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---
title: "Hoisting - Glossary | MDN"
chunk: 1/3
source: "https://developer.mozilla.org/en-US/docs/Glossary/Hoisting"
category: "reference"
tags: "web, html, css, javascript, documentation"
date_saved: "2026-05-05T05:34:01.449559+00:00"
instance: "kb-cron"
---
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1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
2. [Hoisting](https://developer.mozilla.org/en-US/docs/Glossary/Hoisting)
# Hoisting
JavaScript **Hoisting** refers to the process whereby the interpreter appears to move the _declaration_ of functions, variables, classes, or imports to the top of their [scope](https://developer.mozilla.org/en-US/docs/Glossary/Scope), prior to execution of the code.
_Hoisting_ is not a term normatively defined in the ECMAScript specification. The spec does define a group of declarations as [_HoistableDeclaration_](https://tc39.es/ecma262/multipage/ecmascript-language-statements-and-declarations.html#prod-HoistableDeclaration), but this only includes [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function), [`function*`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function), and [`async function*`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function*) declarations. Hoisting is often considered a feature of [`var`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var) declarations as well, although in a different way. In colloquial terms, any of the following behaviors may be regarded as hoisting:
1. Being able to use a variable's value in its scope before the line it is declared. ("Value hoisting")
2. Being able to reference a variable in its scope before the line it is declared, without throwing a [`ReferenceError`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError), but the value is always [`undefined`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined). ("Declaration hoisting")
3. The declaration of the variable causes behavior changes in its scope before the line in which it is declared.
4. The side effects of a declaration are produced before evaluating the rest of the code that contains it.
The four function declarations above are hoisted with type 1 behavior; `var` declaration is hoisted with type 2 behavior; [`let`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let), [`const`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const), and [`class`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class) declarations (also collectively called _lexical declarations_) are hoisted with type 3 behavior; [`import`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) declarations are hoisted with type 1 and type 4 behavior.
Some prefer to see `let`, `const`, and `class` as non-hoisting, because the [temporal dead zone](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#temporal_dead_zone_tdz) strictly forbids any use of the variable before its declaration. This dissent is fine, since hoisting is not a universally-agreed term. However, the temporal dead zone can cause other observable changes in its scope, which suggests there's some form of hoisting:
If the `const x = 2` declaration is not hoisted at all (as in, it only comes into effect when it's executed), then the `console.log(x)` statement should be able to read the `x` value from the upper scope. However, because the `const` declaration still "taints" the entire scope it's defined in, the `console.log(x)` statement reads the `x` from the `const x = 2` declaration instead, which is not yet initialized, and throws a [`ReferenceError`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError). Still, it may be more useful to characterize lexical declarations as non-hoisting, because from a utilitarian perspective, the hoisting of these declarations doesn't bring any meaningful features.
Note that the following is not a form of hoisting:
There's no "access before declaration" here; it's simply because `var` declarations are not scoped to blocks.
For more information on hoisting, see:
* `var`/`let`/`const` hoisting — [Grammar and types guide](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#variable_hoisting)
* `function` hoisting — [Functions guide](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#function_hoisting)
* `class` hoisting — [Classes guide](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_classes#class_declaration_hoisting)
* `import` hoisting — [JavaScript modules](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules#import_declarations_are_hoisted)
## In this article
* [See also](https://developer.mozilla.org/en-US/docs/Glossary/Hoisting#see_also)
## [See also](https://developer.mozilla.org/en-US/docs/Glossary/Hoisting#see_also)
* [`var` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var)
* [`let` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let)
* [`const` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const)
* [`function` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function)
* [`class` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class)
* [`import` statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import)
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