JavaScript - Garbage Collection Algorithms

Garbage collection algorithms in JavaScript define how the runtime automatically identifies and removes memory that is no longer needed by a program. Since JavaScript manages memory automatically, developers do not manually free memory when objects or variables are no longer required. Instead, the runtime continuously tracks memory usage and applies garbage collection techniques to reclaim unused space, helping programs run efficiently and safely.

One of the most fundamental approaches used in JavaScript garbage collection is reachability analysis. In this method, the runtime determines which values can still be accessed by the program. Objects that are reachable from active execution contexts, global variables, or ongoing function calls are considered in use. Any data that cannot be reached through these references is marked as unused and becomes eligible for removal.

A commonly used algorithm based on reachability is the mark-and-sweep approach. During the marking phase, the runtime starts from known root references and marks all reachable objects. After this marking process, the sweep phase begins, where all unmarked objects are removed from memory. This method is effective because it avoids deleting objects that are still indirectly referenced through complex relationships.

Modern JavaScript engines also use generational garbage collection strategies. Memory is divided into regions based on the expected lifetime of objects. Newly created objects are placed in a young generation area, where collection happens frequently because many objects become unused quickly. Objects that survive multiple collection cycles are moved to an older generation, where garbage collection runs less often to improve performance.

Another important aspect of garbage collection algorithms is incremental and concurrent collection. Instead of stopping program execution entirely during cleanup, the runtime performs garbage collection work in small steps or alongside normal execution. This reduces noticeable pauses and improves responsiveness, especially in applications with real-time interaction such as web interfaces.

Garbage collection algorithms also account for special cases like circular references. In JavaScript, two or more objects may reference each other but still be unreachable from the rest of the program. Modern algorithms are designed to detect and remove such cycles, ensuring that memory is reclaimed even when objects reference each other internally.