DEV Community

Cover image for Garbage collection in JS
Mohammed Awad
Mohammed Awad

Posted on • Updated on

Garbage collection in JS

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

Reachability

The main concept of memory management in JavaScript is reachability.
There’s a base set of inherently reachable values, that cannot be deleted for obvious reasons.
For instance:

  1. The currently executing function, its local variables and parameters.
  2. Other functions on the current chain of nested calls, their local variables and parameters.
  3. Global variables.

These values are called roots.

Any other value is considered reachable if it’s reachable from a root by a reference or by a chain of references. There’s a background process in the JavaScript engine that is called garbage collector. It monitors all objects and removes those that have become unreachable.

let's dive in and understand how it's work

example

// user has a reference to the object
let user = {
  name: "John"
};
Enter fullscreen mode Exit fullscreen mode

Image describes how Garbage collection work

and If the value of user is overwritten, the reference is lost:

Image describes how Garbage collection clean unreachable elements

Now John becomes unreachable. There’s no way to access it, but if we copied the reference from user to admin and overwritten the value

Then the object is still reachable via admin global variable.

mark-and-sweep

garbage collection algorithm is called “mark-and-sweep”
and these steps are regularly performed:

  1. The garbage collector takes roots and “marks” (remembers) them.
    garbage collector steps

  2. Then it visits and “marks” all references from them.

garbage collector steps

  1. And so on until all reachable references are visited.

garbage collector steps

  1. All objects except marked ones are removed.

garbage collector steps

Summary

  1. Global variables are considered root, so be aware when you declare or reference them.
  2. Outgoing references do not matter. Only incoming ones can make an object reachable.

all my images and content from javascript.info. you can learn more and explore more complex examples on
JavaScript info

Top comments (5)

Collapse
 
tesbalo profile image
Slim

@muhmmadawd

Two things to note here:

  1. The title of your article is a little bit lacking. Maybe Garbage collection in JavaScript next time.
  2. Secondly, you should give attributions to images that you use anywhere. It is obvious that you grabbed the images from javascript.info but you did not credit the owner.

Things like this would likely result in your articles being pushed down.

Good luck!

Collapse
 
xmohammedawad profile image
Mohammed Awad

thanks for your notes, I will mention that now

Collapse
 
mohammedzkullab profile image
mohammed kullab

Great topic 👏 keep it up

Collapse
 
xmohammedawad profile image
Mohammed Awad

glad to hear that. follow me for more.

Collapse
 
xmohammedawad profile image
Mohammed Awad

any suggestions