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    <p>In your specific case there is no difference (note that the function in your first fragment is invoked with the parameter <code>file</code>, but since you ignore it it shouldn't have any side effects). However, have a look at the following example:</p> <pre><code>var reader = new FileReader(); var my_temporary_var = 42; reader.onload = function(e){ loadData(e.target.result, my_temporary_var); } </code></pre> <p>So far everything is the same. But if you change <code>my_temporary_var</code> it will also be changed <em>inside</em> of your anonymous function. To prevent this you create a closure:</p> <pre><code>reader.onload = (function(some_value) { return function(e){ loadData(e.target.result, some_value); }; })(my_temporary_var); </code></pre> <p>You create an anonymous function which takes the argument you want to bind to another function and then call this function immediately. Note that the dependency on <code>my_temporary_var</code> has been resolved.</p> <h3>Other example</h3> <p>Some people like to create many functions in a for-loop:</p> <pre><code>var i; for(i = 0; i &lt; myObjects.length; ++i) myObjects[i].onload = function(e) { myObjects[i].doSomething(); }; </code></pre> <p>Lets assume that <code>myObjects[0].onload</code> will be handled first, but <em>after</em> the for loop. The given handler <code>function(e) { myObjects[i].doSomething(); };</code> uses the object <code>myObjects[i]</code>. However, <code>i == myObjects.length</code>: we access an object which isn't even there! And since we try to access an attribute of that function, we'll get an exception and the script will stop.</p> <p>This happened because <code>i</code> in the anonymous function is a reference, and not used as a value. In order to prevent this you must use a closure:</p> <pre><code>var i; for(i = 0; i &lt; myObjects.length; ++i) myObjects[i].onload = (function(index){ return function(e) { myObjects[index].doSomething(); }; })(i); </code></pre>
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    1. POAssigning anonymous function that returns anonymous function
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    1. COYes, I understand, thank you ;)
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