40 questions with answers for candidates with Freshers — 22 easy, 18 medium.
What interviewers expect: freshers
Fresher interviews test TypeScript basics, Angular components, data binding, control flow, services, routing and simple reactive forms, along with Express routes and MongoDB CRUD. Expect a live task such as building a list with filtering or a validated form, plus detailed questions on your projects. Interviewers check whether you understand how the pieces connect, not just whether the app runs.
This set is weighted towards Angular Forms and HTTP Client (6), Services, Dependency Injection and Routing (6), MongoDB Data Modelling and Queries (6), TypeScript and Modern JavaScript (5).
TypeScript fundamentals Angular components and binding Reactive forms Project explanation
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01
How do built-in and custom validators work in reactive forms?
Validators are functions that take a control and return null when valid or an error object like { required: true } when invalid. Built-ins include Validators.required, email, minLength, maxLength, pattern, min and max. A custom validator follows the same ValidatorFn signature, for example rejecting weak passwords. Group-level validators can compare fields, such as password and confirm password. Errors are read through control.errors and control.hasError.
02
What is a service in Angular and why should business logic live there?
A service is a class, usually decorated with @Injectable, that holds logic or data not tied to a single view, such as API calls, authentication, caching or shared state. Keeping this out of components makes components thin and focused on presentation, lets several components share the same data, and makes logic easy to unit test by injecting mock services. Services are created and supplied by Angular's dependency injection system.
03
Why is MongoDB a natural fit for a JavaScript stack like MEAN?
MongoDB stores data as JSON-like BSON documents, which map directly to JavaScript and TypeScript objects used in Express and Angular, so there is little translation between layers. Its flexible schema helps when requirements change quickly, and features like rich queries, aggregation, indexes, replication and Atlas cloud hosting cover most web application needs. Mongoose adds schemas, validation and TypeScript support on top of the driver.
04
What are Subject, BehaviorSubject, ReplaySubject and AsyncSubject?
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.
05
Tell me about a time strong typing helped you catch a bug before it reached production.
Situation: we renamed the field amount to totalAmount in the orders API. Task: update the Angular app before release. Action: because our shared Order interface was typed and strict templates were on, the build failed in eleven places, including a payment summary I would have missed. I fixed each usage and added the change to the API changelog. Result: the release went out with no broken screens.
Narrowing is TypeScript refining a broad type to a more specific one inside a code block based on checks. typeof, instanceof, the in operator, equality checks and discriminated union tags all narrow types. A custom type guard is a function returning a predicate like value is Admin, which tells the compiler the type after the check. This makes handling union types, such as API success or error responses, safe.
07
How do you read route parameters and query parameters in an Angular component?
Inject ActivatedRoute and read paramMap or queryParamMap, which are Observables, for example route.paramMap.pipe(map(p => p.get('id'))), so the component updates when the same route is reused with a new id. A snapshot gives a one-time value. With withComponentInputBinding enabled in provideRouter, route params, query params and resolver data can bind directly to component inputs, which is the simplest modern approach.
08
Write a zod validation middleware for an Express route in TypeScript.
The Angular router handles that path in the browser, but the web server looks for a real file at /orders/42 and finds none. Configure the server to return index.html for unknown routes: try_files $uri $uri/ /index.html in Nginx, a rewrite rule in IIS or static hosting, or a catch-all route in Express that serves the built index.html after API routes. Alternatively use server-side rendering.
10
Write a functional auth guard that redirects unauthenticated users to the login page.
MongoDB Atlas is the fully managed cloud service for MongoDB on AWS, Azure and Google Cloud. It handles provisioning, replica sets, automated backups, scaling, monitoring, security features like network access lists and encryption, and performance advice such as index suggestions. It also offers Atlas Search, Vector Search and data federation. A free tier makes it popular for learning projects and MEAN prototypes.
13
Explain the four types of data binding in Angular.
Interpolation, {{ value }}, displays component data as text. Property binding, [src]='imageUrl', sets a DOM or component property. Event binding, (click)='save()', calls a method when an event fires. Two-way binding, [(ngModel)]='name' or [(value)] with a model input, combines property and event binding so the view and component stay in sync. Data flows one way by default, which keeps changes predictable.
