do one-time test to return a typed array where supported.

gistfile1.js

var stringToByteArray = (function(){
var func = function (str) {
    var b = [], i, unicode;
    for(i = 0; i < str.length; i++) {
        unicode = str.charCodeAt(i);
        // 0x00000000 - 0x0000007f -> 0xxxxxxx
        if (unicode <= 0x7f) {
            b.push(unicode);
        // 0x00000080 - 0x000007ff -> 110xxxxx 10xxxxxx
        } else if (unicode <= 0x7ff) {
            b.push((unicode >> 6) | 0xc0);
            b.push((unicode & 0x3F) | 0x80);
        // 0x00000800 - 0x0000ffff -> 1110xxxx 10xxxxxx 10xxxxxx
        } else if (unicode <= 0xffff) {
            b.push((unicode >> 12) | 0xe0);
            b.push(((unicode >> 6) & 0x3f) | 0x80);
            b.push((unicode & 0x3f) | 0x80);
        // 0x00010000 - 0x001fffff -> 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
        } else {
            b.push((unicode >> 18) | 0xf0);
            b.push(((unicode >> 12) & 0x3f) | 0x80);
            b.push(((unicode >> 6) & 0x3f) | 0x80);
            b.push((unicode & 0x3f) | 0x80);
        }
    }

    return b;
};
var typed = window.Uint8ClampedArray || window.Uint8Array;
return typed ? function(str){ return new typed(func(str)); } : func;
})();