Skip to content

Instantly share code, notes, and snippets.

@branneman
Last active April 13, 2024 05:10
Show Gist options
  • Star 49 You must be signed in to star a gist
  • Fork 13 You must be signed in to fork a gist
  • Save branneman/7fb06d8a74d7e6d4cbcf75c50fec599c to your computer and use it in GitHub Desktop.
Save branneman/7fb06d8a74d7e6d4cbcf75c50fec599c to your computer and use it in GitHub Desktop.
Primitive Types & Reference Types in JavaScript

Primitive Types & Reference Types in JavaScript

An explanation of JavaScript's pass-by-value, which is unlike pass-by-reference from other languages.

Facts

  • JavaScript has 2 kinds of variable types: primitive and reference.
  • A fixed amount of memory is reserved after creation of every variable.
  • When a variable is copied, it's in-memory value is copied.
  • Passing a variable to a function via a call also creates a copy of that variable.

Primitive Types

The in-memory value of a primitive type is it's actual value (e.g. boolean true, number 42). A primitive type can be stored in the fixed amount of memory available.

  • null
  • undefined
  • Boolean
  • Number
  • String

Primitive types are also known as: scalar types or simple types.

Reference Types

A reference type can contain other values. Since the contents of a reference type can not fit in the fixed amount of memory available for a variable, the in-memory value of a reference type is the reference itself (a memory address).

  • Array
  • Object
  • Function

Reference types are also known as: complex types or container types.

Code Examples

Copying a primitive:

var a = 13         // assign `13` to `a`
var b = a          // copy the value of `a` to `b`
b = 37             // assign `37` to `b`
console.log(a)     // => 13

The original was not changed, we can only change the copy.

Copying a reference:

var a = { c: 13 }  // assign the reference of a new object to `a`
var b = a          // copy the reference of the object inside `a` to new variable `b`
b.c = 37           // modify the contents of the object `b` refers to
console.log(a)     // => { c: 37 }

The original was also changed, since the reference got copied.

@katherinedragieva
Copy link

Very helpful! Thanks ๐Ÿ‘

@basantkumarpogeyan
Copy link

basantkumarpogeyan commented Dec 20, 2018

โœŒExcellent explanation

@EugeCos
Copy link

EugeCos commented Feb 22, 2019

Great explanation, thank you.
Wish I knew this before I spent a few hours trying to figure out why my object changes when I change its copy.

@bakhti-uzb
Copy link

Thanks , simple and clear explanation .

@sobhardwaj
Copy link

wonderful article it is....thanks dude

@AlisonYoon
Copy link

very clear, thanks!

@jessonjs
Copy link

Concise, thanks!

@Julitree
Copy link

Clear to understand, Thanks!

@AhmadIbrahiim
Copy link

Thank you

@alfasin
Copy link

alfasin commented Jan 7, 2020

Thanks!

null is technically an object, try: typeof null
Arrays are also of type object.

What about Symbols?

@mshuber1981
Copy link

Thank you, I was having trouble understanding this concept and this explanation really helped.

@SpyrosKo
Copy link

I always have to refresh on what the types are and how they work and this was clear and easy to understand. Thank you.

@msindelic
Copy link

Great explanation.Thank you ๐Ÿ‘๐Ÿ‘

@babak-bekhrad
Copy link

Thank, It was so clear ๐Ÿ‘

@shahalizada
Copy link

VERY HELPFUL, THANK YOU...

@taesubyun
Copy link

That's a good explanation! Thank you!

@JenniMont
Copy link

GOod Explanation!! ;)

@berkesayin
Copy link

That's so resourceful, thank you very much!

@onkar-singh
Copy link

thanks

@NAZIMUDHEEN267
Copy link

Great content ๐Ÿ˜Š๐Ÿ‘

@eyucherin
Copy link

๐Ÿ‘

@AbbosbekDev08
Copy link

๐Ÿ‘

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment