Define Function in JavaScript with Example

Updated on September 20, 2015     Kisan Patel

JavaScript functions provide a way to encapsulate a block of code in order to reuse the code several times.

A JavaScript function is a block of code designed to perform a particular task.

A JavaScript function created using the function statement and syntax similar to the following.

function functionName(parameters) {
    code to be executed
}

Create a simple, named, parameter-less function using the function statement.

function simpleFunction() {
    alert("Hello from function!");
};
simpleFunction();

A function created using the function keyword and given a name is known as both a declarative function and a static function.

Provide arguments for the incoming data, and return the result.

function makeHello(strName) {
   return ("Hello " + strName);
}

window.onload=function() {
   var name = prompt("What's your name?","");
   var result = makeHello(name);
   alert(result);
}

Function arguments are a way to pass data to a function. The arguments are separated by commas and included in the parentheses following the function name.

makeHello(firstName, lastName);

The function then processes the arguments, as needed.

function makeHello(firstName, lastName) {
   alert("Hello " + firstName + " " + lastName);
}

Data is returned from the function to the calling program using the return statement.

function makeHello(firstName, lastName) {
   return "Hello " + firstName + " " + lastName;
}

You can pass several arguments to the function, but only one return value.

Passing Objects as argument to a Function

You can use objects, such as arrays, as function arguments.

function makeHello(name) {
   name[name.length] = "Hello " + name[0] + " " + name[1];
}

var name = new Array('from','Function');
makeHello(name);
alert(name[2]); // displays "Hello from Function"