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JavaScript function Statement

JavaScript Statement Reference Manual

Example

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

function myFunction() { // Declare a function
    document.getElementById("demo").innerHTML = "Hello World!";
}
myFunction(); // Call the function

More examples are included at the bottom of this article.


Definition and Usage

The function statement is used to declare a function.

After a function is declared, it can be called whenever needed.

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

Functions can also be defined by expressions (see Function Definition).

For more information about functions, please read our JavaScript tutorial. Start by understanding JavaScript Functions and JavaScript Scope. For detailed content, see Function Definition, Parameters, Invocation, and Closures.

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


Browser Support

Statement Chrome Edge Firefox Safari Opera
function Yes Yes Yes Yes Yes

Syntax

Parameter Values

Parameter Description
functionName Required. Specifies the function's name. The function name can include letters, digits, underscores, and dollar signs (same rules as variable names).
parameters Optional. Specifies one or more parameter names, separated by commas. The function receives actual values when called. Inside the function, parameters act as local variables. Note: If a parameter is not specified when the function is called, its value is set to undefined.

Technical Details

| JavaScript Version: | 1.0 | | --- | --- |


More Examples

Example

Return the value of PI:

function myFunction() {
    return Math.PI;
}

Output:

3.141592653589793

Example

Return the value of a multiplied by b:

function myFunction(a, b) {
    return a * b;
}

Example

Call the function with different arguments to get different results.

Convert Fahrenheit to Celsius:

function toCelsius(fahrenheit) {
    return (5/9) * (fahrenheit-32);
}

Example

Functions can be used as variables.

Instead of:

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

You can also do this:

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

Example

JavaScript functions have a built-in object called arguments.

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

function myFunction(a, b) {
    return arguments.length;
}

Example

Click a button to call a function that outputs "Hello World" on 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>

Example

JavaScript functions can be defined as expressions.

A function expression can be stored in a variable:

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

Example

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

var x = function (a, b) {return a * b};
var z = x(4, 3);

Related Pages

JavaScript Tutorial: JavaScript Functions

JavaScript Tutorial: JavaScript Scope

JavaScript Tutorial: JavaScript Function Definition

JavaScript Tutorial: JavaScript Function Parameters

JavaScript Tutorial: JavaScript Function Invocation

JavaScript Tutorial: JavaScript Function Closures

JavaScript Reference Manual: JavaScript return Statement


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