14
Write Mongoose queries to filter, sort, paginate and select fields for a products listing.
signal() creates writable state. computed() derives a read-only value from other signals, is lazily evaluated and memoised, and recalculates only when its dependencies change, for example a cart total from cart items. effect() runs side effects whenever signals it reads change, such as logging or syncing to local storage. Use computed for derived data and keep effects for non-UI side effects, not for setting other state.
16
Define a typed Mongoose model in TypeScript with an instance method.
Create a root-provided FilterService holding the city in a signal, with methods to update it, and inject the service into both components so they share the same instance. For persistence across reloads or shareable links, sync the value to a query parameter or local storage. If many features need shared state with complex updates, consider NgRx SignalStore or NgRx Store.
18
How do parent and child components communicate in Angular?
A parent passes data down through inputs, declared with the input() function or the @Input decorator, and the child notifies the parent through outputs, declared with output() or @Output with an EventEmitter. For two-way binding, model() creates a writable input. A parent can access a child directly using viewChild. For unrelated components, use a shared service holding a signal or a BehaviorSubject.
19
How do you set up an Express project with TypeScript?
Install express and types with npm i express and npm i -D typescript @types/express @types/node. Create a tsconfig with strict mode and a module setting like NodeNext, write code in src and compile to dist with tsc. For development, run TypeScript directly with tsx watch, and use node dist/server.js in production. Type request bodies, params and responses with Request and Response generics or validated schemas.
20
Users lose a long half-filled form when they accidentally click a menu link. How would you prevent this?
Add a canDeactivate guard on the form route. The guard checks whether the form is dirty and unsaved through a method or signal on the component, and if so asks the user to confirm leaving with a dialog. Return true to allow or false to stay. Also handle browser refresh and tab close with a beforeunload listener, and consider auto-saving drafts to local storage or the server.
21
How should HTTP errors be handled in an Angular application?
Handle them at two levels. A global error interceptor catches HttpErrorResponse, redirects to login on 401, shows a toast for 500 errors and logs details. Component or service code uses catchError to provide fallbacks or show specific messages, such as field errors from a 422 response. Use retry with a delay for transient network failures, and never leave errors unhandled so the UI shows a blank state.
22
Tell me about a complex form you built and the challenges you solved.
Situation: I built a loan application form with five steps and over sixty fields. Task: make it reliable and user friendly. Action: I used typed reactive forms split into step groups, cross-field validators for income rules, a FormArray for co-applicants, auto-saved drafts every thirty seconds and mapped server validation errors back to fields. Result: drop-offs reduced noticeably and QA found very few form bugs.
23
A teammate uses any everywhere to silence TypeScript errors. How would you address it?
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.
24
OTP codes must automatically expire ten minutes after creation. How would you implement this with MongoDB?
Store each OTP in its own collection with a createdAt date field and create a TTL index, db.otps.createIndex({ createdAt: 1 }, { expireAfterSeconds: 600 }). MongoDB's background task removes expired documents automatically, though deletion can lag by about a minute, so also check the timestamp during verification. Store a hash of the code rather than plain text, and limit verification attempts.
25
What is an Angular component and what are its main parts?
A component controls a piece of the screen. It is a TypeScript class decorated with @Component, which provides a selector used as an HTML tag, a template or templateUrl for the view, styles or styleUrl for scoped CSS, and imports for other standalone components, directives and pipes it uses. The class holds data and event handlers, and the template binds to them.
26
What are databases, collections and documents in MongoDB?
A MongoDB deployment holds multiple databases, each database contains collections, and each collection contains documents. A document is a BSON object with fields, arrays and nested sub-documents, identified by a unique _id. Collections are roughly like tables but do not force all documents to share the same fields, although schema validation can enforce rules. A single document can be up to 16 MB in size.
