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POSynchronous / asynchronous nature of browser rendering and javascript execution
 

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  1. POSynchronous / asynchronous nature of browser rendering and javascript execution
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    <p>I have some processing that is going to take a few seconds so I want to add a visual indicator while it is in progress.</p> <pre><code>.processing { background-color: #ff0000; } &lt;div id="mydiv"&gt; Processing &lt;/div&gt; </code></pre> <p>Script:</p> <pre><code>$("#mydiv").addClass("processing"); // Do some long running processing $("#mydiv").removeClass("processing"); </code></pre> <p>I naively thought that the class would be applied to the div and the UI would be updated. However, running this in the browser (in Firefox at least) the div is never highlighted. Can someone explain to me why my my div never gets highlighted? The class is added, the processing takes place and then the class is removed; the UI is not updated in between and the user never sees the red background.</p> <p>I know JS is single-threaded but I'd always presumed the browser rendering would run synchronously as and when the DOM is updated. Is my assumption incorrect?</p> <p>And more importantly, what is the recommended way to achieve this effect? Do I have to result to using <code>setTimeout</code> to make the slow processing asynchronous and work with a callback? There must be a better way as I really don't have any need for async behaviour here; I just want the UI to refresh.</p> <p>EDIT:</p> <p>JSFiddle: <a href="http://jsfiddle.net/SE8wD/5/" rel="nofollow">http://jsfiddle.net/SE8wD/5/</a></p> <p>(Note, you may need to tweak the number of loop iterations to give you a reasonable delay on your own PC)</p>
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    1. COI believe such changes will make it directly to the DOM, but the browser will wait until the JavaScript execution is "idle" before doing any repainting.
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    2. CO"Do some long running processing" has some asynchronous call? (ajax, settimeout etc)
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    3. COI can't really see where your code goes wrong, because it seems fine of what you have posted. Maybe there is some `!important` in your external css file. Are you sure the class get's added ( have you inspected it via firebug) and does the process really take that long ? maybe it happens too fast
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