Example implementation for extending queue.reducer (Guppy)

queue.reducer.js

/* ... */    
case QUEUE_MODIFY_PROJECT: {
  const { projectId, settings } = action;

  return produce(state, draftState => {
    // get existing project queue, or create it if this
    // is the first entry
    const projectQueue = draftState[projectId] || [];

    // get existing modification queue for this project, or
    // create it if it doesn't exist
    let modificationQueue = projectQueue.find(
      q => q.action === 'modify' && !q.active
    );
    if (!modificationQueue) {
      modificationQueue = {
        action: 'modify',
        active: false,
        settings: {},
      };
      projectQueue.push(modificationQueue);
    }

    // maintain existing settings and override duplicate
    // keys with the newer values
    modificationQueue.settings = {
      ...modificationQueue.settings,
      ...settings,
    };

    // update the project's modify queue
    draftState[projectId] = projectQueue;
  });
}

case INSTALL_DEPENDENCIES_START:
case UNINSTALL_DEPENDENCIES_START:
case MODIFY_PROJECT_START: {
  const { projectId } = action;

  return produce(state, draftState => {
    // mark the next item in the queue as active
    draftState[projectId][0].active = true;
  });
}

case INSTALL_DEPENDENCIES_ERROR:
case INSTALL_DEPENDENCIES_FINISH:
case UNINSTALL_DEPENDENCIES_ERROR:
case UNINSTALL_DEPENDENCIES_FINISH:
case MODIFY_PROJECT_ERROR:
case MODIFY_PROJECT_FINISH: {
  const { projectId } = action;

  return produce(state, draftState => {
    // remove oldest item in queue (it's just been
    // completed by the dependency saga)
    draftState[projectId].shift();

    // remove the project's queue if this was the
    // last entry
    if (draftState[projectId].length === 0) {
      delete draftState[projectId];
    }
  });
}
/* ... */