What do JavaScript objects outlast and when are they destroyed?

Question

I'm quite new to Javascript and I was just reading following article.

you can define an ajax connection once, and reuse it multiple times, and start and stop it later on. Here's an example:

var myAjaxRequest = A.io.request('test.html', {
    method: 'POST',
    data: {
      key1: 'value1'
    }
});

Now later on, if I want to make that same ajax call again, all I have to do is call:

myAjaxRequest.start();

What if I had a very frequently used auction page and I wanted to use the myAjaxRequest connection for all actions a user does from his browser. What are the rules for lifetime of the myAjaxRequest instance ? I suppose it is destroyed on page refresh. But is it anything else that destroys it ? Let say that the object is created within YUI sandbox, but it doesn't matter.

No. If you don't change the `myAjaxRequest` object manually, it will be available for the lifetime of the document. (assuming that the variable is a global variable) — Šime Vidas, Dec 12 '10 at 14:36
Set myAjaxRequest = null. It should do gc sometime depends on browser. — WaiLam, Dec 12 '10 at 14:38
And what about time, browser memory management or something. I think in this case the variable is within a scope of a callback function of YUI sandbox. It loads an instance of a module for this variable. But let suppose it's a global variable.. — lisak, Dec 12 '10 at 14:40
You can read this link for more info about gc in javascript. http://stackoverflow.com/questions/864516/what-is-javascript-garbage-collection — WaiLam, Dec 12 '10 at 14:43
So why aren't any of these _answers_ actual answers instead of just comments? :P — Jani Hartikainen, Dec 12 '10 at 15:13

score 2 · Accepted Answer · edited May 23 '17 at 12:04

Its a shame this was answered in comments because nobody gets closure (sorry, terrible pun). @Šime Vidas and @WaiLam deserve the credit but I will at least attempt to craft an answer:

While you have a reference to the object (though the variable myAjaxRequest) it will remain in memory until the document is unloaded. If you assign null to your variable (myAjaxRequest = null), and there are no other references to the object, then the garbage collector will reclaim the memory used to store it.

A reference can exist even if myAjaxRequest is a local variable within a function. The function can return a reference to the local variable, for example as a object property e.g:

function sandbox () {
    var myAjaxRequest = A.io.request(/* constructor... */);

    return {
        myRequest: myAjaxRequest
    };
}

var mySandbox = sandbox();
mySandbox.myRequest.start();

or it can return a reference through a closure (excellent explanation here), e.g:

function sandbox () {
    var myAjaxRequest = A.io.request(/* constructor... */);

    return {
        getRequest: function () {
            return myAjaxRequest;
        } 
    };
}

var mySandbox = sandbox();
mySandbox.getRequest().start();

As long as you have a reference to your object it will not be garbage collected. You can safely call the start method until the page is unloaded.

What do JavaScript objects outlast and when are they destroyed?

1 Answers1