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MEAN Stack Mock Interview: 20 Questions in 50 Minutes

Four rounds that copy a real interview. Answer aloud, then check the model answers.

Mock interview format

  1. Warm-up · 5 minutes — 4 questions
  2. Technical · 20 minutes — 7 questions
  3. Scenario · 15 minutes — 5 questions
  4. HR & behavioural · 10 minutes — 4 questions

Tip: record yourself and listen back. Most people talk too fast and skip the key idea.

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01

What is middleware in Express, and what kinds of middleware are there?

Warm-upNode.js and Express APIsEasy
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Middleware is a function with access to req, res and next that runs in order for matching requests. Kinds include application-level middleware registered with app.use, router-level middleware on an express.Router, built-in middleware like express.json and express.static, third-party middleware such as cors, helmet and morgan, and error-handling middleware with four arguments. Middleware can end the response or pass control with next().
02

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

Warm-upTypeScript 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.
03

What are databases, collections and documents in MongoDB?

Warm-upMongoDB Data Modelling and QueriesEasy
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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.
04

Why is Node.js a good fit for the backend in a MEAN application?

Warm-upNode.js and Express APIsEasy
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Node.js runs JavaScript or TypeScript on the server with a non-blocking, event-driven model, so it handles many concurrent I/O-heavy requests efficiently. The team uses one language family across Angular and the API, can share TypeScript models, and has access to the large npm ecosystem. It is ideal for REST APIs, real-time features and microservices, but CPU-heavy tasks should move to workers or separate services.
05

Define a typed Mongoose model in TypeScript with an instance method.

TechnicalMongoDB Data Modelling and QueriesMedium
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Interface, methods type and schema together. import { Schema, model, Model } from 'mongoose'; import bcrypt from 'bcrypt'; interface IUser { name: string; email: string; password: string; role: 'user' | 'admin' } interface IUserMethods { comparePassword(plain: string): Promise<boolean> } type UserModel = Model<IUser, {}, IUserMethods>; const userSchema = new Schema<IUser, UserModel, IUserMethods>({ name: { type: String, required: true }, email: { type: String, required: true, unique: true, lowercase: true }, password: { type: String, required: true, select: false }, role: { type: String, enum: ['user', 'admin'], default: 'user' } }, { timestamps: true }); userSchema.method('comparePassword', function (plain: string) { return bcrypt.compare(plain, this.password); }); export const User = model<IUser, UserModel>('User', userSchema);
06

Write a type-ahead search using a reactive FormControl and RxJS operators.

TechnicalRxJS, Signals and State ManagementMedium
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switchMap cancels stale requests. export class SearchComponent { private api = inject(ProductService); search = new FormControl('', { nonNullable: true }); results = toSignal( this.search.valueChanges.pipe( debounceTime(300), map((t) => t.trim()), distinctUntilChanged(), filter((t) => t.length >= 2), switchMap((t) => this.api.search(t).pipe(catchError(() => of([])))) ), { initialValue: [] } ); } // template: <input [formControl]='search'> @for (p of results(); track p._id) {...}
07

Serve the built Angular app and the API from one Express server.

TechnicalNode.js and Express APIsMedium
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API first, static files next, fallback last. import express from 'express'; import path from 'node:path'; const app = express(); app.use(express.json()); app.use('/api', apiRouter); const dist = path.join(process.cwd(), 'dist/shop/browser'); app.use(express.static(dist, { maxAge: '1y', index: false })); app.get('/*splat', (req, res) => { res.sendFile(path.join(dist, 'index.html')); }); app.listen(process.env.PORT || 3000); The named wildcard follows Express 5 path syntax.
08

Write a custom attribute directive that highlights an element on hover.

TechnicalAngular Components and TemplatesMedium
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Host listeners handle the events. import { Directive, ElementRef, HostListener, inject, input } from '@angular/core'; @Directive({ selector: '[appHighlight]' }) export class HighlightDirective { private el = inject(ElementRef); color = input('yellow', { alias: 'appHighlight' }); @HostListener('mouseenter') onEnter() { this.el.nativeElement.style.backgroundColor = this.color(); } @HostListener('mouseleave') onLeave() { this.el.nativeElement.style.backgroundColor = ''; } } // usage: <p appHighlight='lightblue'>Hover me</p>
09

Configure routes with lazy loading, a child route and a wildcard 404 route.

TechnicalServices, Dependency Injection and RoutingMedium
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app.routes.ts: import { Routes } from '@angular/router'; export const routes: Routes = [ { path: '', loadComponent: () => import('./home/home.component').then(m => m.HomeComponent) }, { path: 'products', loadChildren: () => import('./products/product.routes').then(m => m.PRODUCT_ROUTES) }, { path: 'login', loadComponent: () => import('./auth/login.component').then(m => m.LoginComponent) }, { path: '**', loadComponent: () => import('./not-found.component').then(m => m.NotFoundComponent) } ]; // app.config.ts providers: [provideRouter(routes, withComponentInputBinding())]
10

What are directives in Angular and what types exist?

