Menu
×
   ❮     
HTML CSS JAVASCRIPT SQL PYTHON JAVA PHP HOW TO W3.CSS C C++ C# BOOTSTRAP REACT MYSQL JQUERY EXCEL XML DJANGO NUMPY PANDAS NODEJS R TYPESCRIPT ANGULAR GIT POSTGRESQL MONGODB ASP AI GO KOTLIN SASS VUE DSA GEN AI SCIPY AWS CYBERSECURITY DATA SCIENCE
     ❯   

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Object Properties JS Object Methods JS Object Display JS Object Constructors JS Events JS Strings JS String Methods JS String Search JS String Templates JS Numbers JS BigInt JS Number Methods JS Number Properties JS Arrays JS Array Methods JS Array Search JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS If Else JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Iterables JS Sets JS Set Methods JS Maps JS Map Methods JS Typeof JS Type Conversion JS Destructuring JS Bitwise JS RegExp JS Precedence JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS Modules JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS 2019 JS 2020 JS 2021 JS 2022 JS 2023 JS 2024 JS IE / Edge JS History

JS Objects

Object Definitions Object Prototypes Object Methods Object Properties Object Get / Set Object Protection

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Bind Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON Server JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor JS Exercises JS Quiz JS Website JS Interview Prep JS Bootcamp JS Certificate

JS References

JavaScript Objects HTML DOM Objects


JavaScript Function Definitions


JavaScript functions are defined with the function keyword.

You can use a function declaration or a function expression.


Function Declarations

Earlier in this tutorial, you learned that functions are declared with the following syntax:

function functionName(parameters) {
  // code to be executed
}

Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are invoked (called upon).

Example

function myFunction(a, b) {
  return a * b;
}
Try it Yourself »

Semicolons are used to separate executable JavaScript statements.
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.


Function Expressions

A JavaScript function can also be defined using an expression.

A function expression can be stored in a variable:

Example

const x = function (a, b) {return a * b};
Try it Yourself »

After a function expression has been stored in a variable, the variable can be used as a function:

Example

const x = function (a, b) {return a * b};
let z = x(4, 3);
Try it Yourself »

The function above is actually an anonymous function (a function without a name).

Functions stored in variables do not need function names. They are always invoked (called) using the variable name.

The function above ends with a semicolon because it is a part of an executable statement.



The Function() Constructor

As you have seen in the previous examples, JavaScript functions are defined with the function keyword.

Functions can also be defined with a built-in JavaScript function constructor called Function().

Example

const myFunction = new Function("a", "b", "return a * b");

let x = myFunction(4, 3);
Try it Yourself »

You actually don't have to use the function constructor. The example above is the same as writing:

Example

const myFunction = function (a, b) {return a * b};

let x = myFunction(4, 3);
Try it Yourself »

Most of the time, you can avoid using the new keyword in JavaScript.


Function Hoisting

Earlier in this tutorial, you learned about "hoisting" (JavaScript Hoisting).

Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.

Hoisting applies to variable declarations and to function declarations.

Because of this, JavaScript functions can be called before they are declared:

myFunction(5);

function myFunction(y) {
  return y * y;
}

Functions defined using an expression are not hoisted.


Self-Invoking Functions

Function expressions can be made "self-invoking".

A self-invoking expression is invoked (started) automatically, without being called.

Function expressions will execute automatically if the expression is followed by ().

You cannot self-invoke a function declaration.

You have to add parentheses around the function to indicate that it is a function expression:

Example

(function () {
  let x = "Hello!!";  // I will invoke myself
})();
Try it Yourself »

The function above is actually an anonymous self-invoking function (function without name).


Functions Can Be Used as Values

JavaScript functions can be used as values:

Example

function myFunction(a, b) {
  return a * b;
}

let x = myFunction(4, 3);
Try it Yourself »

JavaScript functions can be used in expressions:

Example

function myFunction(a, b) {
  return a * b;
}

let x = myFunction(4, 3) * 2;
Try it Yourself »

Functions are Objects

The typeof operator in JavaScript returns "function" for functions.

But, JavaScript functions can best be described as objects.

JavaScript functions have both properties and methods.

The arguments.length property returns the number of arguments received when the function was invoked:

Example

function myFunction(a, b) {
  return arguments.length;
}
Try it Yourself »

The toString() method returns the function as a string:

Example

function myFunction(a, b) {
  return a * b;
}

let text = myFunction.toString();
Try it Yourself »

A function defined as the property of an object, is called a method to the object.
A function designed to create new objects, is called an object constructor.


Arrow Functions

Arrow functions allows a short syntax for writing function expressions.

You don't need the function keyword, the return keyword, and the curly brackets.

Example

// ES5
var x = function(x, y) {
  return x * y;
}

// ES6
const x = (x, y) => x * y;
Try it Yourself »

Arrow functions do not have their own this. They are not well suited for defining object methods.

Arrow functions are not hoisted. They must be defined before they are used.

Using const is safer than using var, because a function expression is always constant value.

You can only omit the return keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them:

Example

const x = (x, y) => { return x * y };
Try it Yourself »

Arrow functions are not supported in IE11 or earlier.


×

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
sales@w3schools.com

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
help@w3schools.com

W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. While using W3Schools, you agree to have read and accepted our terms of use, cookie and privacy policy.

Copyright 1999-2024 by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3.CSS.