const counter = (state = 0, action) => {

  switch (action.type) {

    case 'INCREMENT':

      return state + 1;

    case 'DECREMENT':

      return state - 1;

    default:

      return state;

  }

}

​

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);

​

expect(

  counter(undefined, {})

).toEqual(0);

console.log('Tests passed!');