Does following code cause memory leak in JavaScript?

Question

Following is part of very big logic, function track is just for simplification. Track loads object from database and stores in cache.

var cache = []; function newObject(a){ var b = {}; b.tracker = a; cache.push(b); } var t = {}; track(t); track(t); track(t); track(t); ... cache.length = 0; track(t); track(t); track(t); track(t); .... cache.length = 0;

After clearing cache, does it cause memory leak because b.tracker is holding reference to t? For clarification, tracker t does not store any reference to any object created.

Do I need following method?

for(var i=0;i<cache.length;i++){ cache[i].tracker = null; } cache.length = 0;

Or JavaScript engine is smart enough to remove all instance of b because no body references b anymore?

Not the downvoter, but I can only assume you're getting downvotes because you haven't indicated if you've even tested the code yet or not to see if it causes memory leaks. You're effectively saying, "Here's some code, test it for me." — LittleBobbyTables - Au Revoir
– LittleBobbyTables - Au Revoir, Commented May 6, 2013 at 12:25
Well there is no way to test this, I have created 1000s of objects but objects are of hardly any size to see any noticeable difference in memory management of chrome. — Akash Kava
– Akash Kava, Commented May 6, 2013 at 12:26

flavian · Accepted Answer · 2013-05-09 16:22:28Z

When a variable goes out of scope, it gets garbage collected. Now to remove that cache, you would have three ways:

delete cache; // which is not possible because it's defined with var. cache = []; cache.length = 0;

For the last one, cache.length = 0; I noticed the effect is the desired one. It results in n delete operations of the type delete cache[i]. As soon as the delete operation happens, the value held in cache[i] is de-referenced, goes out of scope, and the garbage collector kicks in.

So yes, the JavaScript engine is smart enough to do that. I tested with 4 sets of profiling tools(Chrome, Safari, Firefox and Opera). I lack the extreme patience required to own a Windows machine, so I couldn't test in Internet Explorer.

Update

According to @Renan the above works just as well in Internet Explorer, so cache.length = 0 is definitely the way to go.

It works just as well in IE 9+. I loath any and all who use older versions (it's already in 10 FFS), though I have a hunch that it would work in IE8 as well.

Geeky Guy · Accepted Answer · 2013-05-06 12:25:53Z

Javascript should reclaim the memory occupied by your t if that's all you're doing. You don't have to set an array item to null if you're setting the array's length to zero.

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