reducer test

package.json

{
  "scripts": "mocha -r should '*.spec.js'"
}

reducer.js

export default (action, state = { sum: 0 }) => {
  switch (action.type) {
    case 'add': {
      return {
        ...state,
        sum: state.sum + action.payload.num
      };
    };
    case 'subtract': {
      return {
        ...state,
        sum: state.sum - action.payload.num
      };
    };
    default: {
      return state;
    }
};

reducer.spec.js

import reducer from './reducer';
describe('reducer', () => {
  it('should add the number to the sum', () => {
    reducer({
      type: 'add',
      payload: { num: 2 }
    }, { sum: 4 }).should.eql({
      sum: 6
    });
  });
  
  it('should not add fail with no state', () => {
    reducer({
      type: 'add',
      payload: { num: 2 }
    }).should.eql({
      sum: 2
    });
  });
  
  it('should subtract the number', () => {
    reducer({
      type: 'subtract',
      payload: { num: 2 }
    }, { sum: 4 }).should.eql({
      sum: 2
    });
  });
  
  it('should not subtract fail with no state', () => {
    reducer({
      type: 'subtract',
      payload: { num: 2 }
    }).should.eql({
      sum: -2
    });
  });
});