Simple JavaScript Captcha Example (Client Side Captcha)

Captcha is a technique to protect web forms from auto submission. Hackers use various tools to insert spam records in to your business. Validating Captcha is a proof that the user is a real human. Today inside this growing network many hackers are sitting around the world. They have several tools to hack your blog or business websites.

Where we need JavaScript Captcha?

Let’s talk about a simple login form. In a login form basically we have 2 input fields (username & password) with 1 submit button. To hack if we will pass random strings to username & password using a program there can be a chance in some point of time submit will successful. While manually it is quite difficult. Another scenario where let you have a signup form. If you are not taking protection 1000+ spammers can signup per day. Which is practically brings difficulties in maintainance. That’s why JavaScript Captcha. Captcha protects our forms from illegal submission.

Captcha is generating a code in the format of an image or string. Something like “DSERTT” or “D9K22A” or “325889”. User need to provide Captcha code before submitting a Form online. Here my intention to tell you adding Captcha to reduce the chance of unnatural form submissions. Captcha can be created at both the end server-side and client side. Compare to server-side captcha client-side captcha is much good for your websites. Client-side captcha reduce network load as well as performance rich.

In this example I created a Simple JavaScript Captcha. Client side Captcha helps to reduce http requests. It gives better performance compair to other type of Captcha techniques.

In the below code using math class I am generating 7 numeric characters randomly. Concatenating them with space in-between. Then assigning this value to txtCaptcha value. While user entering the text to txtCaptcha I am comparing these two values. If it is equal I am returning true or else returning false.

Simple JavaScript Captcha Example

<!DOCTYPE html>
<title>Simple JavaScript Captcha Example</title>
<script type="text/javascript">
/* Function to Generat Captcha */
function GenerateCaptcha() {
var chr1 = Math.ceil(Math.random() * 10)+ '';
var chr2 = Math.ceil(Math.random() * 10)+ '';
var chr3 = Math.ceil(Math.random() * 10)+ '';
var chr4 = Math.ceil(Math.random() * 10)+ '';
var chr5 = Math.ceil(Math.random() * 10)+ '';
var chr6 = Math.ceil(Math.random() * 10)+ '';
var chr7 = Math.ceil(Math.random() * 10)+ '';

var captchaCode = chr1 + ' ' + chr2 + ' ' + chr3 + ' ' + chr4 + ' ' + chr5 + ' '+ chr6 + ' ' + chr7;
document.getElementById("txtCaptcha").value = captchaCode

/* Validating Captcha Function */
function ValidCaptcha() {
var str1 = removeSpaces(document.getElementById('txtCaptcha').value);
var str2 = removeSpaces(document.getElementById('txtCompare').value);

if (str1 == str2) return true;
return false;

/* Remove spaces from Captcha Code */
function removeSpaces(string) {
return string.split(' ').join('');
<body onload="GenerateCaptcha();">
<h2>Generating Captcha Demo</h2>
<input type="text" id="txtCaptcha" style="text-align: center; border: none; font-weight: bold; font-family: Modern" />
<input type="button" id="btnrefresh" value="Refresh" onclick="GenerateCaptcha();" />
<input type="text" id="txtCompare" />
<input id="btnValid" type="button" value="Check" onclick="alert(ValidCaptcha());" />

Captch helps to protect your websites from promotional spam, registration spam, and data scraping.