function declaration

The function declaration 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

The name of an argument to be passed to the function. Maximum number of arguments varies in different engines.

statements

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. Most browsers other than Mozilla will treat such conditional declarations as an unconditional declaration and create the function whether the condition is true or not, see this article for an overview. For this reason, they should not be used — for conditional creation use function expressions instead.

Function declaration hoisting

Function declarations in JavaScript are hoisting the function definition. 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 compatibility

See also

• Functions and function scope
• Function
• function expression
• function* statement
• function* expression
• Arrow functions
• GeneratorFunction
• async function
• async function expression