Quick string hashing Javascript function

gistfile1.js

// This is a modified version of similar functions found here:
// http://stackoverflow.com/questions/7616461/generate-a-hash-from-string-in-javascript-jquery
// http://jsperf.com/hashing-strings

String.prototype.hash = function () {
    
    var res = 0,
        len = this.length;
    
    for (i = 0; i < len; i++) {
        res = res * 31 + this.charCodeAt(i);
        res = res & res;
    }
    
    return res;
}