JavaScript function Statement | Rookie Tutorial

JavaScript Statements Reference Manual

Example

Declare a function that outputs "Hello World" to the element with id="demo" when called:

function myFunction() { // Declare a function

 document.getElementById("demo").innerHTML = "Hello World!";

}

myFunction(); // Call the function

Try it yourself »

More examples are included at the bottom of this article.

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Definition and Usage

The function statement is used to declare a function.

After declaring a function, we can call it whenever needed.

In JavaScript, functions are objects and also have properties and methods.

Functions can also be defined using expressions (see Function Definitions).

Read our JavaScript tutorial to learn more about functions. Start with JavaScript Functions and JavaScript Scope. For more detailed information, see Function Definitions, Parameters, Invocation, and Closures.

Tips: Use the return statement to return a value from a function.

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Browser Support

Statement
function	Yes	Yes	Yes	Yes	Yes

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Syntax

function functionName(parameters) {
Code to be executed
}

Parameter Values

Parameter	Description
functionName	Required. Specifies the name of the function. Function names can contain letters, digits, underscores, and dollar signs (same rules as variable names).
parameters	Optional. Specifies one or more parameter names, separated by commas (,). When the function is called, it receives actual values. Inside the function, parameters act as local variables. Note: If a parameter is not provided when calling the function, its value will be set to undefined.

Technical Details

JavaScript Version:	1.0

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More Examples

Example

Return the value of PI:

function myFunction() {

 return Math.PI;

}

Output:

3.141592653589793

Try it yourself »

Example

Return the product of a and b:

function myFunction(a, b) {

 return a * b;

}

Try it yourself »

Example

Call the function with different arguments to produce different results.

Convert Fahrenheit to Celsius:

function toCelsius(fahrenheit) {

 return (5/9) * (fahrenheit-32);

 }

Try it yourself »

Example

Functions can be used as values.

Instead of:

temp = toCelsius(32);
text = "The temperature is " + temp + " Centigrade";

You can also do this:

text = "The temperature is " + toCelsius(32) + " Centigrade";

Try it yourself »

Example

JavaScript functions have a built-in object called arguments.

The arguments.length property returns the number of arguments received when the function was called:

function myFunction(a, b) {

 return arguments.length;

}

Try it yourself »

Example

Call a function by clicking a button; after execution, "Hello World" will be displayed in the element with id="demo":

<button onclick="myFunction()">Click me</button>

<p id="demo"></p>

<script>

function myFunction() {

 document.getElementById("demo").innerHTML = "Hello World";

}

</script>

Try it yourself »

Example

JavaScript functions can be defined as expressions.

Function expressions can be stored in variables:

var x = function (a, b) {return a * b};

Try it yourself »

Example

After storing a function expression in a variable, the variable can be used as a function:

var x = function (a, b) {return a * b};

 var z = x(4, 3);

Try it yourself »

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Related Pages

JavaScript Tutorial: JavaScript Functions

JavaScript Tutorial: JavaScript Scope

JavaScript Tutorial: JavaScript Function Definitions

JavaScript Tutorial: JavaScript Function Parameters

JavaScript Tutorial: JavaScript Function Invocation

JavaScript Tutorial: JavaScript Function Closures

JavaScript Reference: JavaScript return Statement

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JavaScript Statements Reference Manual