Question

What is JavaScript garbage collection? What’s important for a web programmer to understand about JavaScript garbage collection, in order to write better code?

Answer 1

Eric Lippert wrote a detailed blog post about this subject a while back (additionally comparing it to VBScript). More accurately, he wrote about JScript, which is Microsoft’s own implementation of ECMAScript, although very similar to JavaScript. I would imagine that you can assume the vast majority of behaviour would be the same for the JavaScript engine of Internet Explorer. Of course, the implementation will vary from browser to browser, though I suspect you could take a number of the common principles and apply them to other browsers.

Quoted from that page:

JScript uses a nongenerational
mark-and-sweep garbage collector. It
works like this:

• Every variable which is “in scope”
  is called a “scavenger”. A scavenger
  may refer to a number, an object, a
  string, whatever. We maintain a list
  of scavengers — variables are moved
  on to the scav list when they come
  into scope and off the scav list when
  they go out of scope.

• Every now and then the garbage
  collector runs. First it puts a
  “mark” on every object, variable,
  string, etc – all the memory tracked
  by the GC. (JScript uses the VARIANT
  data structure internally and there
  are plenty of extra unused bits in
  that structure, so we just set one of
  them.)

• Second, it clears the mark on the
  scavengers and the transitive closure
  of scavenger references. So if a
  scavenger object references a
  nonscavenger object then we clear the
  bits on the nonscavenger, and on
  everything that it refers to. (I am
  using the word “closure” in a
  different sense than in my earlier
  post.)

• At this point we know that all the
  memory still marked is allocated
  memory which cannot be reached by any
  path from any in-scope variable. All
  of those objects are instructed to
  tear themselves down, which destroys
  any circular references.

The main purpose of garbage collection is to allow the programmer not to worry about memory management of the objects they create and use, though of course there’s no avoiding it sometimes – it is always beneficial to have at least a rough idea of how garbage collection works.

Historical note: an earlier revision of the answer had an incorrect reference to the delete operator. In JavaScript the delete operator removes a property from an object, and is wholly different to delete in C/C++.