Form Validator with JavaScript, HTML and CSS

In this tutorial, we will learn how to develop a Form Validator Project with plain JavaScript, HTML, and CSS.

Form Validator (Intro Project)

Simple client-side form validation. Check required, length, email, and password match.

Project Specifications

  • Create form UI
  • Show error messages under specific inputs
  • checkRequired() to accept array of inputs
  • checkLength() to check min and max length
  • checkEmail() to validate email with regex
  • checkPasswordsMatch() to match confirm password

JavaScript Breakout Game Development

Create a folder called form-validator as the project workspace and we will create all the project files inside this folder.

1. index.html

Let's create index.html and add the following code to it:

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <link rel="stylesheet" href="style.css" />
    <title>Form Validator</title>
    <div class="container">
      <form id="form" class="form">
        <h2>Register With Us</h2>
        <div class="form-control">
          <label for="username">Username</label>
          <input type="text" id="username" placeholder="Enter username" />
          <small>Error message</small>
        <div class="form-control">
          <label for="email">Email</label>
          <input type="text" id="email" placeholder="Enter email" />
          <small>Error message</small>
        <div class="form-control">
          <label for="password">Password</label>
          <input type="password" id="password" placeholder="Enter password" />
          <small>Error message</small>
        <div class="form-control">
          <label for="password2">Confirm Password</label>
            placeholder="Enter password again"
          <small>Error message</small>
        <button type="submit">Submit</button>

    <script src="script.js"></script>

2. script.js

Let's create a JavaScript file named script.js and add the following JavaScript code to it:

const form = document.getElementById('form');
const username = document.getElementById('username');
const email = document.getElementById('email');
const password = document.getElementById('password');
const password2 = document.getElementById('password2');

// Show input error message
function showError(input, message) {
  const formControl = input.parentElement;
  formControl.className = 'form-control error';
  const small = formControl.querySelector('small');
  small.innerText = message;

// Show success outline
function showSuccess(input) {
  const formControl = input.parentElement;
  formControl.className = 'form-control success';

// Check email is valid
function checkEmail(input) {
  const re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
  if (re.test(input.value.trim())) {
  } else {
    showError(input, 'Email is not valid');

// Check required fields
function checkRequired(inputArr) {
  let isRequired = false;
  inputArr.forEach(function(input) {
    if (input.value.trim() === '') {
      showError(input, `${getFieldName(input)} is required`);
      isRequired = true;
    } else {

  return isRequired;

// Check input length
function checkLength(input, min, max) {
  if (input.value.length < min) {
      `${getFieldName(input)} must be at least ${min} characters`
  } else if (input.value.length > max) {
      `${getFieldName(input)} must be less than ${max} characters`
  } else {

// Check passwords match
function checkPasswordsMatch(input1, input2) {
  if (input1.value !== input2.value) {
    showError(input2, 'Passwords do not match');

// Get fieldname
function getFieldName(input) {
  return +;

// Event listeners
form.addEventListener('submit', function(e) {

  if(!checkRequired([username, email, password, password2])){
    checkLength(username, 3, 15);
    checkLength(password, 6, 25);
    checkPasswordsMatch(password, password2);

3. style.css

Let's create a CSS file named style.css and add the following CSS code to it:

@import url('');

:root {
  --success-color: #2ecc71;
  --error-color: #e74c3c;

* {
  box-sizing: border-box;

body {
  background-color: #f9fafb;
  font-family: 'Open Sans', sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  margin: 0;

.container {
  background-color: #fff;
  border-radius: 5px;
  box-shadow: 0 2px 10px rgba(0, 0, 0, 0.3);
  width: 400px;

h2 {
  text-align: center;
  margin: 0 0 20px;

.form {
  padding: 30px 40px;

.form-control {
  margin-bottom: 10px;
  padding-bottom: 20px;
  position: relative;

.form-control label {
  color: #777;
  display: block;
  margin-bottom: 5px;

.form-control input {
  border: 2px solid #f0f0f0;
  border-radius: 4px;
  display: block;
  width: 100%;
  padding: 10px;
  font-size: 14px;

.form-control input:focus {
  outline: 0;
  border-color: #777;

.form-control.success input {
  border-color: var(--success-color);

.form-control.error input {
  border-color: var(--error-color);

.form-control small {
  color: var(--error-color);
  position: absolute;
  bottom: 0;
  left: 0;
  visibility: hidden;

.form-control.error small {
  visibility: visible;

.form button {
  cursor: pointer;
  background-color: #3498db;
  border: 2px solid #3498db;
  border-radius: 4px;
  color: #fff;
  display: block;
  font-size: 16px;
  padding: 10px;
  margin-top: 20px;
  width: 100%;

Open index.html in Browser

Let's open the index.html file in the browser and you will be able to see the following screen:

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