captcha in javascript

How to make captcha in javascript

<html>

<head><title>JavaScript Captcha Example</title></head>

<body onload="generateCaptcha()">

<script>

var captcha;

function generateCaptcha() {

var a = Math.floor((Math.random() * 10));

var b = Math.floor((Math.random() * 10));

var c = Math.floor((Math.random() * 10));

var d = Math.floor((Math.random() * 10));

captcha=a.toString()+b.toString()+c.toString()+d.toString();

document.getElementById("captcha").value = captcha;

}

function check(){

var input=document.getElementById("inputText").value;

if(input==captcha){

alert("Equal");

}

else{

alert("Not Equal");

}

</script>

<input type="text" id="captcha" disabled/><br/><br/>

<input type="text" id="inputText"/><br/><br/>

<button onclick="generateCaptcha()">Refresh</button>

<button onclick="check()">Submit</button>

</body>

</html>