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function

The function declaration (function statement) defines a function with the specified parameters.

You can also define functions using the Function constructor and a function expression.

Syntax

function name([param[, param,[..., param]]]) {
   [statements]
}
name
The function name.
param Optional
The name of an argument to be passed to the function. Maximum number of arguments varies in different engines.
statements Optional
The statements which comprise the body of the function.

Description

A function created with a function declaration is a Function object and has all the properties, methods and behavior of Function objects. See Function for detailed information on functions.

A function can also be created using an expression (see function expression).

By default, functions return undefined. To return any other value, the function must have a return statement that specifies the value to return.

Conditionally created functions

Functions can be conditionally declared, that is, a function statement can be nested within an if statement, however the results are inconsistent across implementations and therefore this pattern should not be used in production code. For conditional function creation, use function expressions instead.

var hoisted = "foo" in this;
console.log(`'foo' name ${hoisted ? "is" : "is not"} hoisted. typeof foo is ${typeof foo}`);
if (false) {
  function foo(){ return 1; }
}

// In Chrome: 
// 'foo' name is hoisted. typeof foo is undefined
// 
// In Firefox:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Edge:
// 'foo' name is not hoisted. typeof foo is undefined
// 
// In Safari:
// 'foo' name is hoisted. typeof foo is function

The results are exactly the same for a condition that evaluates to true

var hoisted = "foo" in this;
console.log(`'foo' name ${hoisted ? "is" : "is not"} hoisted. typeof foo is ${typeof foo}`);
if (true) {
  function foo(){ return 1; }
}

// In Chrome: 
// 'foo' name is hoisted. typeof foo is undefined
// 
// In Firefox:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Edge:
// 'foo' name is not hoisted. typeof foo is undefined
// 
// In Safari:
// 'foo' name is hoisted. typeof foo is function

Function declaration hoisting

Function declarations in JavaScript are hoisted to the top of the enclosing function or global scope. You can use the function before you declared it:

hoisted(); // logs "foo"

function hoisted() {
  console.log('foo');
}

Note that function expressions are not hoisted:

notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {
   console.log('bar');
};

Examples

Using function

The following code declares a function that returns the total amount of sales, when given the number of units sold of products a, b, and c.

function calc_sales(units_a, units_b, units_c) {
   return units_a * 79 + units_b * 129 + units_c * 699;
}

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
function 1 12 1 3 3 1
Trailing comma in parameters 58 14 52 No 45 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
function 1 18 4 10.1 1 1.0
Trailing comma in parameters 58 58 52 43 10 7.0
Server
Node.js
function 0.1.100
Trailing comma in parameters 8.0.0

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function