Edit in JSFiddle

var initialState = {counter: 0}
var store = Redux.createStore(reducer, initialState)

function render(state) {
  document.getElementById('counter').textContent = state.counter;
}

document.getElementById('button').addEventListener('click', function() {
  incrementCounter()
})

function incrementCounter() {
  store.dispatch({
    type: 'INCREMENT'
  })
}

function reducer(state, action) {
  if (action.type === 'INCREMENT') {
    state = Object.assign({}, state, {counter: state.counter + 1})
  }
  return state
}

store.subscribe(function() {
  render(store.getState())
})

// Render the initial state
render(store.getState())

<div id="counter">-</div>
<button id="button">Inc</button>