// needed to create bytes array

var encoder = new TextEncoder();

// string length, multiple of 4 - vary to get different results

var n = 8;

var arr = encoder.encode(Array((n / 4) | 0).fill('abcd'));

var decoder = new TextDecoder();

function decodeNative(arr) {

  return decoder.decode(arr);

}

​

function decoderJs(buffer) {

  var start = 0;

  var end = buffer.byteLength;

​

  if (end - start < 1) {

    return "";

  }

​

  var str = "";

  for (var i = start; i < end;) {

    var t = buffer[i++];

​

    if (t <= 0x7F) {

      str += String.fromCharCode(t);

    } else if (t >= 0xC0 && t < 0xE0) {

      str += String.fromCharCode((t & 0x1F) << 6 | buffer[i++] & 0x3F);

    } else if (t >= 0xE0 && t < 0xF0) {

      str += String.fromCharCode((t & 0xF) << 12 | (buffer[i++] & 0x3F) << 6 | buffer[i++] & 0x3F);

    } else if (t >= 0xF0) {

      var t2 = ((t & 7) << 18 | (buffer[i++] & 0x3F) << 12 | (buffer[i++] & 0x3F) << 6 | buffer[i++] & 0x3F) - 0x10000;

      str += String.fromCharCode(0xD800 + (t2 >> 10));

      str += String.fromCharCode(0xDC00 + (t2 & 0x3FF));

    }

  }

  return str;

}