U = function (a, b) {
        if (!a) {
            return '';
        }
        b = b || 'x';
        var c = '';
        var d = 0;
        var e;

        for (d; d < a.length; d += 1) a.charCodeAt(d) >= 55296 && a.charCodeAt(d) <= 56319 ? (e = (65536 + 1024 * (Number(a.charCodeAt(d)) - 55296) + Number(a.charCodeAt(d + 1)) - 56320).toString(16), d += 1) : e = a.charCodeAt(d).toString(16),
        c += b + e;

        return c.substr(b.length);
    }
@Keyboard.js
@author zhangxinxu
@version
Created: 17-06-13