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Garbage Collection in Javascript

dillionmegida profile image Dillion Megida ・3 min read

This is most probably not the first time you're hearing this term. But in this article, I'd be explaining in simple words, exactly what it means, specifically in Javascript.

So what's garbage collection?

As "garbage collection" literally implies, collecting garbage which refers to irrelevant or unnecessary things.

What is irrelevant in Javascript?


Examine the following code:

let name = "Dillion";
name = "Javascript";

When name was declared, memory space was created to hold the value "Dillion". name is just a label to that memory space. Later on, name was assigned another value, "Javascript", of which another memory space was created for it.

So what happens to "Dillion"?

It is collected as garbage. This can also be seen as a memory management technique, though not 100% effective. Hence, developers should dearly consider memory management during development and not rely on garbage collection.

Processes of Memory Usage

There are three processes involved in memory usage.

1. Memory Initialization

This refers to assigning space for a value.

var variable1 = 54;
var variable2 = {name: "Javascript"}

From the above, memory spaces are assigned for the number type variable1 and the object type (which is a collection of values) variable2.

2. Memory Usage

This refers to any form of usage of those values. For example:

var result = variable1 + 23;

As seen above, variable1 which was initialized is now been used.

3. Garbage Collection

This is the point where a memory space becomes irrelevant and needs to be collected as garbage.

In lower-level languages, developers would have to explicitly release a memory space but in higher-level languages, this is done automatically in the background with the garbage collection feature.

But the question now is, how does Javascript know when a space is irrelevant?

The general way of doing this is by checking if a value is REFERENCED or in another term, REACHABLE. The garbage collector engine checks to see if a memory space is referenced and if it isn't, it collects it as garbage.

For example:

var name = "javascript";
name = "James";
var obj = {
    name: "Object"
var anotherObj = {
    name: "Another object"
var newObj = anotherObj;
anotherObj = 43;
  1. name is declared and a memory space is allocated to the value "javascript". name is reassigned another value - "James". All through the program, there is no reference to "javascript" any longer, so it is collected as garbage.

  2. obj is declared and assigned an object value. The value is always reachable via the obj variable, so it is retained.

  3. anotherObj is declared and assigned an object value. newObj is assigned the reference value of anotherObj (remember how objects works - mutability). anotherObj is assigned a new value - 43. But the object value remains. Why? Because we can still access the object (newObj.name).

Wrap Up

The garbage collection process (which is done automatically and cannot be controlled) is not a 100% method of memory management. Hence, other methods have to be put in place manually.

There are several other algorithms that the garbage collector engine follows to determine if spaces are irrelevant, but the general thing you should note it tries to verify is "Is the memory space reachable or irrelevant?"

Check out these resources for more details:

  1. Garbage Collection | Javascript.info
  2. Memory Management | MDN

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Dillion Megida


Front-end Developer | Technical Writer | Javascript Enthusiast | Graphics Designer


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