1. What is Node.js?
Node.js is a JavaScript runtime built on Chrome’s V8 JavaScript engine that allows you to run JavaScript code on servers and computers, not just in web browsers.
Example:
// hello.js - Run this file with: node hello.js
console.log("Hello from Node.js!");
console.log("Current time:", new Date());2. What is npm?
npm (Node Package Manager) is the default package manager for Node.js that helps you install, manage, and share code libraries and tools.
Example:
# Install a package globally
npm install -g nodemon
# Install a package locally in your project
npm install express
# Install development dependencies
npm install --save-dev jest3. What is a module in Node.js?
A module is a reusable block of code whose exports can be imported into other modules using require() or import.
Example:
// math.js - Creating a module
function add(a, b) {
return a + b;
}
function subtract(a, b) {
return a - b;
}
module.exports = { add, subtract };
// app.js - Using the module
const { add, subtract } = require("./math");
console.log(add(5, 3)); // 8
console.log(subtract(10, 4)); // 64. How do you create a simple HTTP server in Node.js?
You can create an HTTP server using the built-in http module to handle web requests and responses.
Example:
const http = require("http");
const server = http.createServer((req, res) => {
res.writeHead(200, { "Content-Type": "text/html" });
res.end("<h1>Hello World from Node.js Server!</h1>");
});
server.listen(3000, () => {
console.log("Server running on http://localhost:3000");
});5. What is package.json?
package.json is a configuration file that contains metadata about your Node.js project, including dependencies, scripts, and project information.
Example:
{
"name": "my-node-app",
"version": "1.0.0",
"description": "A sample Node.js application",
"main": "app.js",
"scripts": {
"start": "node app.js",
"dev": "nodemon app.js",
"test": "jest"
},
"dependencies": {
"express": "^4.18.0"
},
"devDependencies": {
"nodemon": "^2.0.0"
}
}6. What is the difference between require() and import?
require() is CommonJS syntax (traditional Node.js), while import is ES6 module syntax (modern JavaScript).
Example:
// CommonJS (require)
const fs = require("fs");
const { readFile } = require("fs");
// ES6 Modules (import) - Need "type": "module" in package.json
import fs from "fs";
import { readFile } from "fs";7. What is a callback in Node.js?
A callback is a function passed as an argument to another function, executed after an asynchronous operation completes.
Example:
const fs = require("fs");
// Callback function
fs.readFile("data.txt", "utf8", (err, data) => {
if (err) {
console.error("Error reading file:", err);
return;
}
console.log("File content:", data);
});
console.log("This runs before file is read!");8. What is asynchronous programming?
Asynchronous programming allows multiple operations to run concurrently without blocking the main thread.
Example:
console.log("Start");
// Asynchronous operation
setTimeout(() => {
console.log("This runs after 2 seconds");
}, 2000);
console.log("End"); // This runs immediately
// Output: Start, End, This runs after 2 seconds9. What is the event loop?
The event loop is Node.js’s mechanism for handling asynchronous operations by continuously checking for and executing pending callbacks.
Example:
console.log("1: Start");
setTimeout(() => console.log("2: Timeout"), 0);
setImmediate(() => console.log("3: Immediate"));
process.nextTick(() => console.log("4: Next Tick"));
console.log("5: End");
// Output: 1: Start, 5: End, 4: Next Tick, 3: Immediate, 2: Timeout10. What is blocking vs non-blocking code?
Blocking code stops execution until completion, while non-blocking code allows other operations to continue.
Example:
const fs = require("fs");
// Blocking (synchronous)
console.log("Before blocking read");
const data = fs.readFileSync("file.txt", "utf8");
console.log(data);
console.log("After blocking read");
// Non-blocking (asynchronous)
console.log("Before non-blocking read");
fs.readFile("file.txt", "utf8", (err, data) => {
console.log(data);
});
console.log("After non-blocking read");11. How do you read a file in Node.js?
You can read files using the fs module with both synchronous and asynchronous methods.
Example:
const fs = require("fs");
// Asynchronous reading
fs.readFile("example.txt", "utf8", (err, data) => {
if (err) {
console.error("Error:", err);
return;
}
console.log("File content:", data);
});
// Synchronous reading
try {
const data = fs.readFileSync("example.txt", "utf8");
console.log("Sync content:", data);
} catch (err) {
console.error("Sync error:", err);
}12. What is middleware?
