const username = document.getElementById('username').value;

const password = document.getElementById('password').value;

const email = document.getElementById('email').value;

let isValid = true;

​

if (email.indexOf('@') === -1) {

    console.log('Invalid email address: "@" is missing.');

    isValid = false;

}

​

if (username.indexOf(' ') !== -1) {

    console.log('Invalid username: spaces are not allowed.');

    isValid = false;

}

​

if (password.length < 8) {

    console.log('Invalid password: must be at least 8 characters long.');

    isValid = false;

}

​

if (password.indexOf(username) !== -1) {

    console.log('Invalid password: the password should not contain the username.');

    isValid = false;

}

​

// Use regular expressions for more complex validations

const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;

if (!emailRegex.test(email)) {

    console.log('Invalid email format.');

    isValid = false;

}

​

const usernameRegex = /^\S+$/; // No spaces allowed

if (!usernameRegex.test(username)) {

    console.log('Invalid username: spaces are not allowed.');

    isValid = false;

}

​

if (isValid) {

    console.log('Form is valid!');

    // Submit form or further processing

} else {

    console.log('Form is invalid.');

}

<form id="myForm" onsubmit="return validateForm()">

    <label for="name">Name:</label><br>

    <input type="text" id="name" name="name"><br>

    <label for="email">Email:</label><br>

    <input type="text" id="email" name="email"><br>

    <input type="submit" value="Submit">

</form>

<script>

function validateForm() {

    var name = document.getElementById("name").value;

    var email = document.getElementById("email").value;

    var errorMessages = [];

​

    // Check if name is empty

    if (name.trim() === "") {

        errorMessages.push("Name is required");

    }

​

    // Check if email is empty and if it's a valid email format

    if (email.trim() === "") {

        errorMessages.push("Email is required");

    } else if (!isValidEmail(email)) {

        errorMessages.push("Invalid email format");

    }

​

    // Display error messages if any

    if (errorMessages.length > 0) {

        alert(errorMessages.join("\n"));

        return false; // Prevent form submission

    }

​

    return true; // Allow form submission

}

​

// Function to validate email format

function isValidEmail(email) {

    var emailRegex = /\S+@\S+\.\S+/;

    return emailRegex.test(email);

}

</script>

let input_value = document.forms["myForm"]["myInputField"].value;

if(input_value == undefined || input_value.trim() == "") {

  console.log("InputName is required") 

}

javascript form validation