puyopuyotetr.is/app/javascript/mastodon/reducers/missed_updates.ts

import { Record } from 'immutable';

import type { Action } from 'redux';

import { focusApp, unfocusApp } from '../actions/app';
import { NOTIFICATIONS_UPDATE } from '../actions/notifications';

interface MissedUpdatesState {
  focused: boolean;
  unread: number;
}
const initialState = Record<MissedUpdatesState>({
  focused: true,
  unread: 0,
})();

export function missedUpdatesReducer(
  state = initialState,
  action: Action<string>
) {
  switch (action.type) {
    case focusApp.type:
      return state.set('focused', true).set('unread', 0);
    case unfocusApp.type:
      return state.set('focused', false);
    case NOTIFICATIONS_UPDATE:
      return state.get('focused')
        ? state
        : state.update('unread', (x) => x + 1);
    default:
      return state;
  }
}