1-before.js

// FETCH response sent as result of FETCH request
var msg = this._curReq.fetchCache[info.num],
    keys = Object.keys(info.text),
    keyslen = keys.length,
    toget, msgEmitter, j;

if (msg === undefined) {
  // simple case -- no bodies were streamed
  toget = this._curReq.fetching.slice(0);
  if (toget.length === 0)
    return;

  msgEmitter = new EventEmitter();
  attrs = {};

  this._curReq.bodyEmitter.emit('message', msgEmitter, info.num);
} else {
  toget = msg.toget;
  msgEmitter = msg.msgEmitter;
  attrs = msg.attrs;
}

i = toget.length;
if (i === 0) {
  if (msg && !msg.ended) {
    msg.ended = true;
    process.nextTick(function() {
      msgEmitter.emit('end');
    });
  }
  return;
}

if (keyslen > 0) {
  while (--i >= 0) {
    j = keyslen;
    while (--j >= 0) {
      if (keys[j].toUpperCase() === toget[i]) {
        if (!RE_BODYPART.test(toget[i])) {
          if (toget[i] === 'X-GM-LABELS') {
            var labels = info.text[keys[j]];
            for (var k = 0, lenk = labels.length; k < lenk; ++k)
              labels[k] = (''+labels[k]).replace(RE_ESCAPE, '\\');
          }
          key = FETCH_ATTR_MAP[toget[i]];
          if (!key)
            key = toget[i].toLowerCase();
          attrs[key] = info.text[keys[j]];
        }
        toget.splice(i, 1);
        break;
      }
    }
  }
}

if (toget.length === 0) {
  if (msg)
    msg.ended = true;
  process.nextTick(function() {
    msgEmitter.emit('attributes', attrs);
    msgEmitter.emit('end');
  });
} else if (msg === undefined) {
  this._curReq.fetchCache[info.num] = {
    msgEmitter: msgEmitter,
    toget: toget,
    attrs: attrs,
    ended: false
  };
}

2-after.js

if (keysToFetch.length > 0) {
  let caseMappedPayload = mapObject(payload, (key, value) => {
    return [ key.toUpperCase(), value ];
  });

  let [ existingKeys, missingKeys ] = splitFilter(keysToFetch, (key) => caseMappedPayload[key] != null);
  let relevantKeys = existingKeys.filter((key) => RE_BODYPART.test(key) === false);

  let newAttributes = syncpipe(caseMappedPayload, [
    (_) => pickObject(_, relevantKeys),
    (_) => mapObject(_, (key, value) => {
      return [
        /* key */ mapIncomingAttributeKey(key),
        /* value */ (key === 'X-GM-LABELS')
          // TODO: Why is this special case needed?
          ? value.map((label) => String(label).replace(RE_ESCAPE, '\\'))
          : value
      ];
    })
  ]);

  // console.log({ existingKeys, missingKeys, relevantKeys, newAttributes, caseMappedPayload });

  Object.assign(attributes, newAttributes);
  keysToFetch = missingKeys;

  let isDone = (missingKeys.length === 0);

  if (isDone === true) {
    endHandled = true;

    // FIXME: Why nextTick?
    process.nextTick(function() {
      emitter.emit("attributes", attributes);
      emitter.emit("end");
    });
  }
} else {
  if (endHandled === false) {
    endHandled = true;

    // FIXME: Why nextTick?
    process.nextTick(function() {
      emitter.emit("end");
    });
  }
}