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JavaScript function Statement

< JavaScript Statements Reference

Example

Declare a function and output "Hello World" in an element with id="demo", when the function is called:

function myFunction() { // Declare a function
    document.getElementById("demo").innerHTML = "Hello World!";
}
 
myFunction(); // Call the function
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More "Try it Yourself" examples below.


Definition and Usage

The function statement declares a function.

A declared function is "saved for later use", and will be executed later, when it is invoked (called).

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

A function can also be defined using an expression (See Function Definitions).

Read our JavaScript Tutorial to learn all you need to know about functions. Start with the introduction chapter about JavaScript Functions and JavaScript Scope. For more detailed information, see our Function Section on Function Definitions, Parameters, Invocation and Closures.

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


Browser Support

Statement
function Yes Yes Yes Yes Yes

Syntax

function functionName(parameters) {
    code to be executed
}

Parameter Values

Parameter Description
functionName Required. Specifies the name of the function, which can be "saved for later use". Function names can contain letters, digits, underscores, and dollar signs (same rules as variables)
parameters Optional. Specifies a set of zero or more parameter names, separated by commas.

Function parameters are the names listed in the function definition.

Function arguments are the real values received by the function when it is invoked. Inside the function, the arguments are used as local variables.

Note: If a function is called with a missing argument, the value of the missing argument is set to undefined

Technical Details

JavaScript Version: 1.0

Examples

More Examples

Example

Return the value of PI:

function myFunction() {
    return Math.PI;
}

The result will be:

3.141592653589793
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Example

Return the product of a and b:

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

With functions, you can use the same code many times with different arguments, to produce different results.

Convert Fahrenheit to Celsius:

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

Functions can be used as variables.

Instead of:

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

You can use:

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

JavaScript functions have a built-in object called the arguments object.

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

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

Click on a button to call a function, which will output "Hello World" in an element with id="demo":

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

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

<script>
function myFunction() {
    document.getElementById("demo").innerHTML = "Hello World";
}
</script>
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Example

A JavaScript function can also be defined using an expression.

A function expression can be stored in a variable:

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

After a function expression has been 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);
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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


< JavaScript Statements Reference