reducer-demo.js

let array = [1, 2, 3, 4, 5];
let add = (x, y) => x + y;
let sum = array.reduce(add, 0);

// This will do:
// 0 + 1
// 1 + 2
// 3 + 3
// 6 + 4
// 10 + 5

const initialState = { count: 0, user: {} }

const actions = [
  { type: 'ADD', by: 2 },
  { type: 'MINUS', by: 4 },
  { type: 'LOG_IN', name: 'fish' }
];

function reducer(state, action) {
  if (action.type === 'ADD') {
    return { ...state, count: state.count + action.by };
  } else if (action.type === 'MINUS') {
    return { ...state, count: state.count - action.by };
  } else if (action.type === 'LOG_IN') {
    return { ...state, user: { name: action.name } };
  }
}

console.log(actions.reduce(reducer, initialState)); // count is -2, user's name is fish

let [state, dispatch] = useReducer(
  (state, action) => { ... },
  { count: 0, user: {} }
) 

let [count, setCount] = useState(0);

setCount(5);