10 NgRx Angular Interview Questions and Answers

NgRx is a powerful state management library for Angular applications, leveraging the reactive programming paradigm. It provides a robust framework for managing complex state interactions, making it easier to maintain and scale large applications. By utilizing NgRx, developers can achieve a more predictable state flow, improve performance, and enhance the overall architecture of their Angular projects.

This article offers a curated selection of interview questions focused on NgRx and Angular. Reviewing these questions will help you deepen your understanding of state management in Angular and prepare you to confidently discuss and demonstrate your expertise in NgRx during technical interviews.

NgRx Angular Interview Questions and Answers

1. Describe how actions are used in NgRx and their role in state management.

Actions in NgRx are the primary means of sending data to the store. They are dispatched using the store’s dispatch method and processed by reducers to update the state. Actions are typically defined as classes in Angular, including a type and an optional payload. The type uniquely identifies the action, while the payload contains additional information needed for processing.

Example:

import { createAction, props } from '@ngrx/store';

export const loadItems = createAction(
  '[Item List] Load Items'
);

export const loadItemsSuccess = createAction(
  '[Item List] Load Items Success',
  props<{ items: any[] }>()
);

export const loadItemsFailure = createAction(
  '[Item List] Load Items Failure',
  props<{ error: any }>()
);

In this example, three actions are defined: loadItems, loadItemsSuccess, and loadItemsFailure. These actions manage the state of an item list. The loadItems action is dispatched when items need to be loaded, loadItemsSuccess is dispatched when the items are successfully loaded, and loadItemsFailure is dispatched if there is an error.

2. What is the purpose of reducers in NgRx, and how do they work?

Reducers in NgRx are pure functions that take the current state and an action as arguments and return a new state. They manage state transitions in a predictable way, ensuring consistency and testability. Reducers are typically implemented as switch statements that handle different action types.

Example:

import { Action } from '@ngrx/store';

export interface AppState {
  count: number;
}

export const initialState: AppState = {
  count: 0
};

export function counterReducer(state = initialState, action: Action): AppState {
  switch (action.type) {
    case 'INCREMENT':
      return {
        ...state,
        count: state.count + 1
      };
    case 'DECREMENT':
      return {
        ...state,
        count: state.count - 1
      };
    default:
      return state;
  }
}

In this example, the counterReducer function handles INCREMENT and DECREMENT actions, updating the count property accordingly.

3. Write a basic example of an action, reducer, and selector for managing a list of items.

Below is a basic example of managing a list of items using actions, reducers, and selectors.

Action:

import { createAction, props } from '@ngrx/store';

export const addItem = createAction(
  '[Item List] Add Item',
  props<{ item: string }>()
);

Reducer:

import { createReducer, on } from '@ngrx/store';
import { addItem } from './item.actions';

export const initialState: string[] = [];

const _itemReducer = createReducer(
  initialState,
  on(addItem, (state, { item }) => [...state, item])
);

export function itemReducer(state, action) {
  return _itemReducer(state, action);
}

Selector:

import { createSelector, createFeatureSelector } from '@ngrx/store';

export const selectItems = createFeatureSelector<string[]>('items');

export const selectAllItems = createSelector(
  selectItems,
  (state: string[]) => state
);

4. Explain the concept of effects in NgRx and provide an example of when you would use them.

Effects in NgRx handle side effects, such as asynchronous operations like HTTP requests. They listen for specific actions and perform operations, often dispatching new actions to update the state.

Example:

import { Injectable } from '@angular/core';
import { Actions, createEffect, ofType } from '@ngrx/effects';
import { of } from 'rxjs';
import { catchError, map, mergeMap } from 'rxjs/operators';
import { UserService } from './user.service';
import * as UserActions from './user.actions';

@Injectable()
export class UserEffects {
  loadUsers$ = createEffect(() =>
    this.actions$.pipe(
      ofType(UserActions.loadUsers),
      mergeMap(() =>
        this.userService.getAll().pipe(
          map(users => UserActions.loadUsersSuccess({ users })),
          catchError(error => of(UserActions.loadUsersFailure({ error })))
        )
      )
    )
  );

  constructor(
    private actions$: Actions,
    private userService: UserService
  ) {}
}

In this example, the UserEffects class listens for the loadUsers action, fetches users from an API, and dispatches success or failure actions based on the result.

5. Explain the role of Entity in NgRx and how it simplifies state management.

NgRx Entity simplifies managing collections of entities by providing utility functions for CRUD operations. It normalizes the state structure, leading to more efficient updates and queries. The entity adapter manages the collection of entities, reducing boilerplate code and ensuring maintainability.

Example:

import { createEntityAdapter, EntityState } from '@ngrx/entity';
import { createReducer, on } from '@ngrx/store';
import { addEntity, updateEntity, removeEntity } from './entity.actions';
import { Entity } from './entity.model';

export interface State extends EntityState<Entity> {}

export const adapter = createEntityAdapter<Entity>();

export const initialState: State = adapter.getInitialState();

const entityReducer = createReducer(
  initialState,
  on(addEntity, (state, { entity }) => adapter.addOne(entity, state)),
  on(updateEntity, (state, { update }) => adapter.updateOne(update, state)),
  on(removeEntity, (state, { id }) => adapter.removeOne(id, state))
);

export function reducer(state: State | undefined, action: Action) {
  return entityReducer(state, action);
}

6. How do you implement optimistic updates in NgRx?

Optimistic updates in NgRx involve dispatching actions that immediately update the state, followed by an effect that handles the server request. If the request fails, another action reverts the state to its previous value.