27
Configure an Angular dev server proxy to forward API calls to Express.
proxy.conf.json in the Angular project:
{
"/api": {
"target": "http://localhost:3000",
"secure": false,
"changeOrigin": true,
"logLevel": "debug"
}
}
angular.json, under serve options:
"proxyConfig": "proxy.conf.json"
Then call relative URLs in services, such as this.http.get('/api/products'), and run ng serve. The browser talks only to the dev server, so no CORS setup is needed locally.
28
What are directives in Angular and what types exist?
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.
29
Write a typed Express route in TypeScript that returns a product by id.
Situation: for my final-year project I built a hospital appointment system. Task: let patients book slots and doctors manage schedules. Action: I designed MongoDB collections with indexes for slots, built a TypeScript Express API with JWT and role checks, and an Angular frontend with reactive forms, guards and signals. I added tests and deployed with Docker on a cloud VM. Result: our college clinic used it for a semester.
31
How do toSignal and toObservable bridge RxJS and signals?
toSignal(observable$) subscribes to an Observable and exposes its latest value as a signal, unsubscribing automatically when the injection context is destroyed; you can supply an initialValue or requireSync. toObservable(signal) creates an Observable that emits whenever the signal changes, letting you apply RxJS operators like debounceTime and switchMap. Together they let you keep simple state in signals while using RxJS for async logic.
32
List the Angular CLI commands you use most often in a project.
Common commands:
ng new shop-app --style=scss
ng serve --open
ng generate component features/product-list
ng generate service core/auth
ng generate guard core/auth --functional
ng generate pipe shared/truncate
ng build --configuration production
ng test
ng update @angular/core @angular/cli
ng update applies official migrations automatically, which makes version upgrades much easier.
33
Which security measures should every Express API in a MEAN app have?
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.
34
A component subscribes to a service Observable in ngOnInit, and after navigating back and forth several times, the API is called multiple times per action. What is wrong?
The subscriptions are never cleaned up, so each visit adds another live subscriber that reacts to every emission. Use the async pipe or toSignal in the template, or add takeUntilDestroyed() to manual subscriptions. Also check that the service does not create a new long-lived stream per call and that nested subscribes are replaced with switchMap. Angular DevTools and network logs confirm the fix.
35
What is the difference between template-driven and reactive forms in Angular?
Template-driven forms build the form model from directives in the template, mainly ngModel, and suit small, simple forms. Reactive forms define the model explicitly in the component with FormControl, FormGroup and FormArray, giving synchronous access, strong typing, easier dynamic fields, observable valueChanges and simpler unit testing. Most enterprise Angular applications use reactive forms for complex or validated forms.
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.
37
Write a unit test for an Angular component using TestBed.
Angular's CLI has moved to Vitest as the default unit test runner for new projects, replacing the deprecated Karma, while many existing projects still run Karma with Jasmine or have switched to Jest. TestBed configures components and services for tests, HttpTestingController mocks HTTP calls, and component harnesses from the CDK simplify interaction tests. Playwright or Cypress handle end-to-end tests.
39
Why does Angular use TypeScript, and what benefits does it bring to a MEAN project?
TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. Angular is written in TypeScript and relies on its types and decorators for components, dependency injection and templates. Types catch mistakes at compile time, improve editor autocompletion and make refactoring large codebases safer. In MEAN projects, you can share interfaces such as User or Order between the Express API and the Angular client.
40
Write a functional HTTP interceptor that adds a JWT and handles 401 errors.
What is asked in MEAN Stack interviews for Freshers?
Fresher interviews test TypeScript basics, Angular components, data binding, control flow, services, routing and simple reactive forms, along with Express routes and MongoDB CRUD. Expect a live task such as building a list with filtering or a validated form, plus detailed questions on your projects. Interviewers check whether you understand how the pieces connect, not just whether the app runs.
How many questions should I practise before a MEAN Stack interview?
Work through all 40 questions here, then the company-wise sets for the employers you are targeting. Quality matters more than count: be able to explain why each answer is right.
Which topics matter most for Freshers?
Focus first on TypeScript fundamentals, Angular components and binding, Reactive forms, Project explanation.