Javascript Scope Issue

Question

I'm trying to get this function to work. I think the function is pretty self explanitory:

function FileExists(path){
    var result = false;

    $.ajax({
        url:    "http://mydomain.com/"+path,
        type:   "HEAD",
        success:
                function(){
                    result = true;
                }
    });

    return result;
}

I need the anonymous function that is called upon success of the ajax post to set the variable "result" that was defined inside the FileExists function to true so that I can return that value from the FileExists function. I think I need a closure for this but they confuse the hell out of me.

Please help! Thanks!

Yi Jiang · Accepted Answer · 2011-03-06 05:41:19Z

3

It's not a scoping issue, but rather because $.ajax is asynchronous, meaning that FileExists will return before $.ajax will complete. What you should be doing is to move all code that depends on result to inside the success callback.

answered Mar 6, 2011 at 5:41

Yi Jiang

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2 Comments

philwinkle Over a year ago

But his question about closure brings up a good point, doesn't any assignment without var preceding it assign to global namespace? And if so, his result=true in the anon func assigns to global?

Yi Jiang Over a year ago

@philwinkle No, anon. functions are ran in the scope of the function that creates them. See a simple test case like: jsfiddle.net/txWLj The engine goes up through each scope looking for the variable until it reaches the global scope.

The Scrum Meister · Accepted Answer · 2011-03-06 05:42:02Z

2

Ajax calls are by default asynchronous, you can either use a callback function:

$.ajax({
    url:    "http://mydomain.com/"+path,
    type:   "HEAD",
    success: function(){
        callback(true);
    }
});

Or make the call synchronously.

$.ajax({
    async:   false,
    url:    "http://mydomain.com/"+path,
...

answered Mar 6, 2011 at 5:42

The Scrum Meister

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1 Comment

Yi Jiang Over a year ago

There is almost no reason why one might want to use async: false; its mostly a crutch for lazy programmers, given it's effects.

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