JavaScript: The Definitive Guide, Fourth Edition

Garbage Collection

As explained in Chapter 4, JavaScript uses garbage collection to reclaim the memory occupied by strings, objects, arrays, and functions that are no longer in use. This frees you, the programmer, from having to explicitly deallocate memory yourself and is an important part of what makes JavaScript programming easier than, say, C programming.

A key feature of garbage collection is that the garbage collector must be able to determine when it is safe to reclaim memory. Obviously, it must never reclaim values that are still in use and should collect only values that are no longer reachable; that is, values that cannot be referred to through any of the variables, object properties, or array elements in the program. If you are the curious type, you may be wondering just how a garbage collector distinguishes between garbage to be collected and values that are still being used or that could potentially be used. The following sections explain some of the gory details.

Mark-and-Sweep Garbage Collection

The computer science literature on garbage collection is large and technical; the actual operation of the garbage collector is really an implementation-specific detail that may vary in different implementations of the language. Still, almost all serious garbage collectors use some variation on a basic garbage-collection algorithm known as “mark and sweep.”

A mark-and-sweep garbage collector periodically traverses the list of all variables in the JavaScript environment ...