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    <p>You don't need that extra closure in the first example. This'd work fine:</p> <pre><code>function request(url, callback){ var xhr = new XMLHttpRequest(); xhr.onreadystatechange = function(){ callback(xhr); }; xhr.open('GET', url, true); xhr.send(''); } </code></pre> <p>In modern browsers (or browsers <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind" rel="nofollow">patched with a polyfill</a>) you could also do this:</p> <pre><code>function request(url, callback){ var xhr = new XMLHttpRequest(); xhr.onreadystatechange = callback.bind(null, xhr); xhr.open('GET', url, true); xhr.send(''); } </code></pre> <p><em>edit</em> &mdash; also take note of the answer @Raynos provided. You don't really need to pass the XHR object as a parameter.</p> <p><em>edit again</em> &mdash; in response to a legit comment, you asked why your original idea wouldn't work:</p> <pre><code>function request(url, callback){ var xhr = new XMLHttpRequest(); xhr.onreadystatechange = callback(xhr); xhr.open('GET', url, true); xhr.send(''); } </code></pre> <p>The problem is with the line here:</p> <pre><code> xhr.onreadystatechange = callback(xhr); </code></pre> <p>That assigns to the <code>onreadystatechange</code> the value <em>returned</em> from a <em>call</em> to the callback function right then and there. In other words, instead of establishing something to be called when the state changes, the call is made immediately. That error is really common, because it's easy to read it incorrectly. Any time, however, JavaScript sees a function reference followed by a parenthesized argument list, that's interpreted as a request to perform the function call, and not a request for a function wrapper around a future function call.</p>
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    1. POClarification on closures in Javascript with AJAX
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    1. COSo I could write this, right? function request(url, callback){ var xhr = new XMLHttpRequest(); xhr.onreadystatechange = callback; xhr.open('GET', url, true); xhr.send(''); }
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    2. COYes, you could do that too! The only thing is that the browser won't pass the XHR object as a parameter, but it'll set it up as the `this` value for you.
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    3. COHey Pointy... so it turns out I was right! BTW, I got the example with the closure from "Object Oriented Javascript" by Stoyan Stefanov . A great book, but the example on page 258, with xhr in a closure, really threw me... I was like "OK, *I* wouldn't have used a closure here, HE did, therefore *I* would have stuffed up... but WHY a closure?!?". I banged my head against it for something like 2 hours, trying to figure out why you would possibly need a closure... and it turns out you don't :D
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