requestAddMessage.js

const requestAddMessage = (sources) => {
  const message$ = sources.ACTION
    .filter((action) => action.type === 'ADD_MESSAGE')
    .map((action) => action.payload);

  const request$ = message$.map((payload) => {
    return {
      url: `/api/conversations/${payload.conversationId}/messages`,
      category: 'addMessage',
      method: 'POST',
      withCredentials: true,
      send: payload.formData,
      progress: true,
      references: {
        messageId: payload.messagePreview.id,
      },
    };
  });

  const response$ = sources.HTTP
    .select('addMessage')
    .map(normaliseAPIErrors)
    .flatten();

  const messageResponse$ = response$
    .filter((x) => x.type === 'application/json')
    .map(({ request: { references }) => addMessageSuccess({ messageId: references.messageId }));

  return {
    ACTION: messageResponse$,
    HTTP: request$,
  };
};

I would like to avoid that let, but messageId is set in the request$ and is needed for the messageResponse$.
Or is it best to leave it there to keep the code "simple"?