The form element is one of the most used HTML elements on websites. These forms take input from the user and process it in the browser or server. However, it’s important to validate these inputs to tackle security issues and unwanted bugs.

Understanding DOM Manipulation

Before we proceed to implement client-side form validation with JavaScript, it’s essential to understand the Document Object Model, commonly known as the “DOM”. The DOM is a standardized API that allows JavaScript to interact with elements on an HTML web page.

Learn More: The Hidden Hero of Websites: Understanding the DOM

For adding event listeners and retrieving user inputs, you’ll need to understand the basics of DOM manipulation. Here’s an example that explains how you can change the webpage’s contents using the DOM API and JavaScript:

In the above code, the

tag has an id of paragraph. While writing the JavaScript code, you can access this element by calling the document.getElementById(‘paragraph’) method and manipulating its value.

Now that you’ve understood the basics of DOM manipulation, let’s go ahead and implement form validation.

Form Validation With JavaScript

There are various types of inputs that you can take from a user. Text type, email type, password type, radio buttons, and checkboxes are some of the most common ones that you may encounter. Due to these vast majority of input types, you will need to use different logic to validate each of them.

Before delving into validation, let’s take a moment to understand HTML forms and their importance. HTML forms are one of the primary ways you interact with the website as they allow you to enter your data, apply changes, invoke pop-ups, submit information to a server, and more.

The HTML

element is used to create these user-facing forms. Here’s how you can validate the inputs of an HTML form:

1. Email Validation

Whether you’re building an authentication system or just collecting user emails for your newsletter, it’s important to validate the email before you can store it in your database or process it. To check if an email satisfies all the required conditions, you can use a regular expression.

HTML:

JavaScript:

2. Password Validation

Passwords are a crucial piece of data that needs a special type of validation to ensure its security. Consider a signup form having two fields: password and confirm password fields. To validate these pair of inputs, here are some things you’ll need to consider:

The password must be more than 6 characters long. The value of the password and the confirm password field must be the same.

HTML:

JavaScript:

3. Radio Input Validation

HTML radio input is a special type of graphical control element that allows the user to choose only one of a predefined set of mutually exclusive options. A common use case of such an input would be for selecting gender. To validate such input, you will need to check if at least one of them is selected.

This can be achieved using the logical operators such as the AND (&&) and NOT (!) operator in this manner:

HTML:

JavaScript:

4. Select Input Validation

The

For the default or initial option, you can set its value to be an empty string so that it’ll be deemed as an invalid option. For all the other options, set an appropriate value attribute. Here’s an example of how you can validate a