Do you know what may cause memory leaks in JavaScript?

Question

Do you know what may cause memory leaks in JavaScript? I am interested in browsers: IE 7, FireFox 3, Safari 3

Answer 1

Here is a classic memory leak in IE:-

function body_onload()
{
    var elem = document.getElementById('someElementId');
    // do stuff with elem
    elem.onclick = function() {
        //Some code that doesn't need the elem variable
    }
 }

After this code has run there is circular reference because an element has a function assigned its onclick event which references a scope object which in turn holds a reference to element.

someElement->onclick->function-scope->elem->someElement

In IE DOM elements are COM based reference counting objects that the Javascript GC can't cleanup.

The addition of a final line in the above code would clean it up:-

var elem = null;