Web/JavaScript/Reference/statements/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

Specification
ECMAScript (ECMA-262)The definition of 'Function definitions' in that specification.

Browser compatibility

Update compatibility data on GitHub

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

Legend

Full support: Full support
No support: No support