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copied!<p>Any function in ECMAScript ( core language that JS is based on ) is a separate execution context, and runs individually from one another. Inside of each execution context, <code>this</code> refers to the object in question, defaulting to whatever the function is attached to.</p> <pre><code>function foo() { alert(this===window) } </code></pre> <p>Would alert true, because the window is the object which owns the 'foo' method. Any variables defined in a function become accessed through that function's unique scope chain, environment.</p> <pre><code>function world() { var name = 'global'; alert(name) } </code></pre> <p>would alert 'global' obviously.</p> <pre><code>function world() { var name = 'global'; (function() { var name = 'country'; alert(name) })(); alert(name) } </code></pre> <p>In the latest example, when the first alert is invoked, Javascript determines that in the scope chain of the inner function that the identifier <code>name</code> is defined, so it doesn't have to look up the scope chain to grab it.</p> <p>In the second alert invocation, <code>name</code> is also defined in the same context and alerts 'global';</p> <pre><code>function world() { var name = 'global'; (function() { alert(name) })(); } </code></pre> <p>In this example, the <code>name</code> identifier is not defined in the same context and thus it has to travel up the scope chain to the outer function where name is defined, and it alerts global.</p> <p>Reference:</p> <ul> <li><a href="http://www.digital-web.com/articles/scope_in_javascript/" rel="noreferrer">http://www.digital-web.com/articles/scope_in_javascript/</a></li> <li><a href="http://www.jibbering.com/faq/faq_notes/closures.html" rel="noreferrer">http://www.jibbering.com/faq/faq_notes/closures.html</a></li> </ul>

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