The function definition in Javascript

I highly recommended Douglas Crockford's book "Javascript:The good parts". In the page 133 of the book, the author mentioned 2 ways of defining a function:

• use function statement,
• use function expression

Their results are exactly the same. For example,
Function statement:

	function foo() {
	alert("foo");
	}

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Function expression:

	var foo = function() {
	alert("foo");
	};

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The above two have the exactly same result. they will both define a function foo. But I found there are still one difference: scope.
The function statement is globally scoped, it can be put anywhere in the javascript, and you can call the function before or after it is defined.
For example:

	//call before function foo is defined
	foo(); //=> alert("foo");
	function foo() {
	alert("foo");
	}
	//call after function foo is defined
	foo(); //=> alert("foo");

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But in function expression is different, the function call statement must be after the function is defined, otherwise the function call will be failed.
For example:

	//call before function foo is defined
	foo(); // will fail, because foo() is not defined.
	var foo = function () {
	alert("foo");
	}
	//call after function foo is defined
	foo(); //=> alert("foo");

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When you think a little deeper, the error of function expression make sense, because the function expression means first declare a variable, then assign it with a function object. Why it failed before the function defined is because the variable is not declared and assigned yet.

So if you use the function expression to define a javascript function, you need to aware this issue.