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---
title: "Function - Glossary | MDN"
chunk: 1/3
source: "https://developer.mozilla.org/en-US/docs/Glossary/Function"
category: "reference"
tags: "web, html, css, javascript, documentation"
date_saved: "2026-05-05T05:32:08.608917+00:00"
instance: "kb-cron"
---
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1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
2. [Function](https://developer.mozilla.org/en-US/docs/Glossary/Function)
# Function
A **function** is a code snippet that can be called by other code or by itself, or a [variable](https://developer.mozilla.org/en-US/docs/Glossary/Variable) that refers to the function. When a function is called, [arguments](https://developer.mozilla.org/en-US/docs/Glossary/Argument) are passed to the function as input, and the function can optionally return a value. A function in [JavaScript](https://developer.mozilla.org/en-US/docs/Glossary/JavaScript) is also an [object](https://developer.mozilla.org/en-US/docs/Glossary/Object).
A function name is an [identifier](https://developer.mozilla.org/en-US/docs/Glossary/Identifier) included as part of a function declaration or function expression. The function name's [scope](https://developer.mozilla.org/en-US/docs/Glossary/Scope) depends on whether the function name is a declaration or expression.
## In this article
* [Different types of functions](https://developer.mozilla.org/en-US/docs/Glossary/Function#different_types_of_functions)
* [See also](https://developer.mozilla.org/en-US/docs/Glossary/Function#see_also)
## [Different types of functions](https://developer.mozilla.org/en-US/docs/Glossary/Function#different_types_of_functions)
An **anonymous function** is a function without a function name. Only function expressions can be anonymous, function declarations must have a name:
The following terms are not used in the ECMAScript language specification, they're jargon used to refer to different types of functions.
A **named function** is a function with a function name:
An **inner function** is a function inside another function (`square` in this case). An **outer function** is a function containing a function (`addSquares` in this case):
A **recursive function** is a function that calls itself. See [recursion](https://developer.mozilla.org/en-US/docs/Glossary/Recursion).
An **Immediately Invoked Function Expression** ([IIFE](https://developer.mozilla.org/en-US/docs/Glossary/IIFE)) is a function that is called directly after the function is loaded into the browser's compiler. The way to identify an IIFE is by locating the extra left and right parenthesis at the end of the function's definition.
Function expressions, named or anonymous, can be called immediately.
Declared functions can't be called immediately this way, because IIFEs must be function _expressions_.
If you'd like to know more about IIFEs, check out the following page on Wikipedia: [Immediately Invoked Function Expression](https://en.wikipedia.org/wiki/Immediately_invoked_function_expression)
## [See also](https://developer.mozilla.org/en-US/docs/Glossary/Function#see_also)
* [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions)
* [Arrow Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions)
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