btoa() 只支持从 String.fromCodePoint(0) 到 String.fromCodePoint(255) 的字符。对于代码点大于 256 的 Base64 字符，您需要在编码/解码之前和之后进行处理。
这时就变得棘手了...
每个可能的符号都排列在 Unicode 表中。Unicode 表分为不同的平面（语言、数学符号等...）。一个平面中的每个符号都有一个唯一的代码点编号。理论上，这个编号可以任意地变大。
计算机以字节（8 位，十六进制 0x00 - 0xff，二进制 00000000 - 11111111，十进制 0 - 255）存储数据。这个范围通常用于保存基本字符（Latin1 范围）。
对于代码点大于 255 的字符，存在不同的编码方式。JavaScript 使用每个符号 16 位（UTF-16），称为 DOMString 的字符串。Unicode 可以处理高达 0x10fffff 的代码点。也就是说，必须存在一种方法来将几个位存储在几个单元格中。
String.fromCodePoint(0x10000).length == 2
UTF-16 使用代理对在两个 16 位单元格中存储 20 位。第一个更高的代理以 110110xxxxxxxxxx 开头，第二个较低的代理以 110111xxxxxxxxxx 开头。Unicode 为此保留了自己的平面：https://unicode-table.com/de/#high-surrogates
为了以字节（Latin1 范围）存储字符，标准化过程使用 UTF-8。
很抱歉，但我认为没有其他办法来自己实现这个函数。
function stringToUTF8(str)
{
let bytes = [];
for(let character of str)
{
let code = character.codePointAt(0);
if(code <= 127)
{
let byte1 = code;
bytes.push(byte1);
}
else if(code <= 2047)
{
let byte1 = 0xC0 | (code >> 6);
let byte2 = 0x80 | (code & 0x3F);
bytes.push(byte1, byte2);
}
else if(code <= 65535)
{
let byte1 = 0xE0 | (code >> 12);
let byte2 = 0x80 | ((code >> 6) & 0x3F);
let byte3 = 0x80 | (code & 0x3F);
bytes.push(byte1, byte2, byte3);
}
else if(code <= 2097151)
{
let byte1 = 0xF0 | (code >> 18);
let byte2 = 0x80 | ((code >> 12) & 0x3F);
let byte3 = 0x80 | ((code >> 6) & 0x3F);
let byte4 = 0x80 | (code & 0x3F);
bytes.push(byte1, byte2, byte3, byte4);
}
}
return bytes;
}
function utf8ToString(bytes, fallback)
{
let valid = undefined;
let codePoint = undefined;
let codeBlocks = [0, 0, 0, 0];
let result = "";
for(let offset = 0; offset < bytes.length; offset++)
{
let byte = bytes[offset];
if((byte & 0x80) == 0x00)
{
codeBlocks[0] = byte & 0x7F;
codePoint = codeBlocks[0];
}
else if((byte & 0xE0) == 0xC0)
{
codeBlocks[0] = byte & 0x1F;
byte = bytes[++offset];
if(offset >= bytes.length || (byte & 0xC0) != 0x80) { valid = false; break; }
codeBlocks[1] = byte & 0x3F;
codePoint = (codeBlocks[0] << 6) + codeBlocks[1];
}
else if((byte & 0xF0) == 0xE0)
{
codeBlocks[0] = byte & 0xF;
for(let blockIndex = 1; blockIndex <= 2; blockIndex++)
{
byte = bytes[++offset];
if(offset >= bytes.length || (byte & 0xC0) != 0x80) { valid = false; break; }
codeBlocks[blockIndex] = byte & 0x3F;
}
if(valid === false) { break; }
codePoint = (codeBlocks[0] << 12) + (codeBlocks[1] << 6) + codeBlocks[2];
}
else if((byte & 0xF8) == 0xF0)
{
codeBlocks[0] = byte & 0x7;
for(let blockIndex = 1; blockIndex <= 3; blockIndex++)
{
byte = bytes[++offset];
if(offset >= bytes.length || (byte & 0xC0) != 0x80) { valid = false; break; }
codeBlocks[blockIndex] = byte & 0x3F;
}
if(valid === false) { break; }
codePoint = (codeBlocks[0] << 18) + (codeBlocks[1] << 12) + (codeBlocks[2] << 6) + (codeBlocks[3]);
}
else
{
valid = false; break;
}
result += String.fromCodePoint(codePoint);
}
if(valid === false)
{
if(!fallback)
{
throw new TypeError("Malformed utf-8 encoding.");
}
result = "";
for(let offset = 0; offset != bytes.length; offset++)
{
result += String.fromCharCode(bytes[offset] & 0xFF);
}
}
return result;
}
function decodeBase64(text, binary)
{
if(/[^0-9a-zA-Z\+\/\=]/.test(text)) { throw new TypeError("The string to be decoded contains characters outside of the valid base64 range."); }
let codePointA = 'A'.codePointAt(0);
let codePointZ = 'Z'.codePointAt(0);
