JavaScript block scope vs function

Question

Are following snippets exactly equal? If no what is the deference?

var x = (function() {
    ... //a
    return function(){
        ... //b
    };
})();

vs.

var x;
{
    ... //a
    x = function(){
        ... //b
    };
}

@JohnS please don't paste code into the title like that. Post your code into the body. — JohnP
– JohnP, Commented Apr 30, 2011 at 11:23
@JohnP You are right but your title is too generic, it's like "What's the problem with my code?" — Ali Shakiba
– Ali Shakiba, Commented Apr 30, 2011 at 11:24
@JohnS but that is what you're asking. Pasting the code into the title does not make it readable. Feel free to come up with a better problem statement for your question — JohnP
– JohnP, Commented Apr 30, 2011 at 11:25
@JohnP Have you seen: stackoverflow.com/questions/336859/… — Ali Shakiba
– Ali Shakiba, Commented Apr 30, 2011 at 11:31
@JohnS That's 3 years old. I've only been active for a couple of months. Also that question title is much more readable than what you had put up. Like I said before, please do update your question if you have a better title :) It just needs to be clear and readable — JohnP
– JohnP, Commented Apr 30, 2011 at 11:37

phihag · Accepted Answer · 2011-04-30 11:29:25Z

6

There is a major difference: In JavaScript, blocks don't induce a new variable scope. Therefore, you can't define private variables in the // a code block. Compare

var x = (function() {
    var v = 42;
    return function(){
        return v;
    };
})();
// v; would yield ReferenceError: v is not defined, so you need to call x

and

var x;
{
    var v = 42;
    x = function(){
        return v;
    };
}
// v is 42 here, that's not what's intended.

answered Apr 30, 2011 at 11:29

phihag

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Comments

neeebzz · Accepted Answer · 2011-04-30 11:22:32Z

0

One major difference is that at the time of executing ...//a , x doesn't exist. Now in your case, in both cases it is undefined but generally speaking it's possible to access x variable during ...//a while in the first case it's not.

Otherwise in your circumstances it's pretty same. After all in your case the code is basically refactored into a separate function just like in any other language.

answered Apr 30, 2011 at 11:22

neeebzz

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1 Comment

Dan Manastireanu Over a year ago

One more thing: {var z = 1;} will create a variable z in the window namespace, while (function(){var z = 2;}()) will not pollute the global namespace. That's why the function option is preferred

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