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    <p>I've been programming in JavaScript for a long time, and "closure in a loop" is a very broad topic. I assume you are talking about the practice of using <code>(function(param) { return function(){ ... }; })(param);</code> inside of a for loop in order to preserve the "current value" of the loop when that inner function later executes...</p> <h2>The code:</h2> <pre><code>for(var i=0; i&lt;4; i++) { setTimeout( // argument #1 to setTimeout is a function. // this "outer function" is immediately executed, with `i` as its parameter (function(x) { // the "outer function" returns an "inner function" which now has x=i at the // time the "outer function" was called return function() { console.log("i=="+i+", x=="+x); }; })(i) // execute the "closure" immediately, x=i, returns a "callback" function // finishing up arguments to setTimeout , i*100); } </code></pre> <h2>Output:</h2> <pre><code>i==4, x==0 i==4, x==1 i==4, x==2 i==4, x==3 </code></pre> <p>As you can see by the output, all of the inner callback functions all point to the same <code>i</code>, however, since each had its own 'closure', the value of <code>x</code> is actually stored as whatever <code>i</code> was at the time of the outer function's execution. </p> <p>Commonly when you see this pattern, you would use the same variable name as the parameter and the argument to the outer function: <code>(function(i){ })(i)</code> for instance. Any code inside that function (even if executed later, like a callback function) is going to refer to <code>i</code> at the time you called the "outer function".</p>
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