Declare vars inside javascript function and memory management

Question

For some time I've had a doubt about js potential memory leak in this type code:

function foo() {

    var a = "This is my content";

    $('#myElemId').on('click', function() {

        $(this).html(a);

    });
}

My question is:

When foo is called, I suppose it is created an execution object, allocates memory for var a and assigns an event listener to a dom element. Once foo returns it should free the execution object but I think it won't because there is still a reference to var a from the click listener, right?

Naeem Shaikh · Accepted Answer · 2015-01-05 09:29:39Z

1

you are right. variables will be only freed if it is no longer needed.

In your case the variable a is still needed for the event callback so it is not deallocated. It still exists,

As another answer for this post by elaijuh says,It is a copy of real variable, actually it is not the copy stored in the event callback function.

You can see this fiddle http://jsfiddle.net/3y7qbjav/ , so still you can change the value of var a after binding the click event. So the variable is not freed.

function foo() {

    var a = "This is my content";

    $('#myElemId').on('click', function() {

        $(this).html(a);

    });
    a="new content after the event binding";
}

edited Jan 5, 2015 at 9:29

answered Jan 5, 2015 at 9:21

Naeem Shaikh

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3 Comments

hjl Over a year ago

pls check my words again, im refering to VariableEnvironment which is ECMA-5's concept, that' absolutely not variables... a is changing all the way in foo's execution and update the LexicalEnvironment. but once foo ends execution, a is deleted but another reference is pointing to the same address.

ramaralo Over a year ago

Thanks Naeem, but my question was more about what happens to execution context object, AKA activation object I believe, after foo() returns. Will it be freed from memory?

Naeem Shaikh Over a year ago

Noo.. It will not be freed as it is still needed

hjl · Accepted Answer · 2015-01-05 09:20:52Z

1

it's a typical closure issue. actually the anonymous function will keep a copy of foo's VariableEnvironment but not a. so when foo ennds execution, it's execution context is destroyed as well as a. the anonymous function can still refer to a via it's scope chain.

answered Jan 5, 2015 at 9:20

hjl

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3 Comments

Naeem Shaikh Over a year ago

doesn't really keep the copy of variable!

bitten Over a year ago

Could you call a = null at the closing of foo?

hjl Over a year ago

@NaeemShaikh, you better check the idea of VariableEnvironment and LexicalEnvironment from ECMA. here is the link: ecma-international.org/ecma-262/5.1/#sec-10.3

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