Tiny Redux app with tests for count reducer

redux-counter.js

// A counter reducer

const counter = ( state = 0, action) => {
  switch(action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
}

// Tests for reducer

expect(
  counter(0, {type: 'INCREMENT'})
).toEqual(1);

expect(
  counter(1, {type: 'INCREMENT'})
).toEqual(2);

expect(
  counter(2, {type: 'DECREMENT'})
).toEqual(1);

expect(
  counter(1, {type: 'DECREMENT'})
).toEqual(0);

expect(
  counter(1, {type: 'SOMETHING_ELSE'})
).toEqual(1);

console.log('Test passed!');

// Tiny Redux App

const { createStore } = Redux;
const store = createStore(counter);

const render = () => {
  document.body.innerText = store.getState();
}

store.subscribe(render);
render();

document.addEventListener('click', () => {
  store.dispatch({type: 'INCREMENT'})
});