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copied!<p>On the most basic level, the <code>return</code> keyword defines what a function should return. So if I have this function:</p> <pre><code>function foo() { return 4; } </code></pre> <p>And then call it:</p> <pre><code>var bar = foo(); </code></pre> <p><code>foo()</code> will return <code>4</code>, hence now the value of <code>bar</code> is also <code>4</code>.</p> <p>Onto your specific example:</p> <p>In this use case the <code>return</code> is used to basically limit outside access to variables inside the <code>hash</code> variable.</p> <p>Any function written like so:</p> <pre><code>(function() {...})(); </code></pre> <p>Is self-invoking, which means it runs immediately. By setting the value of <code>hash</code> to a self-invoking function, it means that code gets run as soon as it can.</p> <p>That function then returns the following:</p> <pre><code>return { contains: function(key) { return keys[key] === true; }, add: function(key) { if (keys[key] !== true){ keys[key] = true; } } }; </code></pre> <p>This means we have access to both the <code>contains</code> and <code>add</code> function like so:</p> <pre><code>hash.contains(key); hash.add(key); </code></pre> <p>Inside <code>hash</code>, there is also a variable <code>keys</code> but this is not returned by the self-invoking function that <code>hash</code> is set to, hence we cannot access <code>key</code> outside of <code>hash</code>, so this wouldn't work:</p> <pre><code>hash.keys //would give undefined </code></pre> <p>It's essentially a way of structuring code that can be used to create private variables through the use of JavaScript closures. Have a look at this post for more information: <a href="http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-private-variables-in-javascript/" rel="noreferrer">http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-private-variables-in-javascript/</a></p> <p>Hope this Helps :)</p> <p>Jack.</p>

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