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    <p><strong>Closures</strong> have already been well handled in Stackoverflow here is just a selection:-</p> <p><a href="https://stackoverflow.com/questions/111102/how-does-a-javascript-closure-work">How does a javascript closure work?</a><br> <a href="https://stackoverflow.com/questions/1801957/what-exactly-does-closure-refer-to-in-javascript">What exactly does “closure” refer to in JavaScript?</a><br> <a href="https://stackoverflow.com/questions/2055641/can-you-say-this-is-a-right-example-of-javascript-closure-where-the-places-we-n">can you say this is a right example of Javascript Closure.. Where the places we need to consider avoiding the closures??</a><br> <a href="https://stackoverflow.com/questions/631187/javascript-scope-and-closure">JavaScript scope and closure</a><br> <a href="https://stackoverflow.com/questions/346015/javascript-closures-and-this-context">Javascript Closures and ‘this’ context</a><br> <a href="https://stackoverflow.com/questions/212401/javascript-how-do-i-learn-about-closures-usage">JavaScript - How do I learn about “closures” usage?</a> </p> <p><strong>Callbacks</strong> are a simpler concept. A callback is basically where a function accepts another function as a parameter. At some point during execution the called function will execute the function passed as a parameter, this is a <em>callback</em>. Quite often the callback actually happens as an asynchronous event, in which case the called function may return without having executed the callback, that may happen later. Here is a common (browser based) example:-</p> <pre><code> function fn() { alert("Hello, World"); } window.setTimeout(fn, 5000); </code></pre> <p>Here the function <code>fn</code> is passed as a <em>callback</em> to the <code>setTimeout</code> function. Set timeout returns immediately however 5 seconds later the function passed as a <em>callback</em> is executed.</p> <p><strong>Closures and callbacks</strong></p> <p>Quite often the reason that closures get created (either incidentally, accidentally or deliberately) is the need to create a callback. For example:-</p> <pre><code> function AlertThisLater(message, timeout) { function fn() { alert(message); } window.setTimeout(fn, timeout); } AlertThisLater("Hello, World!", 5000); </code></pre> <p>(Please read some of the linked posts to grasp closures)</p> <p>A closure is created containing in part the <code>message</code> parameter, <code>fn</code> is executed quite some time after the call to <code>AlertThisLater</code> has returned, yet <code>fn</code> still has access to the original content of <code>message</code>.</p>
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    1. COAre callbacks most commonly used with setTimeout?
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    2. COsetTimeout is a common usage but equally common would be `onreadystatechange` on the `XmlHttpRequest` object. You will also see extensive usage of callbacks in code that uses Javascript frameworks such as JQuery. In many of these cases the callback is just where you would provide a little logic to customise a large but otherwise standard process. Also callbacks are frequently used to execute the same function over a set of objects.
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