Since strings, objects, and arrays do not have a fixed size, storage for them must be allocated dynamically, when the size is known. Every time a JavaScript program creates a string, array, or object, the interpreter must allocate memory to store that entity. Whenever memory is dynamically allocated like this, it must eventually be freed up for reuse, or the JavaScript interpreter will use up all the available memory on the system and crash.

In languages like C and C++, memory must be freed manually. It is the programmer's responsibility to keep track of all the objects that are created and to destroy them (freeing their memory) when they are no longer needed. This can be an onerous task and is often the source of bugs.

Instead of requiring manual deallocation, JavaScript relies on a technique called garbage collection. The JavaScript interpreter is able to detect when an object will never again be used by the program. When it determines that an object is unreachable (i.e., there is no longer any way to refer to it using the variables in the program), it knows that the object is no longer needed and its memory can be reclaimed. Consider the following lines of code, for example:

var s = "hello";            // Allocate memory for a string
var u = s.toUpperCase( );  // Create a new string
s = u;                      // Overwrite reference to original string

After this code runs, the original string "hello" is no longer reachable -- there are no references to it in any variables in the program. The system detects this fact and frees up its storage space for reuse.