JavaScript, Garbage collection

Memory management in JavaScript is performed automatically and invisibly to us. We create primitives, objects, functions… All that takes memory.

What happens when something is not needed any more? How does the JavaScript engine discover it and clean it up?

Reachabilty

The main concept of memory management in JavaScript is reachability.

Simply put, “reachable” values are those that are accessible or usable somehow. They are guaranteed to be stored in memory.

1. There’s a base set of inherently reachable values, that cannot be deleted for obvious reasons. For instance:

: The currently executing function, its local variables and parameters.
: Other functions on the current chain of nested calls, their local variables and parameters.
: Global variables.
(there are some other, internal ones as well)

These values are called roots.

1. Any other value is considered reachable if it’s reachable from a root by a reference or by a chain of references.

For instance, if there’s an object in a global variable, and that object has a property referencing another object, that object is considered reachable. And those that it references are also reachable. Detailed examples to follow.