Base64编解码

Base64编码可用于在HTTP环境下传递较长的标识信息,多用于与服务器端的图片、音视频等传输交互。
具体格式如下：前缀＋图片内容
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0lEQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC

友情提醒：注意二进制流字符串中的反斜杠哦，绝对是个毒瘤！

转换算法：
var Base64 = {
            _keyStr: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",
            encode: function (e) {
                var t = "";
                var n, r, i, s, o, u, a;
                var f = 0;
                e = Base64._utf8_encode(e);
                while (f < e.length) {
                    n = e.charCodeAt(f++);
                    r = e.charCodeAt(f++);
                    i = e.charCodeAt(f++);
                    s = n >> 2;
                    o = (n & 3) << 4 | r >> 4;
                    u = (r & 15) << 2 | i >> 6;
                    a = i & 63;
                    if (isNaN(r)) {
                        u = a = 64;
                    } else if (isNaN(i)) {
                        a = 64;
                    }
                    t = t + this._keyStr.charAt(s) + this._keyStr.charAt(o) + this._keyStr.charAt(u) + this._keyStr.charAt(a);
                }
                return t
            },
            decode: function (e) {
                var t = "";
                var n, r, i;
                var s, o, u, a;
                var f = 0;
                e = e.replace(/[^A-Za-z0-9\+\/\=]/g, "");
                while (f < e.length) {
                    s = this._keyStr.indexOf(e.charAt(f++));
                    o = this._keyStr.indexOf(e.charAt(f++));
                    u = this._keyStr.indexOf(e.charAt(f++));
                    a = this._keyStr.indexOf(e.charAt(f++));
                    n = s << 2 | o >> 4;
                    r = (o & 15) << 4 | u >> 2;
                    i = (u & 3) << 6 | a;
                    t = t + String.fromCharCode(n);
                    if (u != 64) {
                        t = t + String.fromCharCode(r);
                    }
                    if (a != 64) {
                        t = t + String.fromCharCode(i);
                    }
                }
                t = Base64._utf8_decode(t);
                return t;
            },
            _utf8_encode: function (e) {
                e = e.replace(/\r\n/g, "\n");
                var t = "";
                for (var n = 0; n < e.length; n++) {
                    var r = e.charCodeAt(n);
                    if (r < 128) {
                        t += String.fromCharCode(r);
                    } else if (r > 127 && r < 2048) {
                        t += String.fromCharCode(r >> 6 | 192);
                        t += String.fromCharCode(r & 63 | 128)
                    } else {
                        t += String.fromCharCode(r >> 12 | 224);
                        t += String.fromCharCode(r >> 6 & 63 | 128);
                        t += String.fromCharCode(r & 63 | 128);
                    }
                }
                return t;
            },
            _utf8_decode: function (e) {
                var t = "";
                var n = 0;
                var r = c1 = c2 = 0;
                while (n < e.length) {
                    r = e.charCodeAt(n);
                    if (r < 128) {
                        t += String.fromCharCode(r);
                        n++;
                    } else if (r > 191 && r < 224) {
                        c2 = e.charCodeAt(n + 1);
                        t += String.fromCharCode((r & 31) << 6 | c2 & 63);
                        n += 2;
                    } else {
                        c2 = e.charCodeAt(n + 1);
                        c3 = e.charCodeAt(n + 2);
                        t += String.fromCharCode((r & 15) << 12 | (c2 & 63) << 6 | c3 & 63);
                        n += 3;
                    }
                }
                return t;
            }
        };