w3schools form validation code example

Example 1: html form validation

<form action="">
    <label for="name">Username</label>
    <input id="name" type="text" required />
    <label for="Email">Email</label>
    <!-- The required attribute will prompt the user to make sure  -->
    <!-- the fields are not empty, and that the email is formatted correctly -->
    <!-- if you want to use custom validation you should use regex in JavaScript -->
    <input id="Email" type="text" required />
</form>

Example 2: javascript login validation

<script language = "JavaScript">
        function validate() {
            var username = document.getElementById("username").value;
            var password = document.getElementById("password").value;
            if (username == null || username == "") {
                alert("Please enter the username.");
                return false;
            }
            if (password == null || password == "") {
                alert("Please enter the password.");
                return false;
            }
            alert('Login successful');
            
        } 
</script>