< 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
Try it Yourself »
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 |
|---|
More Examples
Example
Return the value of PI:
function myFunction() {
return Math.PI;
}
The result will be:
3.141592653589793
Try it Yourself »
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);
}
Try it Yourself »
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";
Try it Yourself »
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;
}
Try it Yourself »
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>
Try it Yourself »
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};
Try it Yourself »
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);
Try it Yourself »
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