js的base64和文本字符串之间互转

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/** Convert data (an array of integers) to a Base64 string. */    var toBase64Table = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';    var base64Pad = '=';    function toBase64(data) {        var result = '';        var length = data.length;        var i;        // Convert every three bytes to 4 ascii characters.                                                         for (i = 0; i < (length - 2); i += 3) {            result += toBase64Table[data.charCodeAt(i) >> 2];            result += toBase64Table[((data.charCodeAt(i) & 0x03) << 4) + (data.charCodeAt(i + 1) >> 4)];            result += toBase64Table[((data.charCodeAt(i + 1) & 0x0f) << 2) + (data.charCodeAt(i + 2) >> 6)];            result += toBase64Table[data.charCodeAt(i + 2) & 0x3f];        }        // Convert the remaining 1 or 2 bytes, pad out to 4 characters.                                             if (length % 3) {            i = length - (length % 3);            result += toBase64Table[data.charCodeAt(i) >> 2];            if ((length % 3) == 2) {                result += toBase64Table[((data.charCodeAt(i) & 0x03) << 4) + (data.charCodeAt(i + 1) >> 4)];                result += toBase64Table[(data.charCodeAt(i + 1) & 0x0f) << 2];                result += base64Pad;            } else {                result += toBase64Table[(data.charCodeAt(i) & 0x03) << 4];                result += base64Pad + base64Pad;            }        }        return result;    }    /** Convert Base64 data to a string */    var toBinaryTable = [-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, 0, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,        15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,        41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1    ];    function base64ToString(data) {        var result = '';        var leftbits = 0; // number of bits decoded, but yet to be appended          var leftdata = 0; // bits decoded, but yet to be appended         // Convert one by one.                                                                                     for (var i = 0; i < data.length; i++) {            var c = toBinaryTable[data.charCodeAt(i) & 0x7f];            var padding = (data.charCodeAt(i) == base64Pad.charCodeAt(0));            // Skip illegal characters and whitespace              if (c == -1) continue;            // Collect data into leftdata, update bitcount              leftdata = (leftdata << 6) | c;            leftbits += 6;            // If we have 8 or more bits, append 8 bits to the result             if (leftbits >= 8) {                leftbits -= 8;                // Append if not padding.                 if (!padding)                    result += String.fromCharCode((leftdata >> leftbits) & 0xff);                leftdata &= (1 << leftbits) - 1;            }        }        // If there are any bits left, the base64 string was corrupted                                              if (leftbits)            throw Components.Exception('Corrupted base64 string');        return result;    }    //toBase64() 将字符串转换为base64    //base64ToString()  将base64 转换为字符串
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