实现utf8解码

function utf8_decode(str_data) {
  var tmp_arr = [],
    i = 0,
    ac = 0,
    c1 = 0,
    c2 = 0,
    c3 = 0;
  str_data += "";
  while (i < str_data.length) {
    c1 = str_data.charCodeAt(i);
    if (c1 < 128) {
      tmp_arr[ac++] = String.fromCharCode(c1);
      i++;
    } else if (c1 > 191 && c1 < 224) {
      c2 = str_data.charCodeAt(i + 1);
      tmp_arr[ac++] = String.fromCharCode(((c1 & 31) << 6) | (c2 & 63));
      i += 2;
    } else {
      c2 = str_data.charCodeAt(i + 1);
      c3 = str_data.charCodeAt(i + 2);
      tmp_arr[ac++] = String.fromCharCode(
        ((c1 & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63)
      );
      i += 3;
    }
  }
  return tmp_arr.join("");
}