JavaScript block scope vs function - Stack Overflow

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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
    };
}

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
    };
}

• javascript

Share Improve this question Follow edited Jan 25, 2016 at 6:56 Ali Shakiba asked Apr 30, 2011 at 11:15 Ali ShakibaAli Shakiba 21.3k1818 gold badges6565 silver badges9090 bronze badges 7

• @JohnS please don't paste code into the title like that. Post your code into the body. – 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 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 e up with a better problem statement for your question – JohnP Commented Apr 30, 2011 at 11:25
• @JohnP Have you seen: stackoverflow./questions/336859/… – 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 Commented Apr 30, 2011 at 11:37

 |  Show 2 more ments

2 Answers 2

Sorted by: Reset to default 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.

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.

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
    };
}

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
    };
}

• javascript

Share Improve this question Follow edited Jan 25, 2016 at 6:56 Ali Shakiba asked Apr 30, 2011 at 11:15 Ali ShakibaAli Shakiba 21.3k1818 gold badges6565 silver badges9090 bronze badges 7

• @JohnS please don't paste code into the title like that. Post your code into the body. – 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 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 e up with a better problem statement for your question – JohnP Commented Apr 30, 2011 at 11:25
• @JohnP Have you seen: stackoverflow./questions/336859/… – 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 Commented Apr 30, 2011 at 11:37

 |  Show 2 more ments

2 Answers 2

Sorted by: Reset to default 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.

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.

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