Base64编码和解码实现

function Base64() {
    // private property
    _keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    // public method for encoding
    this.encode = function (input) {
        var output = "";
        var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
        var i = ;
        input = _utf8_encode(input);
        while (i < input.length) {
            chr1 = input.charCodeAt(i++);
            chr2 = input.charCodeAt(i++);
            chr3 = input.charCodeAt(i++);
            enc1 = chr1 >> ;
            enc2 = ((chr1 & ) << ) | (chr2 >> );
            enc3 = ((chr2 & ) << ) | (chr3 >> );
            enc4 = chr3 & ;
            if (isNaN(chr2)) {
                enc3 = enc4 = ;
            } else if (isNaN(chr3)) {
                enc4 = ;
            }
            output = output +
                    _keyStr.charAt(enc1) + _keyStr.charAt(enc2) +
                    _keyStr.charAt(enc3) + _keyStr.charAt(enc4);
        }
        return output;
    };
    // public method for decoding
    this.decode = function (input) {
        var output = "";
        var chr1, chr2, chr3;
        var enc1, enc2, enc3, enc4;
        var i = ;
        input = input.replace(/[^A-Za-z0-\+\/\=]/g, "");
        while (i < input.length) {
            enc1 = _keyStr.indexOf(input.charAt(i++));
            enc2 = _keyStr.indexOf(input.charAt(i++));
            enc3 = _keyStr.indexOf(input.charAt(i++));
            enc4 = _keyStr.indexOf(input.charAt(i++));
            chr1 = (enc1 << ) | (enc2 >> );
            chr2 = ((enc2 & ) << ) | (enc3 >> );
            chr3 = ((enc3 & ) << ) | enc4;
            output = output + String.fromCharCode(chr1);
            if (enc3 != ) {
                output = output + String.fromCharCode(chr2);
            }
            if (enc4 != ) {
                output = output + String.fromCharCode(chr3);
            }
        }
        output = _utf8_decode(output);
        return output;
    };
    // private method for UTF-8 encoding
    _utf8_encode = function (string) {
        string = string.replace(/\r\n/g, "\n");
        var utftext = "";
        for (var n = ; n < string.length; n++) {
            var c = string.charCodeAt(n);
            if (c < ) {
                utftext += String.fromCharCode(c);
            } else if ((c > ) && (c < )) {
                utftext += String.fromCharCode((c >> ) | );
                utftext += String.fromCharCode((c & ) | );
            } else {
                utftext += String.fromCharCode((c >> ) | );
                utftext += String.fromCharCode(((c >> ) & ) | );
                utftext += String.fromCharCode((c & ) | );
            }
        }
        return utftext;
    };
    // private method for UTF-8 decoding
    _utf8_decode = function (utftext) {
        var string = "";
        var i = ;
        var c = c1 = c2 = ;
        while (i < utftext.length) {
            c = utftext.charCodeAt(i);
            if (c < ) {
                string += String.fromCharCode(c);
                i++;
            } else if ((c > ) && (c < )) {
                c2 = utftext.charCodeAt(i + );
                string += String.fromCharCode(((c & ) << ) | (c2 & ));
                i += ;
            } else {
                c2 = utftext.charCodeAt(i + );
                c3 = utftext.charCodeAt(i + );
                string += String.fromCharCode(((c & ) << ) | ((c2 & ) << ) | (c3 & ));
                i += ;
            }
        }
        return string;
    }
}

//调用和示例显示
var base = new Base64();
var str = document.getElementById("p1").innerHTML;
str = base.decode(str);  //解码
document.getElementById("p1").innerHTML = str;