在声明函数之前定义了javascript函数的typeof？

Question

Could somebody please explain or link to a resource that will tell me why :

<script type=" type="text/javascript">

    if(typeof window.myfunc == "function"){
        alert("Why does myfunc already exist ?");
    }

    function myfunc(){

    }
</script>

will pop up an alert while the myfunc function has not been defined yet ?

I think I found an issue in Chrome, Safari and IE (not FF) which is linked to this behavior. It keeps me from extending the prototype of a function when the js file that contains the function is included more than once in a web page. I'd like to know more about this before calling it a bug and reporting it.

Thank you !

Answer 1

Named function declarations, including the function body, get hoisted to the top of the scope in JavaScript. I'd recommend reading this article about JavaScript scoping and hoisting .

If you did something like this, where you assigned the function to a named variable, only the variable declaration would be hoisted, but it wouldn't have a value until the assignment actually took place:

if (typeof myFunc == 'function') {
    // will not be reached
}

var myFunc = function() { ... }

The above effectively gets treated as:

var myFunc; // myFunc is undefined

if (typeof myFunc == 'function') {
    // will not be reached
}

myFunc = function() { ... }

Answer 2

One word: hoisting

A quote from JavaScript Garden:

"The above function gets hoisted before the execution of the program starts; thus, it is available everywhere in the scope it was defined in, even if called before the actual definition in the source ."

More info here: http://bonsaiden.github.com/JavaScript-Garden/#function.general