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JS Form validation

For the past decades, web forms have become an essential part of web applications. It is mostly used to collect users' data such as name, email address, location, age, and so on. However, it is actually possible that some users might not enter the data you've expected. So the need to save bandwidth and avoid unnecessary strain on your server resources you can validate the form data on the client-side (i.e user's system) using JavaScript before passing it onto the web server for further processing.

The client-side validation most times helps create a better user experience since it is faster because validation occurs within the user's web browser, while server-side validation occurs on the server, which requires the user's input to be first submitted and sent to the server before validation occurs. Also, the user has to wait for the server's response to know what exactly went wrong.

The subsequent sections will enable us to take a closer look at how to perform JavaScript form validation and handle any input errors found.

Note: The client-side validation is never a substitute or alternative for server-side validation. Therefore, you are advised to always validate form data on the server-side even when they are already validated on the client-side, as users can disable JavaScript in their browser.

Form Validation with JavaScript

In this case, the form validation process consists of two parts i.e. the required fields validation which is performed to ensure that all the mandatory fields are filled in, and the data format validation which is performed to ensure that the type and format of the data entered in the form are valid.

The example below will show us how this actually works.

Creating the HTML Form

You can create a simple HTML form that we will validate on the client-side using JavaScript when the user clicks on the submit button. however, we shall create an HTML file named ‘application-form.html’ and place the following code in it, then save it somewhere on our system.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Simple HTML Form</title>
    <link rel="stylesheet" href="style.css">
    <script src="validator.js"></script>
</head>
<body>
<form name="contactForm" onsubmit="return validateForm()" action="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>

Building the Form Validation Script

Here we are going to create a JavaScript file that holds our complete validation script.

So, we shall create a JavaScript file named ‘validator.js’ and place the following code inside it, then save it at the same location where we saved the previous HTML file. Please, carefully check through each line of the following example code to understand how JavaScript validation works:

// 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 
    var name = document.contactForm.name.value;
    var email = document.contactForm.email.value;
    var mobile = document.contactForm.mobile.value;
    var country = document.contactForm.country.value;
    var gender = document.contactForm.gender.value;
    var hobbies = [];
    var checkboxes = document.getElementsByName("hobbies[]");
    for(var 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
    var nameErr = emailErr = mobileErr = countryErr = genderErr = true;
    
    // Validate name
    if(name == "") {
        printError("nameErr", "Please enter your name");
    } else {
        var regex = /^[a-zA-Zs]+$/;                
        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
        var 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 {
        var 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
        var 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);
    }
};

Here in JavaScript, the value of an individual form field can be accessed and retrieved by using the syntax ‘document.formName.fieldName.value’ or ‘document.getElementsByName(name).value’. however, to get the values from a form field that supports multiple selections, like a group of checkboxes, you need to utilize the loop statement as shown in the example above (in lines no-14 to 21).

More so, to check if the format of input data is correct or if we’ve not used the regular expressions. The technique is an effective one in validating the user’s inputs.

Also, the script above will display the data entered by the user in an alert dialog box for preview purposes before submitting the form to the web server.

Tip: But, we can validate email format using regular expression. However, a user might enter an email that is correctly formatted but does not exist. Hence, the need for authentic email validation; send a confirmation email to the user and verify whether the email exists or not.

Adding Style Sheet to Beautify the Form

Last but not the least, create the file named ‘style.css’ and place the code in it, thereafter save it also at the same location where you've saved the previous two files. So, these are the style rules you can use to beautify your form.

body {
    font-size: 16px;
    background: #f9f9f9;
    font-family: "Segoe UI", "Helvetica Neue", Arial, sans-serif;
}
h2 {
    text-align: center;
    text-decoration: underline;
}
form {
    width: 300px;
    background: #fff;
    padding: 15px 40px 40px;
    border: 1px solid #ccc;
    margin: 50px auto 0;
    border-radius: 5px;
}
label {
    display: block;
    margin-bottom: 5px
}
label i {
    color: #999;
    font-size: 80%;
}
input, select {
    border: 1px solid #ccc;
    padding: 10px;
    display: block;
    width: 100%;
    box-sizing: border-box;
    border-radius: 2px;
}
.row {
    padding-bottom: 10px;
}
.form-inline {
    border: 1px solid #ccc;
    padding: 8px 10px 4px;
    border-radius: 2px;
}
.form-inline label, .form-inline input {
    display: inline-block;
    width: auto;
    padding-right: 15px;
}
.error {
    color: red;
    font-size: 90%;
}
input[type="submit"] {
    font-size: 110%;
    font-weight: 100;
    background: #006dcc;
    border-color: #016BC1;
    box-shadow: 0 3px 0 #0165b6;
    color: #fff;
    margin-top: 10px;
    cursor: pointer;
}
input[type="submit"]:hover {
    background: #0165b6;
}

After that, you are almost done, now open the ‘application-form.html’ file in a web browser and try to fill in some data, and see how the script responds when invalid data is entered in a form field.

For a better understanding of server-side validation, please check out the tutorial on PHP form validation

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