<Memory leak patterns in JavaScript ⁄ >

JavaScript is a garbage collected language, meaning that memory is allocated to objects upon their creation and reclaimed by the browser when there are no more references to them. While there is nothing wrong with JavaScript's garbage collection mechanism, it is at odds with the way some browsers handle the allocation and recovery of memory for DOM objects.

Internet Explorer and Mozilla Firefox are two browsers that use reference counting to handle memory for DOM objects. In a reference counting system, each object referenced maintains a count of how many objects are referencing it. If the count becomes zero, the object is destroyed and the memory is returned to the heap. Although this solution is generally very efficient, it has a blind spot when it comes to circular (or cyclic) references.

What's wrong with circular references?

A circular reference is formed when two objects reference each other, giving each object a reference count of 1. In a purely garbage collected system, a circular reference is not a problem: If neither of the objects involved is referenced by any other object, then both are garbage collected. In a reference counting system, however, neither of the objects can be destroyed, because the reference count never reaches zero. In a hybrid system, where both garbage collection and reference counting are being used, leaks occur because the system fails to identify a circular reference. In this case, neither the DOM object nor the JavaScript object is destroyed. Listing 1 shows a circular reference between a JavaScript object and a DOM object.



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