redux-devtools/extension/examples/counter/test/reducers/counter.spec.js

import expect from 'expect';
import counter from '../../reducers/counter';
import { INCREMENT_COUNTER, DECREMENT_COUNTER } from '../../actions/counter';

describe('reducers', () => {
  describe('counter', () => {
    it('should handle initial state', () => {
      expect(counter(undefined, {})).toBe(0);
    });

    it('should handle INCREMENT_COUNTER', () => {
      expect(counter(1, { type: INCREMENT_COUNTER })).toBe(2);
    });

    it('should handle DECREMENT_COUNTER', () => {
      expect(counter(1, { type: DECREMENT_COUNTER })).toBe(0);
    });

    it('should handle unknown action type', () => {
      expect(counter(1, { type: 'unknown' })).toBe(1);
    });
  });
});