redux.vanilla.js

<script src="https://unpkg.com/redux@latest/dist/redux.min.js"></script>

<p>
  Counter: <span id="value">0</span>
  <button id="increment">+</button>
  <button id="decrement">-</button>
</p>

<script>      
function counterReducer(state, action) {
  if (typeof state === 'undefined') {
    return 0;
  }

  switch (action.type) {
   case 'INCREMENT':
     return state + 1
   case 'DECREMENT':
    return state - 1
    default:
    return state
  }
}

const store = Redux.createStore(counterReducer)

store.subscribe(() => {
  const valueElement = document.getElementById('value');
  valueElement.innerHTML = store.getState().toString()
})

document.getElementById('increment')
  .addEventListener('click', function () {
    store.dispatch({ type: 'INCREMENT' })
  })
document.getElementById('decrement')
  .addEventListener('click', function () {
    store.dispatch({ type: 'DECREMENT' })
  })
</script>