page with some UUID in the query string. (Note: it could just as well return the UUID in a hidden field in the response body, or via some other strategy; I have control over this).</p> <p>Regardless <em>how</em> the server page returns the result, my goal is to access the UUID from the server <strong>before</strong> <code>foo()</code> exits. </p> <p><strong>Update: Suggested Unit Test</strong> Though this question <em>appears</em> to be about scope -- and therefore solvable via closures (test pending) -- it's actually about continuity of execution. A successful test would consist of:</p> <ul> <li>Create the <code>foo()</code> function</li> <li>Assign <code>something.onClick = foo()</code></li> <li><code>foo()</code> somehow initiates a server call and retrieves a URL from the response</li> <li><code>foo()</code> then calls <code>window.open(url);</code> using that URL</li> <li>A window opens in all major browsers (critical case: IE 7, 8 &amp; 9)</li> </ul> <p>I do not currently know of a strategy that can pass this test.</p> <p><strong>Non-working sample:</strong></p> <pre><code>&lt;iframe id="aFrame" src="" height="0" width="0"&gt;&lt;/iframe&gt; &lt;script type="text/javascript" language="javascript"&gt; function foo() { var f = document.getElementById("aFrame"); var loc = "http://localhost:8080/redirect.aspx?after=1000"; f.src = loc; var start = new Date().getTime(); while (elapsedSince(start &lt; 5000)) { // allow for server response // FAIL: this is never true until after foo() exits: if (f.src != loc) { alert(encodeURIComponent(f.src)); return true; } } alert("Timeout"); return false; } function elapsedSince(startTime) { // omitted safety checks for brevity: return new Date().getTime() - startTime; } &lt;/script&gt; </code></pre> <p>I'm not an ace at Ajax functions, but according to my understanding they require a callback, which means any return information arrives outside of the initiating function. Fail.</p> <p>The above strategy doesn't work, per comments in the js code.</p> <p>Other strategies might include something like using <code>document.createElement()</code> to create the iframe and <code>.insertBefore()</code> to add it to the DOM, but I'm still not certain I would be able both to initiate that <strong>and</strong> access any response details from within the same iteration of <code>foo()</code>. </p> <p>Does anyone know of <em>any</em> strategy that meets the above criteria?</p>
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    1. COI don't understand how a callback is 'fail'. The callback function has access to the enclosing function's variable's through closure. AJAX has a timeout that you can specify. AJAX has a failure callback to handle timeouts and otherwise...failure. Ajax supports asynchronous...multiple parallel requests. How does AJAX fail to handle the issues you are trying to overcome?
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      1. POAccess server response from same javascript function that initiated the request
      1. USP.Brian.Mackey
    2. COBasically, it's about continuity and a single invocation, not just access to the original function's variables. Would the callback execute while `foo()` is inside the `while` loop? My (imperfect) understanding was that with an ajax callback inside a closure, `foo()` would still exit, but would re-enter via the callback. If that's untrue and `foo()` could indeed loop until the inner callback was processed, then that might be the answer indeed.
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      1. USPaul Smith
    3. CONo, it could not, JavaScript is turn-based. When the callback is called, the enclosing function is not "re-entered". The callback function keeps a reference to the scope of the enclosing function so that it can access its variables. It's called a "closure" and it is a key feature of JavaScript. Actually, it's probably its single most awesome feature.
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