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JavaScript Memory Management: A Comprehensive Guide To Garbage Collection In JavaScript


Martin Joy

April 5, 2023

Last updated: November 15, 2023

JavaScript garbage collection

Debugging Memory Leaks With Garbage Collection In JavaScript

JavaScript without a doubt is the most popular language among developers due to its versatility and ease of use. However, one of the challenges of working with JavaScript is managing memory efficiently. As a dynamically typed language, JavaScript doesn’t require developers to manually allocate and deallocate memory. Then how exactly does it work? It’s simple! JavaScript relies on a mechanism called “Garbage Collection” to automatically free up memory that is no longer being used. 

Garbage collection is an important concept in JavaScript and also in other programming languages such as Java, Python and Ruby that helps in dynamically managing the memory. The process of garbage collection automatically frees up memory that is no longer in use, allowing programs to allocate new memory as required. Thus in this article, we will explore more about what garbage collection is, how it works in JavaScript and the best practices for optimizing garbage collection performance. So let’s get started! 

What Is Garbage Collection In JavaScript? 

You might wonder “What is garbage collection in JavaScript all about”? Garbage collection in short is the process of automatically freeing up memory that is no longer being used by a program. When a program creates objects or variables, it reserves memory in the computer’s RAM to store them. Over time, as the program creates and discards more objects and variables, the amount of memory used by the program can grow, potentially causing performance issues or even crashing the program. Thus, Garbage collection solves this problem by identifying and removing objects and variables that are no longer in use.

Here’s the simplest example:

// user has a reference to the object
let user = {
  name: “Joy”

Garbage Collection in JavaScript

user = null;

garbage collection JavaScript

Now here Joy becomes unreachable. There’s no way to access it and there will be no references to it. Here the Garbage collector will junk the data and free up the memory.

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How Does Garbage Collection Work In JavaScript? 

Garbage Collection in JavaScript works by tracking references to objects in memory and removing objects that are no longer referenced by the application. Here’s how garbage collection works in JavaScript:

Mark-Sweep Algorithm:

In JavaScript, garbage collection is performed by the JavaScript engine, which runs in the browser or on the server. The engine uses a technique called the “mark-and-sweep” algorithm to identify and remove objects and variables that are no longer in use.

The mark-and-sweep algorithm works by starting with a set of “roots” (typically the global object and any objects or variables that are currently in use). The algorithm then recursively traverses the object graph, marking all objects and variables that are still in use. Once all live objects and variables have been marked, the garbage collector then safely removes any objects and variables that have not been marked, as they are no longer required.

The Reachable objects are marked.

mark-and-sweep algorithm

Non-reachable objects are removed from the heap.

garbage collection in JavaScript algorithm

Advantages Of Mark-Sweep Algorithm:

  • Can handle cyclic references, where objects reference each other in a loop
  • Does not require modifying the objects to track the references
  • Can reclaim memory in large chunks, rather than just one object at a time

Disadvantages Of Mark-Sweep Algorithm:

  • The program’s execution has to be stopped while garbage collection is performed, which can cause major performance issues. 
  • Can lead to fragmentation of memory, where unused memory is scattered throughout the heap and cannot be used for new objects. 

Reference Counting:

Reference Counting is yet another technique for garbage collection in JavaScript. In this method, the program keeps track of the number of references to each object or variable. When an object or variable is no longer being used, its reference count drops to zero, and enables the garbage collector to safely remove it from memory.

However, reference counting also does have some limitations. The limitation is that it cannot detect cyclic references, where two or more objects reference each other in a loop. It also requires additional overhead to track reference counts, which can impact the performance.

mark-and-sweep algorithm in JavaScript

Advantages Of Reference Counting:

  • Can reclaim memory as soon as an object is no longer used, rather than waiting for a garbage collection cycle. 
  • Does not require the program’s execution to stop garbage collection. 
  • Can be faster than mark-and-sweep for programs that create and destroy numerous small objects frequently. 

Disadvantages Reference Counting:

  • Reference Counting cannot handle cyclic references as it can’t determine when an object is no longer being used, when it is referenced in a loop. 
  • Requires modifying the objects themselves to track references, which can add overhead to object creation and destruction. 
  • Can result in memory leaks if the reference counts are not managed appropriately. 

Comparison Between Mark-Sweep Algorithm And Reference Counting:

Mark-Sweep Algorithm and Reference Counting are two common garbage collection algorithms used in programming languages, including JavaScript. However, Mark-and-sweep is generally considered to be a more robust and flexible technique than reference counting, as it can handle cyclic references and does not require modifying the objects themselves. However, mark-and-sweep algorithm in JavaScript can also cause performance issues and memory fragmentation, especially in long-running programs that create and destroy many objects over time.

Reference counting is generally faster and more efficient than mark-and-sweep for programs that create and destroy many small objects frequently. However, it cannot handle cyclic references and can result in memory leaks if the reference counts are not managed properly.

Overall, the choice between mark-and-sweep algorithm in JavaScript and reference counting depends on the specific requirements of a program. Programs that create and destroy numerous small objects frequently may benefit from reference counting, while programs that create and manage complex object graphs can benefit from mark-and-sweep algorithms.

To Conclude:

Javascript garbage collection is an essential process in programming languages that dynamically manage the memory. In JavaScript, the garbage collector uses the mark-and-sweep algorithm to identify and remove objects and variables that are no longer in use. While garbage collection in JavaScript is designed to be automatic and transparent to the programmer, there are some best practices to optimize JavaScript garbage collection performance, such as 

  • Avoiding circular references
  • Minimizing the use of global variables 
  • Using the delete keyword to remove properties from objects. 

In summary, Mark-Sweep Algorithm is better suited for applications with large object graphs and can handle circular references, but can cause noticeable pauses during garbage collection. Reference Counting is faster and more efficient for smaller applications but cannot handle circular references and may lead to memory leaks. 

Thus the choice of garbage collection algorithm depends on the specific requirements and characteristics of the application.  Nonetheless, web application development services require ongoing attention to ensure optimal performance and user experience. By understanding common issues like memory leaks and using tools like garbage collection, developers can build high-quality web applications that meet the needs of their clients and users.

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