MEAN Stack Interview Questions in Hyderabad

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Explain that any hides real bugs and removes autocompletion, then show concrete fixes: proper interfaces for API models, unknown with narrowing for untrusted data, and generics for reusable helpers. Enable the noImplicitAny compiler option and the ESLint no-explicit-any rule as a warning, fix the most critical files first, and add a review guideline. Keep the conversation constructive by pairing on one module together.

Write mongosh commands to inspect a slow query and the indexes on a collection.

MongoDB Data Modelling and QueriesMedium

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Useful diagnostic commands: db.orders.getIndexes() db.orders.find({ status: 'pending', city: 'Pune' }) .sort({ createdAt: -1 }) .explain('executionStats') db.setProfilingLevel(1, { slowms: 100 }) db.system.profile.find().sort({ ts: -1 }).limit(5) db.orders.createIndex({ status: 1, city: 1, createdAt: -1 }) Compare totalDocsExamined with nReturned before and after creating the index.

What are typed reactive forms?

Services, Dependency Injection and RoutingHard

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Since Angular 14, reactive forms are strongly typed. A FormControl<string> knows its value type, so form.value and getRawValue return typed objects and the compiler catches wrong field names. Controls are nullable by default because reset sets them to null; using nonNullable: true or NonNullableFormBuilder resets to the initial value instead. Typed forms make it safe to map form values directly to API request interfaces.

What types of indexes does MongoDB support?

MongoDB Data Modelling and QueriesMedium

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MongoDB supports single-field and compound indexes, multikey indexes for array fields, text indexes for keyword search, geospatial 2dsphere indexes for location queries, hashed indexes for sharding, wildcard indexes for unpredictable fields, TTL indexes that delete documents after a time, and unique, partial and sparse options. Atlas also provides Atlas Search for full-text relevance and Vector Search for AI similarity queries.

A third-party chart component inside an Angular page causes performance issues because it triggers too many change detection cycles. What can you do?

Angular Components and TemplatesHard

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In Zone.js apps, run the chart's frequent events outside Angular using NgZone.runOutsideAngular, then re-enter only when the UI must update. Make the host component OnPush, pass immutable inputs and use @defer so the chart loads only when visible. Moving to zoneless change detection solves the root cause, because third-party timers no longer trigger global checks.

Write an injectable service that fetches and caches a list of categories.

Services, Dependency Injection and RoutingMedium

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Cache the Observable with shareReplay so the API is called once. import { Injectable, inject } from '@angular/core'; import { HttpClient } from '@angular/common/http'; import { Observable, shareReplay } from 'rxjs'; export interface Category { _id: string; name: string } @Injectable({ providedIn: 'root' }) export class CategoryService { private http = inject(HttpClient); private categories$?: Observable<Category[]>; getAll(): Observable<Category[]> { this.categories$ ??= this.http.get<Category[]>('/api/categories') .pipe(shareReplay({ bufferSize: 1, refCount: false })); return this.categories$; } }

Tell me about a time you improved the structure of an Angular application.

Services, Dependency Injection and RoutingMedium

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Situation: our app had one huge shared module and every page loaded everything, so the first load was slow. Task: I proposed restructuring as part of a performance goal. Action: I converted features to standalone components, split them into lazy routes with canMatch guards, moved shared logic into root services and removed dead code. Result: the initial bundle shrank by about 45 percent and new developers found features faster.

Admin routes are protected by a guard, but the admin feature code is still downloaded by every user. How do you fix this?

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canActivate does not stop the route from matching, so its lazy chunk can still be downloaded, for example by a preloading strategy. Use a canMatch guard on the lazy admin route instead, so the route definition does not match for non-admin users and its code is never loaded; they fall through to another route or a not-authorised page. Keep the real security on the API, since frontend code is always visible.

A list API that uses Mongoose populate on three references has become slow. How would you improve it?

MongoDB Data Modelling and QueriesMedium

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Check how many queries populate triggers and how much data each returns. Select only needed fields in populate, use lean(), and add indexes on the referenced fields. If the list always needs a customer name, store it with the extended reference pattern to avoid the join entirely. For complex filtering on related data, switch to one aggregation with $lookup and $project, and paginate.

Users complain that the app shows a blank screen whenever the backend is down. How do you improve this?

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Add a global error interceptor that maps network and 5xx errors to a friendly toast or banner, with retry for transient failures. In each data component, handle loading, error and empty states explicitly, using httpResource status signals or catchError with fallback values. Show a retry button, keep cached data visible if available, and log errors to monitoring. A health check banner helps during outages.

What are Angular's lifecycle hooks and when is each typically used?

Angular Components and TemplatesMedium

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ngOnChanges runs when input values change. ngOnInit runs once after the first inputs are set, used for initial data loading. ngDoCheck allows custom change detection. ngAfterContentInit and ngAfterViewInit run after projected content and the view are ready, used for DOM access. ngOnDestroy runs before removal, used for cleanup. With signals, effect, computed and DestroyRef often replace several of these hooks.

What are pipes in Angular and what is the difference between pure and impure pipes?

