reducer-2.spec.js

const reducer = require('./reducer-2');

describe('reducer()', () => {
  it('should not be affected by previous call', () => {
    expect(reducer(undefined, { type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer(undefined, { type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer(undefined, { type: 'DECREMENT' })).toEqual({ count: -1 });
    expect(reducer({ count: 99 }, { type: 'INCREMENT' }))
      .toEqual({ count: 100 });
  });

  it('should handle increment and decrement actions without mutating state', () => {
    const state1 = reducer(undefined, { type: 'INCREMENT' });
    const state2 = reducer(state1, { type: 'INCREMENT' });
    const state3 = reducer(state2, { type: 'DECREMENT' });
    expect(state1).toEqual({ count: 1 });
    expect(state2).toEqual({ count: 2 });
    expect(state3).toEqual({ count: 1 });
    expect(state1).not.toBe(state2);
    expect(state1).not.toBe(state3);
    expect(state2).not.toBe(state3);
  });

  it('should return the state as it came in when unknown action', () => {
    const state = { count: 10 };
    expect(reducer(state, {})).toBe(state);
  });
});