Example:

// actions.ts
import { createAction, props } from '@ngrx/store';

export const updateItemOptimistically = createAction(
  '[Item] Update Item Optimistically',
  props<{ item: any }>()
);

export const updateItemSuccess = createAction(
  '[Item] Update Item Success',
  props<{ item: any }>()
);

export const updateItemFailure = createAction(
  '[Item] Update Item Failure',
  props<{ item: any, error: any }>()
);

// reducer.ts
import { createReducer, on } from '@ngrx/store';
import { updateItemOptimistically, updateItemSuccess, updateItemFailure } from './actions';

const initialState = {
  items: [],
  error: null
};

const itemReducer = createReducer(
  initialState,
  on(updateItemOptimistically, (state, { item }) => ({
    ...state,
    items: state.items.map(i => i.id === item.id ? item : i)
  })),
  on(updateItemSuccess, (state, { item }) => ({
    ...state,
    items: state.items.map(i => i.id === item.id ? item : i)
  })),
  on(updateItemFailure, (state, { item, error }) => ({
    ...state,
    items: state.items.map(i => i.id === item.id ? { ...i, ...item } : i),
    error
  }))
);

// effects.ts
import { Injectable } from '@angular/core';
import { Actions, createEffect, ofType } from '@ngrx/effects';
import { of } from 'rxjs';
import { catchError, map, mergeMap } from 'rxjs/operators';
import { updateItemOptimistically, updateItemSuccess, updateItemFailure } from './actions';
import { ItemService } from './item.service';

@Injectable()
export class ItemEffects {
  updateItem$ = createEffect(() =>
    this.actions$.pipe(
      ofType(updateItemOptimistically),
      mergeMap(action =>
        this.itemService.updateItem(action.item).pipe(
          map(item => updateItemSuccess({ item })),
          catchError(error => of(updateItemFailure({ item: action.item, error })))
        )
      )
    )
  );

  constructor(
    private actions$: Actions,
    private itemService: ItemService
  ) {}
}

7. How would you optimize performance in an NgRx application?

To optimize performance in an NgRx application, consider the following strategies:

• Efficient Use of Selectors: Use memoized selectors to reduce unnecessary recalculations and component re-renders.
• Minimizing Store Usage for Transient State: Manage transient state locally within components to reduce store complexity.
• Using OnPush Change Detection: Employ the OnPush strategy to limit change detection cycles to when component inputs change.
• Avoiding Unnecessary Dispatches: Ensure actions are only dispatched when necessary to prevent redundant state updates.
• Lazy Loading of State: Load feature states only when accessed to reduce initial load time and memory usage.

8. Write a test case for an NgRx effect that makes an HTTP request.

To test an NgRx effect that makes an HTTP request, mock the HTTP service and verify that the effect dispatches the correct actions based on the response.

Example:

import { TestBed } from '@angular/core/testing';
import { provideMockActions } from '@ngrx/effects/testing';
import { Observable, of, throwError } from 'rxjs';
import { MyEffects } from './my-effects';
import { MyService } from '../services/my-service';
import { MyActions } from '../actions/my-actions';
import { cold, hot } from 'jasmine-marbles';

describe('MyEffects', () => {
  let actions$: Observable<any>;
  let effects: MyEffects;
  let myService: jasmine.SpyObj<MyService>;

  beforeEach(() => {
    const spy = jasmine.createSpyObj('MyService', ['getData']);

    TestBed.configureTestingModule({
      providers: [
        MyEffects,
        provideMockActions(() => actions$),
        { provide: MyService, useValue: spy }
      ]
    });

    effects = TestBed.inject(MyEffects);
    myService = TestBed.inject(MyService) as jasmine.SpyObj<MyService>;
  });

  it('should dispatch success action when service call is successful', () => {
    const data = { id: 1, name: 'Test' };
    const action = MyActions.loadData();
    const successAction = MyActions.loadDataSuccess({ data });

    actions$ = hot('-a', { a: action });
    const response = cold('-a|', { a: data });
    myService.getData.and.returnValue(response);

    const expected = cold('--b', { b: successAction });

    expect(effects.loadData$).toBeObservable(expected);
  });

  it('should dispatch failure action when service call fails', () => {
    const error = 'Error';
    const action = MyActions.loadData();
    const failureAction = MyActions.loadDataFailure({ error });

    actions$ = hot('-a', { a: action });
    const response = cold('-#|', {}, error);
    myService.getData.and.returnValue(response);

    const expected = cold('--b', { b: failureAction });

    expect(effects.loadData$).toBeObservable(expected);
  });
});

9. Write a custom meta-reducer to log all actions dispatched in an NgRx application.

A meta-reducer in NgRx is a higher-order reducer that wraps other reducers to enhance their behavior. To log all actions dispatched in an NgRx application, create a custom meta-reducer that intercepts actions and logs them before passing them to the next reducer.

Example:

import { ActionReducer, MetaReducer } from '@ngrx/store';

export function logger(reducer: ActionReducer<any>): ActionReducer<any> {
  return function(state, action) {
    console.log('state before: ', state);
    console.log('action: ', action);

    return reducer(state, action);
  };
}

export const metaReducers: MetaReducer<any>[] = [logger];

In the example above, the logger function logs the current state and the dispatched action, then calls the original reducer to process the action and return the new state.

10. Discuss the role of DevTools in debugging NgRx applications.

DevTools assist in debugging NgRx applications by providing tools to inspect and manipulate the state and actions within the NgRx store. The Redux DevTools Extension offers features like:

• State Inspection: View the current state of the store at any point.
• Action Monitoring: Log every action dispatched to the store.
• Time Travel Debugging: Replay actions to identify where the state diverged from expected behavior.
• State Diffing: Show differences between states before and after an action is dispatched.
• Dispatching Actions: Manually dispatch actions to test application responses.