XSS prevention using JavaScript

xss.js

// *** Prevent Javascript attacks using Javascript...sweet! ***
// Proof of concept for output encoding user input on the client side.
// Works in IE6+

// Some user input 
var malicious_input = "<script>alert(document.cookie)</script>";

// Inserting it safely
var body = document.querySelector('body');
body.innerHTML = malicious_input.xssSafe();

// Encoding function
String.prototype.xssSafe = function() {
    var node = document.createTextNode(this);
    // using the nodeValue property allows us to use it in markup and also in element attributes
    // otherwise we get a [object Text] instead of what we actually want
    return node.nodeValue;
}


// =================================================


// Using from server-side - language agnostic
// For cases when data is added using server variables before access to JS
//
// Simply output the JS function at the start of the body tag
// and then reference it before outputting any user input. For eg:
// <?php
//    echo "<script> String.prototype.xssSafe = function() { ... } </script>";
//    ...
//    // bunch of random php markup
//    ...
//    echo "<script>document.write($malicious.xssSafe());</script>";
//    // the above is too much text, best to write a helper php function
// ?>
// Note: Markup comes out with script tags intact this way,
// doesn't seem to be causing issues with CSS rendering though.

// TO DO:
// Look into documentFragment to see if it can do the same thing more efficiently
// What happens if Javascript is disabled
// Look at why providing the value </script> with document.write gives SyntaxError in Chrome and Firefox