What is a Garbage Collection

| 4 min read
system-design

What is a Garbage Collection?

Managing memory is an essential part of software development. When you write code, your program needs to allocate memory to store variables, objects, and functions. However, once the program no longer needs this memory, it must be released to ensure efficient use of resources. This process of reclaiming unused memory is called garbage collection.

In some programming languages like C and C++, developers must manually allocate and deallocate memory, which can be error-prone and cause issues like memory leaks. However, many modern languages, including JavaScript, automate this process, making it easier for developers to manage memory effectively.

Understanding Memory Management

Memory management typically involves three steps:

  • Memory Allocation: When you create variables, objects, or functions, memory is allocated to store them.
  • Memory Usage: This memory remains allocated as long as the program references the data it holds.
  • Memory Deallocation: Once the program no longer needs the data, the memory should be released for reuse.

Failure to release memory that’s no longer needed can lead to memory leaks, causing your program to consume excessive resources over time.

Garbage Collection in JavaScript

Garbage collection (GC) in JavaScript is the automated process of identifying and reclaiming memory that’s no longer in use. The JavaScript engine determines which pieces of memory are no longer reachable and frees them, ensuring efficient use of resources without requiring manual intervention.

Reachability and Reference Graph

The core concept behind garbage collection in JavaScript is reachability. An object is considered reachable if it can be accessed in some way:

  • The root of reachability is the Global Object (e.g., window in browsers or global in Node.js).
  • Any objects directly or indirectly referenced by global variables, the call stack, or closures are considered reachable.
  • Objects that are not reachable from the root or its descendants are considered unreachable and eligible for garbage collection.

Mark-and-Sweep Algorithm

Most JavaScript engines, such as V8 (used in Chrome and Node.js), use the mark-and-sweep algorithm for garbage collection:

  • Mark Phase: The garbage collector starts at the root and marks all objects reachable from the root as “live.”
  • Sweep Phase: Objects that are not marked as live are considered unreachable and are deallocated, freeing their memory.

This approach ensures unused memory is reclaimed while the program continues to run.

Generational Garbage Collection

Modern JavaScript engines optimize garbage collection by using generational garbage collection:

  • Young Generation: Short-lived objects, such as temporary variables, are collected frequently.
  • Old Generation: Long-lived objects are moved to the old generation space and are collected less frequently.

This optimization reduces the overhead of garbage collection by focusing more on memory areas with high turnover.

Common Memory Management Pitfalls

Even with automated garbage collection, developers can still encounter memory-related issues. Here are some common pitfalls:

Memory Leaks

  • Global Variables: Variables unintentionally declared globally can persist throughout the program’s execution.
  • Detached DOM Elements: Removing a DOM element without nullifying references to it can prevent garbage collection.
  • Circular References: Two objects referencing each other can create a situation where neither is garbage-collected, even if they are no longer reachable from the root.

Preventing Memory Leaks

  • Avoid excessive use of global variables.
  • Use tools like Chrome’s DevTools or Node.js’s -inspect flag to monitor memory usage.
  • Set references to null when an object is no longer needed.

Recap

Garbage collection in JavaScript simplifies memory management, allowing developers to focus on writing code without worrying about allocating and freeing memory manually. However, understanding how it works and being mindful of potential pitfalls is essential to ensure your applications run efficiently.

Resources

Wiki Garbage collection (computer science)

MDN - Memory management

Wiki - Tracing garbage collection (Mark and Sweep)

JavaScript Info Garbage collection

Thank you!

Thank you for your time and for reading this!