Middleware functions are functions that execute during the request-response cycle, having access to request and response objects.
Example:
const express = require("express");
const app = express();
// Custom middleware
function logger(req, res, next) {
console.log(`${req.method} ${req.url} - ${new Date()}`);
next(); // Continue to next middleware
}
app.use(logger); // Use middleware globally
app.get("/", (req, res) => {
res.send("Hello World!");
});
app.listen(3000);13. What is Express.js?
Express.js is a minimal and flexible Node.js web application framework that provides robust features for web and mobile applications.
Example:
const express = require("express");
const app = express();
// Middleware
app.use(express.json());
// Routes
app.get("/", (req, res) => {
res.json({ message: "Welcome to Express!" });
});
app.post("/users", (req, res) => {
const user = req.body;
res.json({ message: "User created", user });
});
app.listen(3000, () => {
console.log("Express server running on port 3000");
});14. What is the difference between setTimeout and setImmediate?
setTimeout schedules execution after a minimum delay, while setImmediate schedules execution on the next iteration of the event loop.
Example:
console.log("Start");
setTimeout(() => {
console.log("setTimeout");
}, 0);
setImmediate(() => {
console.log("setImmediate");
});
console.log("End");
// Output: Start, End, setImmediate, setTimeout15. What is a Promise in Node.js?
A Promise is an object representing the eventual completion or failure of an asynchronous operation.
Example:
const fs = require("fs").promises;
// Creating a Promise
function readFilePromise(filename) {
return new Promise((resolve, reject) => {
fs.readFile(filename, "utf8")
.then((data) => resolve(data))
.catch((err) => reject(err));
});
}
// Using Promise
readFilePromise("example.txt")
.then((data) => console.log("Content:", data))
.catch((err) => console.error("Error:", err));16. What is async/await?
async/await is syntactic sugar for Promises, making asynchronous code look and behave more like synchronous code.
Example:
const fs = require("fs").promises;
async function readMultipleFiles() {
try {
const file1 = await fs.readFile("file1.txt", "utf8");
const file2 = await fs.readFile("file2.txt", "utf8");
console.log("File 1:", file1);
console.log("File 2:", file2);
} catch (error) {
console.error("Error reading files:", error);
}
}
readMultipleFiles();17. What is the global object in Node.js?
The global object in Node.js is similar to window in browsers, providing access to global variables and functions.
Example:
// Global variables
global.myGlobalVar = "Hello Global!";
// Access in another file
console.log(global.myGlobalVar); // Hello Global!
// Common global properties
console.log(global.process.version); // Node.js version
console.log(global.__dirname); // Current directory
console.log(global.__filename); // Current file18. What is process.env?
process.env is an object containing environment variables, commonly used for configuration settings.
Example:
// Set environment variables in .env file or system
// PORT=3000
// DATABASE_URL=mongodb://localhost:27017/mydb
const port = process.env.PORT || 5000;
const dbUrl = process.env.DATABASE_URL || "default-db-url";
console.log("Server will run on port:", port);
console.log("Database URL:", dbUrl);
// Setting environment variable in code
process.env.NODE_ENV = "development";19. How do you handle errors in Node.js?
Error handling in Node.js can be done using try-catch blocks, error callbacks, and Promise rejections.
Example:
const fs = require("fs");
// Callback error handling
fs.readFile("nonexistent.txt", (err, data) => {
if (err) {
console.error("Callback error:", err.message);
return;
}
console.log(data);
});
// Promise error handling
fs.promises
.readFile("nonexistent.txt")
.catch((err) => console.error("Promise error:", err.message));
// Async/await error handling
async function readFileAsync() {
try {
const data = await fs.promises.readFile("nonexistent.txt");
console.log(data);
} catch (err) {
console.error("Async/await error:", err.message);
}
}20. What is nodemon?
nodemon is a development tool that automatically restarts your Node.js application when file changes are detected.
Example:
# Install nodemon globally
npm install -g nodemon
# Or install as dev dependency
npm install --save-dev nodemon
# Use nodemon instead of node
nodemon app.js
# Add to package.json scripts
# "scripts": {
# "dev": "nodemon app.js"
# }
# npm run dev