微信小程序arrayBuffer转base64

参考链接：https://blog.csdn.net/weixin_44116302/article/details/123219369 // arrayBuffer转base64 const arrayBufferToBase64 = (buffer, contentType) => {   // 用以下方法防止内存越界   let str = '';   const bytes = new Uint8Array( buffer );   const len = bytes.byteLength;   for (let i = 0; i < len; i++) {     str += String.fromCharCode(bytes[ i ]);   }   // ArrayBuffer过大时会报：Maximum call stack size exceeded。该错误是String.fromCharCode接收的参数长度超限引起的。   // const str = String.fromCharCode(...new Uint8Array(buffer));   return 'data:' + contentType + `;base64,${btoa(str)}`; }  

关键点btoa的实现
因为该函数，在浏览器中已经实现，所以更多使用在小程序及node环境中。所以总体以module.exports进行方法的输出，以require形式进行引入。

输出方法

module.exports = {
btoa: ...,
atob: ...
}

引入文件

const { btoa } = require('./base64')

新建base64.js

var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
a256 = '',
r64 = [256],
r256 = [256],
i = 0;

var UTF8 = {

/**
* Encode multi-byte Unicode string into utf-8 multiple single-byte characters
* (BMP / basic multilingual plane only)
*
* Chars in range U+0080 - U+07FF are encoded in 2 chars, U+0800 - U+FFFF in 3 chars
*
* @param {String} strUni Unicode string to be encoded as UTF-8
* @returns {String} encoded string
*/
encode: function (strUni) {
// use regular expressions & String.replace callback function for better efficiency
// than procedural approaches
var strUtf = strUni.replace(/[\u0080-\u07ff]/g, // U+0080 - U+07FF => 2 bytes 110yyyyy, 10zzzzzz
function (c) {
var cc = c.charCodeAt(0);
return String.fromCharCode(0xc0 | cc >> 6, 0x80 | cc & 0x3f);
})
.replace(/[\u0800-\uffff]/g, // U+0800 - U+FFFF => 3 bytes 1110xxxx, 10yyyyyy, 10zzzzzz
function (c) {
var cc = c.charCodeAt(0);
return String.fromCharCode(0xe0 | cc >> 12, 0x80 | cc >> 6 & 0x3F, 0x80 | cc & 0x3f);
});
return strUtf;
},

/**
* Decode utf-8 encoded string back into multi-byte Unicode characters
*
* @param {String} strUtf UTF-8 string to be decoded back to Unicode
* @returns {String} decoded string
*/
decode: function (strUtf) {
// note: decode 3-byte chars first as decoded 2-byte strings could appear to be 3-byte char!
var strUni = strUtf.replace(/[\u00e0-\u00ef][\u0080-\u00bf][\u0080-\u00bf]/g, // 3-byte chars
function (c) { // (note parentheses for precence)
var cc = ((c.charCodeAt(0) & 0x0f) << 12) | ((c.charCodeAt(1) & 0x3f) << 6) | (c.charCodeAt(2) & 0x3f);
return String.fromCharCode(cc);
})
.replace(/[\u00c0-\u00df][\u0080-\u00bf]/g, // 2-byte chars
function (c) { // (note parentheses for precence)
var cc = (c.charCodeAt(0) & 0x1f) << 6 | c.charCodeAt(1) & 0x3f;
return String.fromCharCode(cc);
});
return strUni;
}
};

while (i < 256) {
var c = String.fromCharCode(i);
a256 += c;
r256[i] = i;
r64[i] = b64.indexOf(c);
++i;
}

function code(s, discard, alpha, beta, w1, w2) {
s = String(s);
var buffer = 0,
i = 0,
length = s.length,
result = '',
bitsInBuffer = 0;

while (i < length) {
var c = s.charCodeAt(i);
c = c < 256 ? alpha[c] : -1;

buffer = (buffer << w1) + c;
bitsInBuffer += w1;

while (bitsInBuffer >= w2) {
bitsInBuffer -= w2;
var tmp = buffer >> bitsInBuffer;
result += beta.charAt(tmp);
buffer ^= tmp << bitsInBuffer;
}
++i;
}
if (!discard && bitsInBuffer > 0) result += beta.charAt(buffer << (w2 - bitsInBuffer));
return result;
}

var Plugin = function (dir, input, encode) {
return input ? Plugin[dir](input, encode) : dir ? null : this;
};

Plugin.btoa = Plugin.encode = function (plain, utf8encode) {
plain = Plugin.raw === false || Plugin.utf8encode || utf8encode ? UTF8.encode(plain) : plain;
plain = code(plain, false, r256, b64, 8, 6);
return plain + '===='.slice((plain.length % 4) || 4);
};

Plugin.atob = Plugin.decode = function (coded, utf8decode) {
coded = coded.replace(/[^A-Za-z0-9\+\/\=]/g, "");
coded = String(coded).split('=');
var i = coded.length;
do {
--i;
coded[i] = code(coded[i], true, r64, a256, 6, 8);
} while (i > 0);
coded = coded.join('');
return Plugin.raw === false || Plugin.utf8decode || utf8decode ? UTF8.decode(coded) : coded;
};
module.exports = {
btoa: Plugin.btoa,
atob: Plugin.atob
}

整个流程如下：

得到一个ArrayBuffer —> 转成类型化数组以正常读取(Uint8Array) --> 转成普通字符串(String.fromCharCode) --> 转成base64字符串(btoa)

const { btoa } = require('./base64')
const arrayBufferToBase64Img = (buffer) => {
const str = String.fromCharCode(...new Uint8Array(buffer));
return `data:image/jpeg;base64,${btoa(str)}`;
}