Functions

Understand how to create and use functions in JavaScript.

Introduction

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

There are two kinds of functions in JavaScript: built-in and developer created.

Some examples of built-in JavaScript functions include: concat(), date(), length() etc.

A JavaScript function is executed when "something" invokes or calls it.

Calling a Function

The code inside a function will execute when something invokes or calls the function.

When an event occurs (ex. when a user clicks a button)
When it is called from JavaScript code

Function Example

<!DOCTYPE html>
<html>
  <body>
    <button onclick="myFunction()">Call Function</button>
    <script>
      function myFunction() {
        window.alert("Hello World!");
      }
    </script>
  </body>
</html>

Function Syntax

A JavaScript function is defined with the function keyword, followed by a name, then followed by parentheses ().
Function names can contain letters, digits, underscores, and dollar signs.
The parentheses may include parameter names separated by commas.
- (parameter1, parameter2, ....)
The code to be executed, by the function, is placed inside curly brackets .

function name(parameter1, parameter2, parameter3) {
  // code to be executed
}

Function parameters are listed inside the parentheses () in the function definition.
Function arguments are the values received by the function when it is called.
Inside the function, the arguments (parameters) behave as local variables.
Outputs of the operations done using parameters are sent back to the program using the return statement.

Function With Paramter and Return

<!DOCTYPE html>
<html>
  <body>
    <p id="demo"></p>
    <script text="text/javascript">
      function product(num1, num2) {
        return num1 * num2;
      }
      document.getElementById("demo").innerHTML = "The return value for function is " + product(12, 3);
    </script>
  </body>
</html>

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History

Brief history of JavaScript

Validation

Learn how to perform form validation in JavaScript.