Javascript parse/evaluation order?

This is probably a nub question, but I don't understand why this works:

<script type="text/javascript">
    alert(foo);
    function foo() { }
</script>

This alerts "function foo() { }", but I expected开发者_如何学Python the alert to be evaluated before the function foo was defined. Can someone explain what I don't understand about parse/evaluation order or point me to a resource that does?

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JavaScript, like PHP, tracks top-level function declarations before the code runs. However, you can bypass the auto-function by using assignments:

var a = function a() { }

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A must read about the types of function definitions in JavaScript.

Named Function Expressions Demystified

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Function declarations are hoisted to the top, and therefore declared first and foremost.

You can change this behavior by assigning them to a variable like so

var a = function() {
   // do it
};

This assigns the variable a to an anonymous function.