MEAN Stack Interview Questions in Bangalore

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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.

How do you set up an Express project with TypeScript?

Node.js and Express APIsEasy

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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.

How do you approach learning a new framework version or language feature on the job?

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Situation: our team decided to adopt Angular signals in new features. Task: I had to use them in a sprint while still delivering. Action: I read the official guide, built a small sample, compared it with our RxJS patterns, then implemented one component and asked a senior to review. I shared a short note with examples for the team. Result: my component became the reference pattern for later features.

You are building an e-commerce product catalogue where each category has different attributes, such as size for clothes and RAM for laptops. How would you model it in MongoDB?

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Store common fields like name, price, category and stock at the top level, and keep variable specifications in an attributes array of { key, value } pairs, the attribute pattern. Create a compound index on attributes.key and attributes.value so filters work on any attribute. This avoids dozens of sparse fields and lets new categories be added without schema changes, while Mongoose validates the common fields.

What is MongoDB Atlas and what features does it offer?

MongoDB Data Modelling and QueriesEasy

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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.

How does change detection work in Angular, and what does OnPush do?

Angular Components and TemplatesMedium

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Change detection checks component bindings and updates the DOM when data changes. Traditionally Zone.js patched async APIs and triggered a check of the whole tree after events, timers and HTTP calls. With ChangeDetectionStrategy.OnPush, a component is checked only when an input reference changes, an event fires inside it, an async pipe emits or a signal it reads changes. OnPush with immutable data greatly reduces work.

What is type narrowing and what are type guards?

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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.

Create a text index and run a relevance-sorted search in MongoDB.

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Text search with scores: db.articles.createIndex({ title: 'text', body: 'text' }, { weights: { title: 5, body: 1 } }) db.articles.find( { $text: { $search: 'angular signals' } }, { score: { $meta: 'textScore' }, title: 1 } ).sort({ score: { $meta: 'textScore' } }).limit(10) Weights make title matches rank higher. A collection can have only one text index, so include all searchable fields in it.

How do you use Mongoose with TypeScript?

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Define a TypeScript interface for the document, then create a Schema typed with that interface and a model with model<IUser>('User', userSchema). Mongoose infers types for queries, so findById returns a typed document. For lean queries, results are plain typed objects. Schema options like timestamps add createdAt and updatedAt, and you can type instance methods, statics and virtuals through additional generic parameters.

What is the difference between signal, computed and effect?

RxJS, Signals and State ManagementEasy

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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.

Write an aggregation that finds the top five customers by total spend with their names.

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Group, sort, limit, then join. db.orders.aggregate([ { $match: { status: 'paid' } }, { $group: { _id: '$customerId', spend: { $sum: '$total' }, orders: { $sum: 1 } } }, { $sort: { spend: -1 } }, { $limit: 5 }, { $lookup: { from: 'users', localField: '_id', foreignField: '_id', as: 'customer' } }, { $unwind: '$customer' }, { $project: { _id: 0, name: '$customer.name', spend: 1, orders: 1 } } ]) Joining after $limit means only five lookups run.

Your MongoDB collection of sensor readings is growing by millions of documents per day. How would you design for it?

MongoDB Data Modelling and QueriesHard

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Use a time series collection, which stores readings efficiently in compressed buckets with a timeField and metaField such as deviceId. Set expireAfterSeconds to drop old raw data, and keep hourly or daily aggregates in a separate collection for dashboards. Index on metaField and time, write in batches with insertMany, and plan sharding by device or a hashed key if a single replica set cannot keep up.

What is the difference between template-driven and reactive forms in Angular?

Angular Forms and HTTP ClientEasy

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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.

How do you validate request data in an Express API, and why is it necessary even with Angular validation?

Node.js and Express APIsEasy

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Frontend validation improves user experience, but anyone can call the API directly with tools like Postman or curl, so the server must validate everything. Use a schema library such as zod, Joi or express-validator in middleware to check types, required fields, lengths and formats, reject unknown fields and return 400 with clear field errors. Mongoose validation is a second layer close to the data.

Show the tsconfig settings you would enable for a strict Node.js Express API.

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A typical strict configuration: { "compilerOptions": { "target": "ES2022", "module": "NodeNext", "moduleResolution": "NodeNext", "outDir": "dist", "rootDir": "src", "strict": true, "noUncheckedIndexedAccess": true, "esModuleInterop": true, "skipLibCheck": true, "sourceMap": true }, "include": ["src"] } noUncheckedIndexedAccess adds undefined to indexed lookups, catching missing keys.

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 standalone Angular component that displays a list of products using @for.

