Form Validation in JavaScript Code

This JavaScript code snippet helps you to create a form validation feature on the form submit. It validates username, email, and password and displays the inline error message in case of invalid input. You can integrate this vanilla JavaScript code for registration/signup forms to validate inputs on submit.

This form validation snippet doesn’t require any additional library or plugin. It uses JavaScript regular expressions to validate emails. It allows setting min/max length rules for input validation.  Moreover, it can be integrated with your existing HTML forms.

How to Create Form Validation in JavaScript

1. First you need to create the HTML form element with username, email, and password input with validation attributes.

 <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>
        <div class="form-control">
            <label for="email">Email</label>
            <input type="text" id="email" placeholder="Enter email">
            <small>Error Message</small>
        </div>
        <div class="form-control">
            <label for="password">Password</label>
            <input type="password" id="password" placeholder="Enter password">
            <small>Error Message</small>
        </div>
        <div class="form-control">
            <label for="password2">Confirm Password</label>
            <input type="password" id="password2" placeholder="Enter password again">
            <small>Error Message</small>
        </div>
        <button>Submit</button>
    </form>
</div>

2. After that, define the CSS styles for HTML form inputs, success, and error messages as follows.

:root{
    --succes-color: #2ecc71;;
    --error-color: #e74c3c;
}
.container{
    background-color: #fff;
    border-radius: 5px;
    box-shadow: 0 2px 10px rgba(0,0,0,0.3);
    width: 400px;
    margin: 10px auto;
}
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(--succes-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;
    padding: 10px;
    font-size: 16px;
    margin-top:20px;
    width:100%;
}

3. Finally, include the form validation JavaScript code in your project and done.

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

//show success colour
function showSucces(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())) {
        showSucces(input)
    }else {
        showError(input,'Email is not invalid');
    }
}

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

//check input Length
function checkLength(input, min ,max) {
    if(input.value.length < min) {
        showError(input, `${getFieldName(input)} must be at least ${min} characters`);
    }else if(input.value.length > max) {
        showError(input, `${getFieldName(input)} must be les than ${max} characters`);
    }else {
        showSucces(input);
    }
}

//get FieldName
function getFieldName(input) {
    return input.id.charAt(0).toUpperCase() + input.id.slice(1);
}

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

//Event Listeners
form.addEventListener('submit',function(e) {
    e.preventDefault();

    checkRequired([username, email, password, password2]);
    checkLength(username,3,15);
    checkLength(password,6,25);
    checkEmail(email);
    checkPasswordMatch(password, password2);
}); 

That’s all! hopefully, this JavaScript validation plugin is helpful for you. If you have any questions or suggestions, let me know by comment below.

Asif Mughal

I code and create web elements for amazing people around the world. I like work with new people. New people new Experiences.
I truly enjoy what I’m doing, which makes me more passionate about web development and coding. I am always ready to do challenging tasks whether it is about creating a custom CMS from scratch or customizing an existing system.