Question 1

What is the role of the describe() function in Jest?

Accepted Answer

The describe() function groups multiple related tests together in a test suite. It improves readability by organizing tests by feature or component. Each describe() can contain multiple it() or test() to describe individual behaviors.

Question 2

What is the official npm package used by NestJS for managing scheduled tasks?

Accepted Answer

@nestjs/schedule is the official package integrated with NestJS for managing cron jobs, intervals, and timeouts. It relies on the node-cron package under the hood. Installing this package enables the use of @Cron, @Interval, and @Timeout decorators in providers.

Question 3

What is the event loop in Node.js?

Accepted Answer

The event loop is the central mechanism in Node.js that enables asynchronous execution. It monitors the call stack and callback queue, executing callbacks when the call stack is empty. This mechanism allows Node.js to handle thousands of simultaneous connections without creating additional threads, making the model very efficient for I/O operations.

Question 4

Which method from the fs module allows reading a file asynchronously?

Accepted Answer

fs.readFile() reads the entire file asynchronously and returns its content in a callback. The fs.read() method exists but works with file descriptors for partial reads. fs.readFileSync() is the synchronous version. This asynchronous API avoids blocking the event loop during expensive I/O operations.

Question 5

What is a module in NestJS?

Accepted Answer

A module is a class annotated with the @Module() decorator that organizes code in a cohesive way. It encapsulates a set of providers, controllers, and other modules, allowing the application to be structured into modular components. Each NestJS application has at least one root module that serves as the entry point.

Question 6

What is the ConfigModule in NestJS?

Accepted Answer

ConfigModule is an official NestJS module that centralizes application configuration management. It allows loading environment variables from .env files, validating them with schemas, and injecting them into services via ConfigService. Unlike direct access to process.env, it provides strong typing and facilitates unit testing.

Question 7

What is a JWT (JSON Web Token)?

Accepted Answer

A JWT is an open standard (RFC 7519) that defines a compact and secure format for transmitting information between parties as a JSON object. It is digitally signed to ensure its integrity and can be encrypted. JWTs are commonly used for stateless authentication in REST APIs because they contain all necessary information without requiring server-side sessions.

Question 8

What is a modular monolith?

Accepted Answer

A modular monolith is an application deployed as a single unit but structured into strongly decoupled modules with clear boundaries. Unlike a classic monolith where everything is mixed, the modular monolith organizes code into independent modules that communicate via well-defined interfaces. This approach combines monolith simplicity (single deployment, simple ACID transactions) with microservices maintainability (separation of concerns, clear boundaries).

Question 9

What is Helmet in the context of NestJS?

Accepted Answer

Helmet is middleware that automatically configures HTTP security headers to protect applications against common vulnerabilities. It enables headers like X-Frame-Options, Content-Security-Policy, X-Content-Type-Options to prevent XSS, clickjacking, and MIME sniffing. Helmet is an essential production best practice to strengthen the security surface.

Question 10

Which approach optimizes NestJS container deployment in AWS ECS?

Accepted Answer

To optimize ECS deployment of NestJS containers: use multi-stage Docker builds to reduce final image size (build stage + minimal production stage), leverage Docker layer caching to accelerate builds, use ECR (Elastic Container Registry) in same region to reduce latency, configure appropriate health checks, use Fargate to simplify management, define optimal resource limits (CPU/memory), implement graceful shutdown for SIGTERM. An optimized Node.js alpine image can be <100MB vs >1GB unoptimized.

Node.js / NestJS interview questions

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