Functions

A JavaScript function is a code block that performs a task or calculates a value. A function can be called anywhere from your code and as many times as needed, allowing for code re-usability.

Function Syntax

To create a function, first write the keyword function, followed by the name of the function. This is then followed by a group of comma separated parameters enclosed by parentheses - ( and ). Two curly braces { and } then follow, which contain code that the function will execute.

function functionname (param1, param2, ....) {
    //Function's code goes here
}

Examples

Following example shows a function with no parameters.

function Hello () {
    alert("Hello World");
}

Following example shows a function that accepts two parameters.

function Add (value1, value2) {
    alert(value1 + value2);
}

Calling a Function

To call a function you will just write the function name and pass any values, if needed.

In the following example, we create a function called Add. Then we call the function by writing the function name and passing values to it.

function Add (value1, value2) {
    document.write(value1 + value2);
} 

Add(2,3);
document.write("<br>");
Add(8,1);

The above example will have the below output:

5
9

Following example has a button which when clicked, calls the HelloWorld function and then shows an alert box displaying "Hello World!".

<html>
    <head>
        <script type="text/javascript"> 
            function HelloWorld() {
                alert("Hello World!");
            } 
        </script>
    </head>
    <body>
        <form>
            <input type="button" value="Hello World!" onclick="HelloWorld();" >
        </form>
    </body>
</html>

Below is the output of above code: