Widely available Supported across all major browsers. Safe to use in production.

Let and const

The const declaration declares block-scoped local variables. The value of a constant can't be changed through reassignment using the assignment operator, but if a constant is an object, its properties can be added, updated, or removed.

Language core

Browser support

Feature Desktop Mobile
Chrome
Edge
Firefox
Safari
Chrome Android
Safari iOS
const
21
12
36
5.1
25
5
Other
javascript.statements.let

The let declaration declares re-assignable, block-scoped local variables, optionally initializing each to a value.
49
14
44
10
49
10
1+Supported (version) Not supported ※Has note Sub-feature descriptions sourced from MDN Web Docs (CC BY-SA 2.5)
Notes 2 item(s)
Implementation note
  • Before Firefox 13, `const` is implemented, but re-assignment is not failing.
  • Before Firefox 46, a `TypeError` was thrown on redeclaration instead of a `SyntaxError`.
Notes 3 item(s)
Limitation
  • This browser only partially implements this feature
Removed
  • This feature was removed in a later browser version (49)
Implementation note
  • Strict mode is required.
Notes 3 item(s)
Limitation
  • This browser only partially implements this feature
Removed
  • This feature was removed in a later browser version (14)
Implementation note
  • In Edge 12 and 13, `let` within a `for` loop initializer does not create a separate variable for each loop iteration as defined by ES2015. Instead, it behaves as though the loop were wrapped in a scoping block with the `let` immediately before the loop.
Notes 3 item(s)
Implementation note
  • Before Firefox 44, `let` is only available to code blocks in HTML wrapped in a `<script type="application/javascript;version=1.7">` block (or higher version) and has different semantics (e.g. no temporal dead zone).
  • Before Firefox 46, a `TypeError` is thrown on redeclaration instead of a `SyntaxError`.
  • Firefox 54 adds support of `let` in workers.
Notes 3 item(s)
Limitation
  • This browser only partially implements this feature
Removed
  • This feature was removed in a later browser version (49)
Implementation note
  • Strict mode is required.

Syntax

JAVASCRIPT 
// let: can be reassigned
let count = 0;
count = 1; // OK

// const: cannot be reassigned
const PI = 3.14;
// PI = 3; // Error

// Object properties can be changed even with const
const user = { name: 'Taro' };
user.name = 'Hanako'; // OK

Live demo

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Const and not-

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Use cases

  • Using Let and const

    The const declaration declares block-scoped local variables.

Cautions

  • No specific concerns. Stable across all major browsers.

Accessibility

  • When updating the DOM dynamically, announce important changes to assistive technology with aria-live regions.

Related links