code: function getChunk(readerClient, slice, opConfig, logger, metadata) {
      const delimiter = opConfig.line_delimiter;
  
      async function getMargin(offset, length) {
          let margin = '';
          return new Promise(async (resolve) => {
              while (margin.indexOf(delimiter) === -1) {
                  // reader clients must return false-y when nothing more to read.
                  const chunk = await readerClient(offset, length); // eslint-disable-line no-await-in-loop, max-len
                  if (!chunk) {
                      resolve(margin.split(delimiter)[0]);
                      return;
                  }
                  margin += chunk;
                  offset += length; // eslint-disable-line no-param-reassign, max-len
              }
              // Don't read too far - next slice will get it.
              resolve(margin.split(delimiter)[0]);
          });
      }
  
      let needMargin = false;
      if (slice.length) {
          // Determines whether or not to grab the extra margin.
          if (slice.offset + slice.length !== slice.total) {
              needMargin = true;
          }
      }
  
      return readerClient(slice.offset, slice.length)
          .then(async (data) => {
              if (data.endsWith(delimiter)) {
                  // Skip the margin if the raw data ends with the delimiter since
                  // it will end with a complete record.
                  needMargin = false;
              }
              if (needMargin) {
                  // Want to minimize reads since will typically be over the
                  // network. Using twice the average record size as a heuristic.
                  const avgSize = _averageRecordSize(data.split(delimiter));
                  data += await getMargin(slice.offset + slice.length, 2 * avgSize); // eslint-disable-line no-param-reassign, max-len
              }
              return data;
          })
          .then(data => chunkFormatter[opConfig.format](data, logger, opConfig, metadata, slice));
  }
docs: This function will grab the chunk of data specified by the slice plus an extra margin if the slice does not end with the delimiter.