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    <p>Side-effect:</p> <p>Think of a side-effect as something that does <b>two things</b> at once. For example:</p> <p>Classic example of a side effect:</p> <pre><code>var i = 1; var j = i++; </code></pre> <p>The side effect happens at <code>i++</code>. What happens here is <code>j</code> becomes 1 <b>and then</b> <code>i</code> gets incremented and becomes 2. In other words, two things happened and the side-effect was that <code>i</code> became 2. </p> <p>Closure:</p> <p>Visualize a chain of links like this: &lt;>&lt;>&lt;>&lt;>&lt;>&lt;>&lt;>. Imagine that the name of this chain of links is called the <b>scope chain</b>. Then imagine that all these links connect <b>objects</b> together like this: &lt;>object&lt;>object&lt;>object&lt;>. Now, keep in mind the following:</p> <p>(1) <b>All scope chains begin with the global object</b>.</p> <p>(2) <b>When a function is defined, a scope chain for that function is stored</b>.</p> <p>(3) <b>When a function is invoked, it creates a new object and adds that to the scope chain.</b></p> <p>Now, please look at the following example:</p> <pre><code>function counter () { // define counter var count = 0; return function () { return count + 1;}; // define anonymous function }; var count = counter(); // invoke counter </code></pre> <p>In this example, when <code>counter()</code> is defined, the scope chain for counter looks like this: &lt;>global object&lt;>. Then, when <code>counter()</code> is invoked, the scope chain looks like this: &lt;>global object&lt;>counter object&lt;>. After that, the function with no name (called an anonymous function) inside counter is defined and invoked. The scope chain for the anonymous function once invoked looks like this: &lt;>global object&lt;>counter object&lt;>anonymous function object&lt;></p> <p>Heres were the closure part comes in. If you notice, the anonymous function is using the variable <code>count</code> which was defined outside of it. The reason is because <b> the anonymous function can access any variables defined in its scope chain</b>. This is what a closure is, a function along with references to any variables in its stored scope chain.</p> <p>However, in the above example, once the functions return, the objects created at invocation are discarded so there really is no point. Now look at the following:</p> <pre><code>function counter () { // define counter var count = 0; function f() { return count + 1;}; // define f return f; // return f }; var count = counter(); // invoke counter </code></pre> <p>In this example, I am returning a function named <code>f</code> and assign that to the variable <code>count</code>. Now the variable <code>count</code> holds a reference to the entire scope chain and it does not get discarded. In other words the variable count stores the scope chain like this: &lt;>global object&lt;>counter object&lt;>anonymous function object&lt;>. This is the power of closures, you can hold a reference to a scope chain, and call it like this: <code>count()</code>.</p>
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    1. POJavascript closures and side effects in plain English? (separately)
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    1. CO**The closure portion of this answer is incorrect**. JavaScript has *static scope*, which means that the scope chain of a function is defined at function *definition*, **not** function *invocation*. The example works because the anonymous function is defined each time `counter` is invoked, meaning that each returned function will have a scope chain with a different *activation object* for `counter`, and thus a different set of local variables (such as `count`). [See this answer for more info.](http://stackoverflow.com/questions/7721200/using-javascript-closures-in-settimeout/7722057#7722057)
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    2. CO@josh3736: Thanks for the comment. I changed my answer, please advise if anything is wrong. Also, I wanted to comment that, for a global function declaration, the function will only have a reference to a scope chain that contains the global object. Not until you invoke it will it have a reference to the activation object.
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