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. A function can have up to 255 arguments.
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. Therefore they should not be used, for conditional creation use function expressions.
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
Specification | Status | Comment |
---|---|---|
ECMAScript 2017 Draft (ECMA-262) The definition of 'Function definitions' in that specification. |
Draft | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Function definitions' in that specification. |
Standard | |
ECMAScript 5.1 (ECMA-262) The definition of 'Function definition' in that specification. |
Standard | |
ECMAScript 3rd Edition (ECMA-262) The definition of 'Function definition' in that specification. |
Standard | |
ECMAScript 1st Edition (ECMA-262) The definition of 'Function definition' in that specification. |
Standard | Initial definition. Implemented in JavaScript 1.0. |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Allowed in sloppy mode | 49.0 |
Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|
Basic support | No support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Allowed in sloppy mode | No support |
49.0 |
49.0 |