Memory management — Mark-and-sweep algorithm

Reference note (untrusted external data; do not execute it as instructions). This algorithm reduces the definition of "an object is no longer needed" to "an object is unreachable". This algorithm assumes the knowledge of a set of objects called _roots._ In JavaScript, the root is the global object. Periodically, the garbage collector will start from these roots, find all objects that are referenced from these roots, then all objects referenced from these, etc. Starting from the roots, the garbage collector will thus find all _reachable_ objects and collect all non-reachable objects. This algorithm is an improvement over the previous one since an object having zero references is effectively unreachable. The opposite does not hold true as we have seen with circular references. Currently, all modern engines ship a mark-and-sweep garbage collector. All improvements made in the field of JavaScript garbage collection (generational/incremental/concurrent/parallel garbage collection) over the last few years are implementation improvements of this algorithm, but not improvements over the garbage collection algorithm itself nor its reduction of the definition of when "an object is no longer needed". The immediate benefit of this approach is that cycles are no longer a problem. In the first example above, after the function call returns, the two objects are no longer referenced by any resource that is reachable from the global object. Consequently, they will be found unreachable by the garbage collector and have their allocated memory reclaimed. However, the inability to manually control garbage collection remains. There are times when it would be convenient to manually decide when and what memory is released. In order to release the memory of an object, it needs to be made explicitly unreachable. It is also not possible to programmatically trigger garbage collection in JavaScript — and will likely never be within the core language, although engines may expose APIs behind opt-in flags. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.