Angular Components and TemplatesEasy

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Signals and built-in control flow keep it concise. import { Component, signal } from '@angular/core'; import { CurrencyPipe } from '@angular/common'; @Component({ selector: 'app-product-list', imports: [CurrencyPipe], template: ` @for (p of products(); track p.id) { <div class='card'>{{ p.name }} - {{ p.price | currency:'INR' }}</div> } @empty { <p>No products found</p> } ` }) export class ProductListComponent { products = signal([{ id: 1, name: 'Laptop', price: 55000 }]); }

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

Angular Forms and HTTP ClientMedium

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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) { ... }

What are Mongoose virtuals, instance methods and statics?

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Virtuals are computed properties not stored in MongoDB, such as fullName from firstName and lastName, or a virtual populate that links related documents. Instance methods are functions available on each document, like user.comparePassword(). Statics are functions on the model itself, like User.findByEmail(email). Virtuals are excluded from JSON output unless you enable toJSON: { virtuals: true } in schema options.

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

RxJS, Signals and State ManagementMedium

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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.

Write a multi-stage Dockerfile that builds Angular and serves it with Nginx.

Security, Testing and DeploymentMedium

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Build in Node, ship only static files. FROM node:22-alpine AS build WORKDIR /app COPY package*.json ./ RUN npm ci COPY . . RUN npx ng build --configuration production FROM nginx:alpine COPY nginx.conf /etc/nginx/conf.d/default.conf COPY --from=build /app/dist/shop/browser /usr/share/nginx/html EXPOSE 80 The final image contains no Node.js or source code. nginx.conf should include try_files $uri /index.html for Angular routes.

Users can search products by name, and a regex query has become very slow. What would you do?

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A regex without an anchored prefix cannot use a normal index efficiently and scans the collection. For simple word search, create a text index and use $text with a relevance score. For a better search experience with typo tolerance, autocomplete and facets, use Atlas Search or a dedicated search engine. Paginate results, limit fields with projection and debounce requests from the Angular search box.

Your app must call different API base URLs for development, staging and production. How do you configure this in Angular?

Services, Dependency Injection and RoutingMedium

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Use environment files such as environment.ts and environment.prod.ts with fileReplacements in angular.json build configurations, or load a config.json at startup with provideAppInitializer so one build works in every environment. Expose the URL through an InjectionToken and inject it in services or an interceptor. Never put secrets in these files, because everything in the Angular bundle is public.

Why does Angular use TypeScript, and what benefits does it bring to a MEAN project?

TypeScript and Modern JavaScriptEasy

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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.

Users lose a long half-filled form when they accidentally click a menu link. How would you prevent this?

Services, Dependency Injection and RoutingMedium

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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.

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

RxJS, Signals and State ManagementMedium

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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.

What is the difference between valueChanges and statusChanges, and what does updateOn do?

Angular Forms and HTTP ClientMedium

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valueChanges is an Observable that emits the new value whenever a control or group changes, useful for dependent fields or type-ahead search. statusChanges emits when validation status changes between VALID, INVALID, PENDING and DISABLED. The updateOn option controls when value and validation update: change by default, blur to validate when the field loses focus, or submit to validate only on form submission.

How do you test an Angular service that makes HTTP calls?

Security, Testing and DeploymentMedium

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Provide provideHttpClient() and provideHttpClientTesting() in TestBed, then inject the service and HttpTestingController. Call the service method and subscribe, use httpMock.expectOne('/api/products') to capture the request, assert its method and body, and flush a fake response. Finally call httpMock.verify() to ensure no unexpected requests remain. This tests the service logic without a real backend.

Your Express API is used by thousands of users and occasionally crashes due to an unhandled promise rejection. How do you make it more resilient?

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Find the root cause from logs and fix missing await or catch blocks. Use Express 5, which forwards async errors to error middleware, or an async wrapper in Express 4. Add a central error handler, log unhandledRejection and uncaughtException and then exit cleanly. Run under PM2 or a container orchestrator with multiple instances and health checks so a crash restarts one instance while others keep serving.

How does the Node.js event loop affect the way you write Express route handlers?

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Every request handler runs on the same main thread, so any synchronous heavy work, such as a large loop, JSON parsing of huge payloads, sync file reads or bcrypt with sync methods, blocks all other requests. Write handlers with async I/O, keep CPU work small, stream large data, and offload heavy jobs to worker threads or a queue. Monitor event-loop lag in production to catch blocking code.

How do you serve an Angular build and an Express API from the same Node server?

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Build Angular with ng build, which outputs static files to dist/project/browser. In Express, register API routes under /api first, then serve the static files with express.static, and finally add a catch-all GET route that returns index.html so Angular routing works on refresh. In larger deployments, Nginx or a CDN usually serves the Angular files while Express handles only the API.

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

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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.

Write a generic TypeScript function that groups an array of items by a key.