---
title: "Immutable - Glossary | MDN"
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source: "https://developer.mozilla.org/en-US/docs/Glossary/Immutable"
category: "reference"
tags: "web, html, css, javascript, documentation"
date_saved: "2026-05-05T05:35:02.716246+00:00"
instance: "kb-cron"
---

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  1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
  2. [Immutable](https://developer.mozilla.org/en-US/docs/Glossary/Immutable)

# Immutable
An **immutable** value is one whose content cannot be changed without creating an entirely new value, in comparison with [mutable values](https://developer.mozilla.org/en-US/docs/Glossary/Mutable).
In [JavaScript](https://developer.mozilla.org/en-US/docs/Glossary/JavaScript), [primitive values](https://developer.mozilla.org/en-US/docs/Glossary/Primitive) are immutable — once a primitive value is created, it cannot be changed, although the variable that holds it may be reassigned another value. By contrast, [objects](https://developer.mozilla.org/en-US/docs/Glossary/Object) and [arrays](https://developer.mozilla.org/en-US/docs/Glossary/Array) are mutable by default — their properties and elements can be changed without reassigning a new value.
It can be beneficial to use immutable objects for several reasons:
  * To improve performance (no planning for the object's future changes)
  * To reduce memory use (make [object references](https://developer.mozilla.org/en-US/docs/Glossary/Object_reference) instead of cloning the whole object)
  * Thread-safety (multiple threads can reference the same object without interfering with one other)
  * Lower developer mental burden (the object's state won't change and its behavior is always consistent)

Note that you can easily prove mutability: an object is mutable as long as it offers one way to change its properties. On the other hand, _immutability_ is hard to prove if there are no language semantics to secure it — it is a matter of developer contract. For example, [`Object.freeze()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze) is a language-level method to make an object immutable in JavaScript.
## In this article
  * [See also](https://developer.mozilla.org/en-US/docs/Glossary/Immutable#see_also)

##  [See also](https://developer.mozilla.org/en-US/docs/Glossary/Immutable#see_also)
  * [Immutable object](https://en.wikipedia.org/wiki/Immutable_object) on Wikipedia
  * Related glossary terms: 
    * [Mutable](https://developer.mozilla.org/en-US/docs/Glossary/Mutable)