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    <p>Your <code>setTimeout</code> argument function is declared to take a single parameter, <code>i</code>. When <code>setTimeout</code> calls the function with no arguments, as it does, the parameter is thus set to <code>undefined</code>.</p> <p>This would seem to be be slightly better, as it no longer shadows the outer <code>i</code> variable you were originally trying to reference...</p> <pre><code>setTimeout(function() { console.log(i); }, 2000) </code></pre> <p>...but if you run it, you'll find it prints <code>10</code> 5 times, because every function you create is referencing the same <code>i</code> variable, whose value will be <code>10</code> when the loop exit condition becomes <code>true</code> and it terminates.</p> <p>Creating a <em>closure</em> which holds the value of <code>i</code> as it was during the loop in which each <code>setTimout</code> argument function was created will do the trick:</p> <pre><code>setTimeout((function(i) { return function() { console.log(i); } })(i), 2000) </code></pre> <p>Renaming the argument to the <a href="http://benalman.com/news/2010/11/immediately-invoked-function-expression/">Immediately-Invoked Function Expression</a> we just used might help make things clearer:</p> <pre><code>setTimeout((function(loopIndex) { return function() { console.log(loopIndex); } })(i), 2000) </code></pre> <p>We're:</p> <ul> <li>Creating a function which takes a single argument and returns another function.</li> <li><em>Immediately</em> calling the "outer" function, passing it the current value of <code>i</code> (which is not an object, so is effectively passed by value).</li> <li>The "outer" function returns the "inner" function, which is passed as the argument to <code>setTimeout</code>.</li> </ul> <p>This works because:</p> <ol> <li><p>Creating a function in JavaScript creates a new <em>scope</em> to hold the function's argument variables and any other variables declared within it using the <code>var</code> keyword. Think of this like an invisible object with properties corresponding to the variable names. </p></li> <li><p>All functions hold a reference to the scope in which they were defined, as part of their <em>scope chain</em>. They still have access to this scope even if it's no longer "active" (e.g. when the function it was created for returns). The "inner" function was created in a scope which contained a <code>loopIndex</code> variable, set to the value of <code>i</code> at the time the IIFE was called. So when you try to reference a variable named <code>loopIndex</code> from inside the inner function, it first checks its own scope (and doesn't find a <code>loopIndex</code> there) <em>then</em> starts walking up its scope chain - first, it checks the scope in which it was defined, which <em>does</em> contain a <code>loopIndex</code> variable, the value of which is passed to <code>console.log()</code>.</p></li> </ol> <p>That's all a closure is - a function which has access to the scope in which it was defined, even if the function the scope was created for has finished executing.</p>
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