reducers.md

Reducers

Multiple events with same payload and state change

When multiple events have the same payload and state change, they can be handled in a single reducer handler.

A typical example of this is handling multiple failure events which deliver an error message (or other error as a value).

const someReducer = createReducer(
  initialState,
  on(someEventA, someEventB, someEventC, (state, { error }) => ({
    ...state,
    error
  }))
);

Testing

When testing reducers, we can always fall back to initialState as the base mock state and override only relevant properties. Then mock actual events with the full payload and assert only the relevant properties of the resulting state.

describe('someReducer', () => {
  it('updates state in a specific way when someEventA is dispatched', () => {
    const event = SomeEvents.someEventA({ payload: 'some value' });
    const result = someReducer({
      ...initialState,
      someOverriddenProp: 'some value'
    }, event);
    expect(result.changedPropA).toBe('some expected value');
  });
});