Angular Components and TemplatesMedium

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Pipes transform values in templates, such as {{ price | currency:'INR' }}, date, uppercase, json and async. A pure pipe, the default, runs only when its input reference or primitive value changes, so it is very efficient. An impure pipe runs on every change detection cycle, which can hurt performance, and is needed only when output depends on mutable data. Prefer pure pipes or computed signals.

What is the inject() function and how does it compare to constructor injection?

Services, Dependency Injection and RoutingMedium

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inject() retrieves a dependency inside an injection context, such as a field initialiser, constructor, factory or functional guard. It removes long constructor parameter lists, works in plain functions like functional guards, interceptors and resolvers, and makes inheritance simpler because subclasses do not need to pass dependencies to super. Constructor injection still works, but modern Angular code and CLI templates favour inject().

Write an exhaustive switch over a union type using never.

TypeScript and Modern JavaScriptHard

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The never check makes the compiler flag unhandled cases. type Shape = { kind: 'circle'; r: number } | { kind: 'square'; side: number }; function area(s: Shape): number { switch (s.kind) { case 'circle': return Math.PI * s.r ** 2; case 'square': return s.side ** 2; default: { const unreachable: never = s; throw new Error(`Unknown shape ${unreachable}`); } } } Adding a triangle variant now causes a compile error until it is handled.

Users double-click the Pay button and two payment requests are sent. How would you prevent it on the frontend?

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Disable the button while the request is pending, using an isSubmitting signal. In RxJS, model clicks as a stream and use exhaustMap, which ignores new clicks until the current payment request completes. Show a loading state and handle errors so the button re-enables. The backend must still use idempotency keys, because frontend protection alone cannot stop network retries or duplicate tabs.

What are the main steps to deploy a MEAN application to production?

Security, Testing and DeploymentEasy

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Build Angular with ng build to produce optimised static files and compile the TypeScript API. Host the Angular files on a CDN, static host or Nginx, and run the API on a VM, container service or Kubernetes with environment variables. Use MongoDB Atlas or a managed replica set. Configure HTTPS, domain DNS, CORS, logging and monitoring, and automate everything through a CI/CD pipeline.

What are directives in Angular and what types exist?

Angular Components and TemplatesMedium

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Directives add behaviour to elements. Components are directives with templates. Attribute directives change appearance or behaviour of an existing element, such as ngClass, ngStyle or a custom appHighlight using host bindings. Structural directives change the DOM layout by adding or removing elements, like the older *ngIf and *ngFor, now largely replaced by built-in control flow. Custom directives keep repeated DOM logic out of components.

Write a group validator that checks password and confirm password match.

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Group validators see all child controls. export const passwordsMatch: ValidatorFn = (group: AbstractControl) => { const pass = group.get('password')?.value; const confirm = group.get('confirm')?.value; return pass && confirm && pass !== confirm ? { passwordMismatch: true } : null; }; form = this.fb.group( { password: ['', Validators.required], confirm: ['', Validators.required] }, { validators: passwordsMatch } ); // @if (form.hasError('passwordMismatch') && form.controls.confirm.touched) { ... }

Explain the difference between switchMap, mergeMap, concatMap and exhaustMap.

RxJS, Signals and State ManagementHard

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All map each source value to an inner Observable but differ in concurrency. switchMap cancels the previous inner Observable, ideal for search. mergeMap runs all inner Observables in parallel, good for independent uploads. concatMap queues them and runs one at a time in order, good for sequential saves. exhaustMap ignores new values while one is active, ideal for preventing duplicate login or submit clicks.

How do real-time features work in a MEAN application?

Node.js and Express APIsMedium

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Use WebSockets, commonly through Socket.IO on the Express server and its client library in an Angular service that exposes events as Observables or signals. The server emits events, such as new chat messages or order status changes, to rooms of interested users. For scaling across instances, add a Redis adapter. Server-sent events are a simpler option for one-way updates like notifications or live dashboards.

A dashboard needs data from three independent APIs before it can render. How would you load them?

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Use forkJoin to run all three requests in parallel and emit once when all complete, giving an object or array of results. Handle errors per request with catchError returning a fallback value, so one failed widget does not block the whole dashboard. If any source emits repeatedly, use combineLatest instead. With signals, you could use three httpResource instances and a computed for the combined loading state.

A form must show fields based on earlier answers, such as company name only when employment type is salaried. How would you build it?

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Subscribe to the employment type control's valueChanges, or use a signal, and enable or disable the dependent controls accordingly; disabled controls are excluded from value and validation. Show the fields in the template with @if. For forms driven by configuration, build controls dynamically from a JSON schema into a FormGroup. Remember to clear stale values when a field is hidden.

The security team asks you to move JWTs out of localStorage in the Angular app. What design would you propose?

Security, Testing and DeploymentHard

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Issue a short-lived access token kept only in memory in an Angular auth service, and a refresh token in an httpOnly, Secure, SameSite cookie scoped to the refresh path. On page load, call the refresh endpoint to get a new access token. An interceptor attaches the access token and refreshes on 401. Rotate refresh tokens and store their hashes server-side so they can be revoked.