TechnicalAngular 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.
11

Write a cart service using signals with computed totals.

TechnicalRxJS, Signals and State ManagementMedium
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Expose read-only state and mutation methods. @Injectable({ providedIn: 'root' }) export class CartService { private items = signal<CartItem[]>([]); readonly cart = this.items.asReadonly(); readonly count = computed(() => this.items().reduce((n, i) => n + i.qty, 0)); readonly total = computed(() => this.items().reduce((s, i) => s + i.qty * i.price, 0)); add(product: Product) { this.items.update((list) => { const found = list.find((i) => i.id === product.id); return found ? list.map((i) => (i.id === product.id ? { ...i, qty: i.qty + 1 } : i)) : [...list, { ...product, qty: 1 }]; }); } remove(id: string) { this.items.update((l) => l.filter((i) => i.id !== id)); } }
12

Every API request needs a JWT header, and expired tokens should trigger a refresh without logging the user out. How do you implement this?

ScenarioAngular Forms and HTTP ClientHard
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Write an auth interceptor that clones each request with the Authorization header from the auth service. In the response path, catch 401 errors, call the refresh endpoint once, store the new access token, and retry the original request. Use a shared refresh Observable with shareReplay or a flag so parallel failed requests wait for one refresh. If refresh fails, clear the session and redirect to login.
13

You see ExpressionChangedAfterItHasBeenCheckedError in development. What does it mean and how do you fix it?

ScenarioAngular Components and TemplatesHard
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In development Angular runs an extra check to make sure a binding did not change after it was already checked in the same cycle. The error usually appears when a child or a lifecycle hook like ngAfterViewInit changes parent data during rendering. Fix it by moving the update earlier, such as into ngOnInit, deriving the value with a computed signal, or restructuring data flow, rather than hiding it with setTimeout.
14

You are migrating a large JavaScript Node.js codebase to TypeScript. How would you approach it?

ScenarioTypeScript and Modern JavaScriptHard
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Migrate gradually. Add TypeScript with allowJs so JS and TS files coexist, start with loose settings and turn on strict options step by step. Convert leaf modules such as utilities and models first, then services and routes. Install type packages like @types/express, define shared interfaces for request and response shapes, and run type checks in CI. Avoid a big-bang rewrite that blocks feature work.
15

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?

ScenarioMongoDB Data Modelling and QueriesMedium
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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.
16

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

ScenarioAngular Forms and HTTP ClientMedium
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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.
17

Tell me about a time you improved test coverage or quality in a project.

HR & behaviouralSecurity, Testing and DeploymentMedium
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Situation: our MEAN project had almost no tests and every release broke something. Task: I volunteered to improve this. Action: I set up Supertest with an in-memory MongoDB for critical APIs, added component tests for checkout, wrote a short testing guide and made tests required in CI. I started with the five most bug-prone areas. Result: production bugs per release dropped sharply within three months.
18

Describe how you handled a situation where requirements changed late in a sprint.

HR & behaviouralServices, Dependency Injection and RoutingMedium
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Situation: two days before release, the client asked to add role-based access to a reporting page. Task: deliver without breaking the release. Action: I clarified the minimum needed with the product owner, implemented a canMatch guard and hid admin actions, and coordinated with the backend developer for API checks, while moving extra filters to the next sprint. Result: we released on time and the client approved the change.
19

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

HR & behaviouralServices, 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.
20

How do you decide where state should live in a large Angular application?

HR & behaviouralServices, Dependency Injection and RoutingHard
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Situation: our team argued over putting everything in a global store. Task: as tech lead I had to set guidelines. Action: I proposed rules: local UI state in components with signals, feature state in feature-scoped services or SignalStore, truly global data like the user and settings in root services, and server data cached in services. I documented examples and reviewed the first PRs. Result: fewer bugs and simpler components.

Frequently asked questions

How do I use this mock interview?

Set a 50-minute timer, answer every question aloud (or ask a friend to read them), and open answers only after each round. Note every question where your answer missed the key idea.

How often should I take a mock?

Once a week while preparing and once two or three days before a real interview. The question order is fixed, so for fresh practice use the company and experience pages too.

More ways to practise

Company-wise MEAN Stack questionsExperience-wise MEAN Stack questionsRole-wise MEAN Stack questionsCity-wise MEAN Stack questionsMost asked MEAN Stack questionsScenario-based questionsPractical / hands-on questionsHR & behavioural questionsOnline assessment questionsTechnical round questionsManagerial round questionsMEAN Stack topic sheets