I'm wondering if it is more efficient to place any vars referenced within a loop, outside of the loop - or can they get garbage collected like vars inside of a function?

var obj = {key:'val'};
for(var i=0; i<10; i++){
    console.log(obj);
}

or

for(var i=0; i<10; i++){
    var obj = {key:'val'};
    console.log(obj);
}

I tried to run some memory test in my browser's profiler but still couldn't tell which method was better.

I'm wondering if it is more efficient to place any vars referenced within a loop, outside of the loop - or can they get garbage collected like vars inside of a function?

var obj = {key:'val'};
for(var i=0; i<10; i++){
    console.log(obj);
}

or

for(var i=0; i<10; i++){
    var obj = {key:'val'};
    console.log(obj);
}

I tried to run some memory test in my browser's profiler but still couldn't tell which method was better.

• javascript
• performance
• garbage-collection

Share Improve this question Follow asked Oct 11, 2011 at 2:16 George HessGeorge Hess 68911 gold badge88 silver badges1515 bronze badges

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3 Answers 3

Sorted by: Reset to default 6

var is function scoped, not blocked scoped, so it does not matter whether they appear inside the loop or not. What is the scope of variables in JavaScript? explains this distinction.

The next version of JavaScript will have let-scoped variables and the value stored in those would bee collectible at the end of a loop body run if declared inside the loop.

Neither will get garbage collected until the variables go out of scope. Scope in Javascript is introduced by functions. A loop construct has no influence on scope whatsoever.

As far as garbage collection, what the other answers say should be true, the browser handles the garbage collection and it doesn't matter if the variable is declared internally, or externally, to a loop.

As for efficiency, your code would be a little more optimized to declare the variable prior to the loop.