What are the resource and httpResource APIs in Angular?

RxJS, Signals and State ManagementHard

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resource and httpResource bring asynchronous data into the signal world. You give them reactive parameters, such as a signal holding a product id, and they fetch data whenever those parameters change, cancelling stale requests. They expose signals like value, isLoading, error and status, and provide reload. httpResource builds on HttpClient, so interceptors still apply. They reduce manual subscribe-and-set code in components.

Describe a time you helped a teammate who was struggling with RxJS.

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Situation: a new teammate kept writing nested subscribes and could not follow our streams. Task: help him become productive quickly. Action: I drew marble diagrams for map, switchMap and combineLatest, paired with him to refactor one of his components, and shared a short list of the operators we use most with examples. Result: within two weeks his pull requests used flattening operators correctly and he started reviewing others.

Implement a type-ahead search box that calls the API only after the user pauses typing and ignores outdated results. Which RxJS approach would you use?

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Take the input stream from a FormControl's valueChanges or toObservable of a signal, then pipe debounceTime(300) to wait for a pause, map to trimmed text, filter out terms shorter than two characters, distinctUntilChanged to skip repeats, and switchMap to call the API, which cancels any in-flight request when a new term arrives. Add catchError inside switchMap so one failed request does not kill the stream.

An Express endpoint generates a large PDF report and the whole API slows down while it runs. How do you fix it?

Node.js and Express APIsMedium

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PDF generation is CPU-heavy and blocks the event loop. Move it to a background job: the endpoint creates a job in a queue such as BullMQ with Redis and immediately returns 202 Accepted with a job id. A separate worker generates the PDF, uploads it to storage and updates the job status. Angular polls the status or receives a websocket notification, then downloads the file.

What is the difference between a cold and a hot Observable?

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A cold Observable starts producing values when each subscriber subscribes, and every subscriber gets its own independent execution; an HttpClient call is cold, so two subscriptions send two requests. A hot Observable produces values regardless of subscribers, and subscribers share the same stream, like mouse events or a websocket. You can make a cold Observable shared with operators like share or shareReplay.

Test an Angular service's HTTP call with HttpTestingController.

Security, Testing and DeploymentMedium

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Capture and flush the request. describe('ProductService', () => { let service: ProductService; let httpMock: HttpTestingController; beforeEach(() => { TestBed.configureTestingModule({ providers: [provideHttpClient(), provideHttpClientTesting()] }); service = TestBed.inject(ProductService); httpMock = TestBed.inject(HttpTestingController); }); it('fetches products', () => { service.getAll().subscribe((p) => expect(p.length).toBe(1)); const req = httpMock.expectOne('/api/products'); expect(req.request.method).toBe('GET'); req.flush([{ _id: '1', name: 'Pen' }]); httpMock.verify(); }); });

How do you set and enforce TypeScript coding standards across a development team?

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Situation: our MEAN team had inconsistent code, with some files strict and others full of any. Task: as a senior developer I was asked to standardise it. Action: I proposed a shared tsconfig with strict mode, ESLint with typescript-eslint rules, Prettier and a pre-commit hook, discussed exceptions with the team, and fixed violations module by module. Result: review comments on style dropped sharply and type-related bugs in QA reduced.

Write a debounced search helper in TypeScript using a closure.

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A typed debounce keeps argument types. function debounce<A extends unknown[]>(fn: (...args: A) => void, ms = 300) { let timer: ReturnType<typeof setTimeout> | undefined; return (...args: A) => { clearTimeout(timer); timer = setTimeout(() => fn(...args), ms); }; } const search = debounce((term: string) => console.log('search', term), 400); search('ang'); In Angular you would usually use the RxJS debounceTime operator instead.

Which security measures should every Express API in a MEAN app have?

Security, Testing and DeploymentEasy

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Use HTTPS, helmet for security headers, a strict CORS allowlist, input validation on every route, rate limiting on auth endpoints, body size limits, parameterised queries and stripped MongoDB operators, bcrypt for passwords, secrets in environment variables, and least-privilege database users. Keep dependencies updated with npm audit or Dependabot, log security events and never return stack traces in production.

Frontend and backend models keep drifting apart, causing runtime bugs. How would you keep them in sync in a MEAN project?

TypeScript and Modern JavaScriptHard

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Share types from a single source. In a monorepo, for example with Nx, put DTO interfaces in a shared library used by both the Express API and Angular. Alternatively generate an OpenAPI spec from the backend and generate Angular client types from it, or define schemas once with zod and infer types. Add contract tests in CI so a breaking change fails the build.

What are Subject, BehaviorSubject, ReplaySubject and AsyncSubject?

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A Subject is both an Observable and an observer, multicasting values to subscribers but giving late subscribers nothing from the past. BehaviorSubject needs an initial value and gives the latest value to new subscribers, popular for state like the current user. ReplaySubject replays a set number of past values. AsyncSubject emits only the final value on completion. In new code, signals often replace BehaviorSubject for state.

What are TypeScript utility types such as Partial, Pick, Omit and Record used for?