Live Demo: JavaScript Form validation

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>JavaScript Form validation</title>
<link rel="stylesheet" href="/examples/css/form-style.css">
<script>
// Defining a function to display error message
function printError(elemId, hintMsg) {
    document.getElementById(elemId).innerHTML = hintMsg;
}

// Defining a function to validate form 
function validateForm() {
    // Retrieving the values of form elements 
    let name = document.contactForm.name.value;
    let email = document.contactForm.email.value;
    let mobile = document.contactForm.mobile.value;
    let country = document.contactForm.country.value;
    let gender = document.contactForm.gender.value;
    let hobbies = [];
    let checkboxes = document.getElementsByName("hobbies[]");
    for(let i=0; i < checkboxes.length; i++) {
        if(checkboxes[i].checked) {
            // Populate hobbies array with selected values
            hobbies.push(checkboxes[i].value);
        }
    }
    
	// Defining error variables with a default value
    let nameErr = emailErr = mobileErr = countryErr = genderErr = true;
    
    // Validate name
    if(name == "") {
        printError("nameErr", "Please enter your name");
    } else {
        let regex = /^[a-zA-Z\s]+$/;                
        if(regex.test(name) === false) {
            printError("nameErr", "Please enter a valid name");
        } else {
            printError("nameErr", "");
            nameErr = false;
        }
    }
    
    // Validate email address
    if(email == "") {
        printError("emailErr", "Please enter your email address");
    } else {
        // Regular expression for basic email validation
        let regex = /^\S+@\S+\.\S+$/;
        if(regex.test(email) === false) {
            printError("emailErr", "Please enter a valid email address");
        } else{
            printError("emailErr", "");
            emailErr = false;
        }
    }
    
    // Validate mobile number
    if(mobile == "") {
        printError("mobileErr", "Please enter your mobile number");
    } else {
        let regex = /^[1-9]\d{9}$/;
        if(regex.test(mobile) === false) {
            printError("mobileErr", "Please enter a valid 10 digit mobile number");
        } else{
            printError("mobileErr", "");
            mobileErr = false;
        }
    }
    
    // Validate country
    if(country == "Select") {
        printError("countryErr", "Please select your country");
    } else {
        printError("countryErr", "");
        countryErr = false;
    }
    
    // Validate gender
    if(gender == "") {
        printError("genderErr", "Please select your gender");
    } else {
        printError("genderErr", "");
        genderErr = false;
    }
    
    // Prevent the form from being submitted if there are any errors
    if((nameErr || emailErr || mobileErr || countryErr || genderErr) == true) {
       return false;
    } else {
        // Creating a string from input data for preview
        let dataPreview = "You've entered the following details: \n" +
                          "Full Name: " + name + "\n" +
                          "Email Address: " + email + "\n" +
                          "Mobile Number: " + mobile + "\n" +
                          "Country: " + country + "\n" +
                          "Gender: " + gender + "\n";
        if(hobbies.length) {
            dataPreview += "Hobbies: " + hobbies.join(", ");
        }
        // Display input data in a dialog box before submitting the form
        alert(dataPreview);
    }
};
</script>
</head>
<body>
<form name="contactForm" onsubmit="return validateForm()" action="/examples/actions/confirmation.php" method="post">
    <h2>Application Form</h2>
    <div class="row">
        <label>Full Name</label>
        <input type="text" name="name">
        <div class="error" id="nameErr"></div>
    </div>
    <div class="row">
        <label>Email Address</label>
        <input type="text" name="email">
        <div class="error" id="emailErr"></div>
    </div>
    <div class="row">
        <label>Mobile Number</label>
        <input type="text" name="mobile" maxlength="10">
        <div class="error" id="mobileErr"></div>
    </div>
    <div class="row">
        <label>Country</label>
        <select name="country">
            <option>Select</option>
            <option>Australia</option>
            <option>India</option>
            <option>United States</option>
            <option>United Kingdom</option>
        </select>
        <div class="error" id="countryErr"></div>
    </div>
    <div class="row">
        <label>Gender</label>
        <div class="form-inline">
            <label><input type="radio" name="gender" value="male"> Male</label>
            <label><input type="radio" name="gender" value="female"> Female</label>
        </div>
        <div class="error" id="genderErr"></div>
    </div>
    <div class="row">
        <label>Hobbies <i>(Optional)</i></label>
        <div class="form-inline">
            <label><input type="checkbox" name="hobbies[]" value="sports"> Sports</label>
            <label><input type="checkbox" name="hobbies[]" value="movies"> Movies</label>
            <label><input type="checkbox" name="hobbies[]" value="music"> Music</label>
        </div>
    </div>
    <div class="row">
        <input type="submit" value="Submit">
    </div>
</form>
</body>
</html>