微信返回的流式数据是 arrayBuffer 格式的

尝试了第一种办法，这种只可以在h5页面上使用，在真机上 decodeURIComponent 这个方法会报一些转化的错误

let resStr = '', chunk = 8 * 1024, i;
const unitStr = new Uint8Array(SSEUnit8ArrayStr[0])
for (i = 0; i < unitStr.length / chunk; i++) {
  resStr += String.fromCharCode.apply(null, unitStr.slice(i * chunk, (i + 1) * chunk))
}
resStr += String.fromCharCode.apply(null, unitStr.slice(i * chunk)).replace(/%/g,'%25')
resStr = decodeURIComponent(escape(resStr))

尝试了使用第二种方式（完美解决）

const data = Utf8ArrayToStr(new Uint8Array(str))
console.log(data)

const Utf8ArrayToStr = (array) => {
  var out, i, len, c;
  var char2, char3;
  out = "";
  len = array.length;
  i = 0;
  while(i < len) {
  c = array[i++];
  switch(c >> 4)
  { 
    case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
      out += String.fromCharCode(c);
      break;
    case 12: case 13:
      char2 = array[i++];
      out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));
      break;
    case 14:
      char2 = array[i++];
      char3 = array[i++];
      out += String.fromCharCode(((c & 0x0F) << 12) | ((char2 & 0x3F) << 6) | ((char3 & 0x3F) << 0));
      break;
    }
  }
  return out;
}