How to Create Captcha using JavaScript

<!DOCTYPE html>
<html>
<head>
    <title>Using JavaScript how to Create Captcha</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 str = new Array(4).join().replace(/(.|$)/g, function () { return ((Math.random() * 36) | 0).toString(36)[Math.random() < .5 ? "toString" : "toUpperCase"](); });
            var captchaCode = str + chr1 + ' ' + chr2 + ' ' + chr3;
            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('');
        }
    </script>

</head>
<body onload="GenerateCaptcha();">
    <div style="border: 2px solid gray; width: 700px;">
        <h2>Generating Captcha Demo</h2>

        Enter the Captcha Text:
        <input type="text" id="txtCompare" />
        <input type="text" id="txtCaptcha" style="text-align: center; border: none; font-weight: bold; font-size: 20px; font-family: Modern" />
        <input type="button" id="btnrefresh" value="Refresh" onclick="GenerateCaptcha();" />
        <input id="btnValid" type="button" value="Check" onclick="alert(ValidCaptcha());" />

        <br />
        <br />
    </div>
</body>
</html>