Why does Google claim that Javascript closures are dangerous?

Question

I just read this point of Google JS style guide: http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml?showone=Closures#Closures

and I do not fully understand it. While element.onclick has function assigned to it everything is fine. Nothing is garbage collected, but that's ok because we are still using the objects.

Once we assign

element.onclick = null; 

Then closure will be garbage collected as element is no longer pointing to it. Where's the problem?

Answer 1

The point they are making is that in this code:

function foo(element, a, b) {
   element.onclick = function() { /* uses a and b */ };
}

element keeps a reference to the closure, and the closure a reference to element.

Now if you set element to null, you would normally expect it to be garbage collected since it seems nothing's referencing it anymore. However that won't happen since the closure is actually still referencing it, so that would create a memory leak.

The solution, as mentioned in the style guide, is to make sure the closure does not keep a reference to element.