Escape String as per EcmaScript spec

escape.js

/*
 * implementation of the alorithm from EcmaScript spec in section 15.1.2.4
 * http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf
 */
(function(global) {
    var allowed = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./,';
    global.escapeString = function(str) {
        str = str.toString();
        var len = str.length, R = '', k = 0, S, chr, ord;
        while(k < len) {
            chr = str[k];
            if (allowed.indexOf(chr) != -1) {
                S = chr;
            } else {
                ord = str.charCodeAt(k);
                if (ord < 256) {
                    S = '%' + ("00" + ord.toString(16)).toUpperCase().slice(-2);
                } else {
                    S = '%u' + ("0000" + ord.toString(16)).toUpperCase().slice(-4);
                }
            }
            R += S;
            k++;
        }
        return R;
    };

})(typeof window == 'undefined' ? global : window);