Question 1

What is Node.js and what are its main features?

Accepted Answer

Node.js is a JavaScript runtime built on Chrome's V8 engine that allows running JavaScript on the server. Main features include non-blocking I/O, event-driven architecture, single-threaded event loop, npm package ecosystem, and ability to build scalable network applications.

Question 2

Explain the Node.js event loop and how it works.

Accepted Answer

The event loop is Node.js's mechanism for handling asynchronous operations. It has phases: timers (setTimeout/setInterval), pending callbacks, idle/prepare, poll (I/O operations), check (setImmediate), and close callbacks. It continuously processes these phases, executing callbacks when operations complete.

Question 3

What is the difference between process.nextTick() and setImmediate()?

Accepted Answer

process.nextTick() executes before the event loop continues to the next phase, with highest priority. setImmediate() executes in the check phase of the next event loop iteration. Despite names, nextTick runs before setImmediate.

Question 4

How does Node.js handle blocking operations?

Accepted Answer

Node.js offloads blocking operations to libuv thread pool (default 4 threads) or system kernel. Operations like file I/O, DNS lookups use thread pool, while network I/O uses kernel async mechanisms. CPU-intensive tasks should use worker threads or child processes.

Question 5

What is the difference between require() and import?

Accepted Answer

require() is CommonJS (synchronous, used in Node.js), loads modules at runtime. import is ES6 modules (asynchronous, can be used with .mjs or type: "module"), supports static analysis, tree-shaking, and top-level await. CommonJS uses module.exports, ES6 uses export.

Question 6

Explain callback hell and how to avoid it.

Accepted Answer

Callback hell occurs when multiple nested callbacks make code hard to read. Avoid it using promises, async/await, named functions, or libraries like async.js. Promises and async/await provide cleaner error handling and flow control.

Question 7

What are Promises and how do they work?

Accepted Answer

Promises represent eventual completion of async operations. States: pending, fulfilled, rejected. They provide .then() for success, .catch() for errors, .finally() for cleanup. Promises chain and can be used with async/await for cleaner code.

Question 8

What is async/await and how does it work?

Accepted Answer

async/await is syntactic sugar over promises. async functions return promises. await pauses execution until promise resolves. Errors are caught with try/catch. Makes asynchronous code look synchronous and easier to read.

Question 9

What is the difference between spawn, exec, and fork in child_process?

Accepted Answer

spawn launches process, returns stream, non-blocking. exec runs command in shell, buffers output, has size limit. fork is spawn for Node.js processes, enables IPC. Use spawn for large output, exec for simple commands, fork for Node.js workers.

Question 10

What are streams in Node.js and their types?

Accepted Answer

Streams handle data in chunks instead of loading all at once. Types: Readable (data source), Writable (data destination), Duplex (both), Transform (duplex with modification). Benefits: memory efficient, composable, handle backpressure. Examples: fs.createReadStream, HTTP requests.

Question 11

What is middleware in Express.js?

Accepted Answer

Middleware are functions that execute during request-response cycle. They can modify req/res, end cycle, call next middleware. Types: application-level, router-level, error-handling, built-in. Order matters - executes sequentially.

Question 12

How do you handle errors in Node.js?

Accepted Answer

Use try/catch for async/await, .catch() for promises, error-first callbacks, error middleware in Express, process.on('uncaughtException'), process.on('unhandledRejection'). Always handle errors explicitly, log them, and provide meaningful messages.

Question 13

What is the difference between cluster and worker_threads?

Accepted Answer

Cluster creates separate Node.js processes, shares nothing, uses IPC. Worker threads share memory via SharedArrayBuffer, lighter weight, better for CPU tasks. Cluster for isolation, worker threads for parallelism within process.

Question 14

What is the purpose of package.json?

Accepted Answer

package.json defines project metadata, dependencies (dependencies, devDependencies), scripts, version, entry point, license, engines. npm uses it for installs, scripts, publishing. Essential for Node.js project management and dependency tracking.

Question 15

What is the difference between dependencies and devDependencies?

Accepted Answer

dependencies are required for production runtime. devDependencies are only needed during development (testing, building, linting). npm install --production installs only dependencies. Examples: express in dependencies, jest in devDependencies.

Question 16

What is Buffer in Node.js?

Accepted Answer

Buffer is global class for handling binary data. Represents fixed-size memory allocation outside V8 heap. Used for file I/O, network operations, crypto. Created with Buffer.from(), Buffer.alloc(), Buffer.allocUnsafe(). Immutable in modern Node.js.

Question 17

What is the difference between setImmediate() and setTimeout()?

Accepted Answer

setImmediate() executes in check phase of current event loop iteration. setTimeout() executes in timers phase after specified delay. In I/O cycle, setImmediate runs before setTimeout(0). Outside I/O, order depends on performance.

Question 18

How do you create a RESTful API in Express?

Accepted Answer

Use Express router, define routes for HTTP methods (GET, POST, PUT, DELETE), use middleware for parsing (express.json()), validation, authentication. Follow REST conventions: resource-based URLs, proper status codes, JSON responses.

Question 19

What is the purpose of __dirname and __filename?

Accepted Answer

__dirname is directory name of current module. __filename is absolute path of current module file. Both are CommonJS globals. In ES modules, use import.meta.url with fileURLToPath() and dirname() from path module.

Question 20

What is event-driven programming in Node.js?

Accepted Answer

Event-driven programming uses events and event handlers. Node.js is built on event-driven architecture with EventEmitter. Components emit events, listeners respond. Enables non-blocking, reactive programming. Examples: HTTP server, file streams, custom events.

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