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
.
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
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
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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