function counter(state = 0){
  if(action.type === "INCREMENT"){
    return state +1
  } else if(action.type === "DECREMENT"){
    return state - 1
  } else {
    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(2, {type: "DECREMENT"})
).toEqual(0)

expect(
  counter(undefined, {})
).toEqual(0)

console.log('test passed')