time-flicker.ts

type State = { ms: number, lightOn: boolean };
const initialState = {ms: 0, lightOn: false};
type Event = { type: 'TIME_PASSED'; ms: number };

const reduce = (event: Event) => (state: State): State => ((newMs) => ((toggle) => toggle ? {
  // ignore remaining time for simplicity
  ms: 0,
  lightOn: !state.lightOn
} : {
  ...state,
  ms: newMs
})(newMs >= 3000))(event.ms + state.ms);

let state = initialState;

setInterval(() => {
  state = reduce({type: 'TIME_PASSED', ms: 1000})(state);
  console.log('lights on:', state.lightOn);
}, 1000);