Koa.js

Koa is a minimalist and expressive web framework for Node.js, designed to build web applications and APIs with a strong emphasis on async/await for handling requests and responses.

Core Concepts

1. Lightweight and Modular:
   Koa provides a minimal core without built-in middleware, encouraging developers to add only what they need.

2. Async/Await Middleware:
   Middleware is written using async functions, making asynchronous code clean and readable.

3. Context Object (ctx):
   The ctx object represents the request and response, providing methods and properties for interacting with them.

Basic Setup

1. Installation:

   npm install koa

2. Simple Koa Server:

   const Koa = require("koa");
   const app = new Koa();
   
   app.use(async (ctx) => {
     ctx.body = "Hello, Koa!";
   });
   
   app.listen(3000, () => {
     console.log("Server running at http://localhost:3000");
   });

Routing Example (Using koa-router)

1. Install koa-router:

   npm install koa-router

2. Basic Routing:

   const Koa = require("koa");
   const Router = require("koa-router");
   
   const app = new Koa();
   const router = new Router();
   
   router.get("/", (ctx) => {
     ctx.body = "Welcome to Koa!";
   });
   
   router.get("/user/:id", (ctx) => {
     ctx.body = `User ID: ${ctx.params.id}`;
   });
   
   app.use(router.routes()).use(router.allowedMethods());
   
   app.listen(3000);

Middleware Examples

1. Logging Middleware:

   app.use(async (ctx, next) => {
     const start = Date.now();
     await next();
     const ms = Date.now() - start;
     console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
   });

2. Error Handling Middleware:

   app.use(async (ctx, next) => {
     try {
       await next();
     } catch (err) {
       ctx.status = err.status || 500;
       ctx.body = { error: err.message };
       ctx.app.emit("error", err, ctx);
     }
   });

3. Body Parsing Middleware (Using koa-bodyparser):

   npm install koa-bodyparser

   const bodyParser = require("koa-bodyparser");
   app.use(bodyParser());
   
   app.use(async (ctx) => {
     if (ctx.method === "POST") {
       ctx.body = `Received: ${JSON.stringify(ctx.request.body)}`;
     }
   });

Common ctx Methods

Method	Description
ctx.body	Sets the response body
ctx.status	Sets the HTTP status code
ctx.method	Access the request method
ctx.url	Access the request URL
ctx.request	Access request object
ctx.response	Access response object
ctx.params	Access route parameters (via router)
ctx.query	Access query string parameters

tip

1. Use Koa Middleware:
   Add essential middleware like koa-router, koa-bodyparser, and koa-static to handle routing, parsing, and serving static files.

2. Embrace Async/Await:
   Koa’s async/await pattern makes it easy to handle asynchronous operations cleanly.

3. Error Handling:
   Implement global error-handling middleware early to catch and manage errors gracefully.

4. Check Koa Documentation:
   Visit the Koa Docs for deeper insights into the framework and advanced usage.