Skip to content

Node_Express_Backend_Notes.md

Latest commit

 

History

History
249 lines (196 loc) · 5.09 KB

Node_Express_Backend_Notes.md

File metadata and controls

249 lines (196 loc) · 5.09 KB
Raw

Node.js and Express.js Backend Notes

1. Node.js Deep Dive

Node.js is a JavaScript runtime built on Chrome's V8 engine. It is designed for fast, non-blocking server-side applications.

Event Loop

  • Node.js uses a single-threaded event loop to manage asynchronous work.
  • The event loop lets Node handle many operations without blocking the main thread.
  • I/O tasks are usually delegated to the system or libuv, then callbacks are queued when ready.

Event Loop Phases

  • timers
  • pending callbacks
  • idle, prepare
  • poll
  • check
  • close callbacks

Important Ideas

  • Synchronous code runs first.
  • Microtasks and promise callbacks are handled with high priority.
  • setTimeout and setImmediate do not always run in the same order.
  • CPU-heavy work can block the event loop and slow the app.

Buffer

  • A Buffer is a raw binary data container.
  • Buffers are useful for files, streams, network packets, and binary APIs.
const buf = Buffer.from("hello");
console.log(buf.toString());

Streams

Streams let you process data piece by piece instead of loading everything into memory at once.

Types of Streams

  • readable
  • writable
  • duplex
  • transform

Why Streams Matter

  • memory efficient
  • good for large files
  • useful for request/response piping

Example

const fs = require("fs");
const readStream = fs.createReadStream("big-file.txt");
readStream.on("data", chunk => {
  console.log(chunk.toString());
});

fs Module

  • fs gives file-system access.
  • It supports synchronous and asynchronous methods.

Common Methods

  • readFile
  • writeFile
  • appendFile
  • unlink
  • mkdir
  • readdir
  • stat

Async Example

const fs = require("fs");

fs.readFile("notes.txt", "utf8", (err, data) => {
  if (err) throw err;
  console.log(data);
});

Sync Example

const data = fs.readFileSync("notes.txt", "utf8");

npm

  • npm is Node's package manager.
  • It installs and manages third-party dependencies.

Important Files

  • package.json: project metadata and scripts
  • package-lock.json: exact dependency tree

Common Commands

  • npm init
  • npm install
  • npm install <package>
  • npm install -D <package>
  • npm uninstall <package>
  • npm run <script>

dotenv

  • dotenv loads environment variables from a .env file into process.env.
  • This keeps secrets out of source code.
require("dotenv").config();
console.log(process.env.PORT);

Best Practice

  • Store secrets in .env
  • Add .env to .gitignore
  • Use environment variables for ports, keys, and database URLs

Modules

Node supports modular code organization.

CommonJS

const fs = require("fs");
module.exports = myValue;

ES Modules

import fs from "fs";
export default myValue;

Notes

  • CommonJS is traditional in Node.
  • ES Modules are the modern standard.
  • Use one module system consistently within a project.

2. Express.js Essentials

Express is a web framework for Node.js that simplifies routing, middleware, and request handling.

Creating a Server

const express = require("express");
const app = express();

app.listen(3000, () => {
  console.log("Server running on port 3000");
});

Routing

  • Routes map HTTP methods and paths to handlers.
  • Common methods: GET, POST, PUT, PATCH, DELETE
app.get("/users", (req, res) => {
  res.send("All users");
});

app.post("/users", (req, res) => {
  res.send("Create user");
});

Middleware Deep Dive

Middleware is a function that runs between request and response.

What Middleware Can Do

  • read or modify req and res
  • end the response
  • call next() to continue
  • handle errors

Common Middleware Types

  • application-level middleware
  • router-level middleware
  • built-in middleware
  • third-party middleware
  • error-handling middleware

Example

app.use((req, res, next) => {
  console.log(req.method, req.url);
  next();
});

Built-in Middleware

  • express.json()
  • express.urlencoded({ extended: true })
  • express.static()

Params and Queries

Route Params

app.get("/users/:id", (req, res) => {
  res.send(req.params.id);
});

Query Strings

app.get("/search", (req, res) => {
  res.send(req.query.term);
});

Cookies and Sessions

  • Cookies are small pieces of data stored in the browser.
  • Sessions store server-side state and usually use a cookie as the session identifier.

Cookie Uses

  • auth tokens
  • preferences
  • tracking identifiers

Session Uses

  • logged-in state
  • temporary user data
  • server-managed carts

Error Handling

Express uses special error middleware.

app.use((err, req, res, next) => {
  console.error(err);
  res.status(500).json({ message: "Something went wrong" });
});

Best Practices

  • validate input early
  • return proper status codes
  • keep errors consistent
  • avoid leaking sensitive stack traces in production

Express Notes People Often Forget

  • order of middleware matters
  • next() is required to continue
  • route handlers can be arrays or separate functions
  • res.status(...).json(...) is a common response pattern