let codePointa = 'a'.codePointAt(0);
let codePointz = 'z'.codePointAt(0);
let codePointZero = '0'.codePointAt(0);
let codePointNine = '9'.codePointAt(0);
let codePointPlus = '+'.codePointAt(0);
let codePointSlash = '/'.codePointAt(0);
function getCodeFromKey(key)
{
let keyCode = key.codePointAt(0);
if(keyCode >= codePointA && keyCode <= codePointZ)
{
return keyCode - codePointA;
}
else if(keyCode >= codePointa && keyCode <= codePointz)
{
return keyCode + 26 - codePointa;
}
else if(keyCode >= codePointZero && keyCode <= codePointNine)
{
return keyCode + 52 - codePointZero;
}
else if(keyCode == codePointPlus)
{
return 62;
}
else if(keyCode == codePointSlash)
{
return 63;
}
return undefined;
}
let codes = Array.from(text).map(character => getCodeFromKey(character));
let bytesLength = Math.ceil(codes.length / 4) * 3;
if(codes[codes.length - 2] == undefined) { bytesLength = bytesLength - 2; } else if(codes[codes.length - 1] == undefined) { bytesLength--; }
let bytes = new Uint8Array(bytesLength);
for(let offset = 0, index = 0; offset < bytes.length;)
{
let code1 = codes[index++];
let code2 = codes[index++];
let code3 = codes[index++];
let code4 = codes[index++];
let byte1 = (code1 << 2) | (code2 >> 4);
let byte2 = ((code2 & 0xf) << 4) | (code3 >> 2);
let byte3 = ((code3 & 0x3) << 6) | code4;
bytes[offset++] = byte1;
bytes[offset++] = byte2;
bytes[offset++] = byte3;
}
if(binary) { return bytes; }
return utf8ToString(bytes, true);
}
function encodeBase64(bytes) {
if (bytes === undefined || bytes === null) {
return '';
}
if (bytes instanceof Array) {
bytes = bytes.filter(item => {
return Number.isFinite(item) && item >= 0 && item <= 255;
});
}
if (
!(
bytes instanceof Uint8Array ||
bytes instanceof Uint8ClampedArray ||
bytes instanceof Array
)
) {
if (typeof bytes === 'string') {
const str = bytes;
bytes = Array.from(unescape(encodeURIComponent(str))).map(ch =>
ch.codePointAt(0)
);
} else {
throw new TypeError('bytes must be of type Uint8Array or String.');
}
}
const keys = [
'A',
'B',
'C',
'D',
'E',
'F',
'G',
'H',
'I',
'J',
'K',
'L',
'M',
'N',
'O',
'P',
'Q',
'R',
'S',
'T',
'U',
'V',
'W',
'X',
'Y',
'Z',
'a',
'b',
'c',
'd',
'e',
'f',
'g',
'h',
'i',
'j',
'k',
'l',
'm',
'n',
'o',
'p',
'q',
'r',
's',
't',
'u',
'v',
'w',
'x',
'y',
'z',
'0',
'1',
'2',
'3',
'4',
'5',
'6',
'7',
'8',
'9',
'+',
'/'
];
const fillKey = '=';
let byte1;
let byte2;
let byte3;
let sign1 = ' ';
let sign2 = ' ';
let sign3 = ' ';
let sign4 = ' ';
let result = '';
for (let index = 0; index < bytes.length; ) {
let fillUpAt = 0;
byte1 = bytes[index++];
byte2 = bytes[index++];
byte3 = bytes[index++];
if (byte2 === undefined) {
byte2 = 0;
fillUpAt = 2;
}
if (byte3 === undefined) {
byte3 = 0;
if (!fillUpAt) {
fillUpAt = 3;
}
}
sign1 = keys[byte1 >> 2];
sign2 = keys[((byte1 & 0x3) << 4) + (byte2 >> 4)];
sign3 = keys[((byte2 & 0xf) << 2) + (byte3 >> 6)];
sign4 = keys[byte3 & 0x3f];
if (fillUpAt > 0) {
if (fillUpAt <= 2) {
sign3 = fillKey;
}
if (fillUpAt <= 3) {
sign4 = fillKey;
}
}
result += sign1 + sign2 + sign3 + sign4;
if (fillUpAt) {
break;
}
}
return result;
}
let base64 = encodeBase64("\u{1F604}");
let str = decodeBase64(base64);
console.log("base64", base64);
console.log("str", str);
document.body.innerText = str;
如何使用它：decodeBase64(encodeBase64("\u{1F604}"))
示例：https://jsfiddle.net/qrLadeb8/
