Learn KOA-JS with Real Code Examples

const Koa = require('koa');
const Router = require('@koa/router');
const bodyParser = require('koa-bodyparser');

const app = new Koa();
const router = new Router();

let count = 0;

router.get('/counter', ctx => {
    ctx.body = { count };
});

router.post('/counter/increment', ctx => {
    count++;
    ctx.body = { count };
});

router.post('/counter/decrement', ctx => {
    count--;
    ctx.body = { count };
});

router.post('/counter/reset', ctx => {
    count = 0;
    ctx.body = { count };
});

app.use(bodyParser());
app.use(router.routes()).use(router.allowedMethods());

app.listen(3000, () => {
    console.log('Koa.js server running on http://localhost:3000');
});

Demonstrates a simple Koa.js REST API with a counter using async functions and middleware.

const Koa = require('koa');
const app = new Koa();

app.use(ctx => {
    ctx.body = { message: 'Hello World' };
});

app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

A basic Koa.js REST API returning 'Hello World'.

const Koa = require('koa');
const Router = require('@koa/router');
const bodyParser = require('koa-bodyparser');

const app = new Koa();
const router = new Router();

router.post('/echo', ctx => {
    ctx.body = ctx.request.body;
});

app.use(bodyParser());
app.use(router.routes()).use(router.allowedMethods());

app.listen(3000, () => {
    console.log('Koa.js server running on http://localhost:3000');
});

A POST endpoint that echoes back JSON data.

const Koa = require('koa');
const Router = require('@koa/router');
const app = new Koa();
const router = new Router();

router.get('/greet', ctx => {
    const name = ctx.query.name || 'Guest';
    ctx.body = { message: `Hello ${name}` };
});

app.use(router.routes()).use(router.allowedMethods());
app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

Demonstrates usage of query parameters in Koa.js.

const Koa = require('koa');
const Router = require('@koa/router');
const app = new Koa();
const router = new Router();

router.get('/users/:id', ctx => {
    ctx.body = { id: ctx.params.id, name: `User ${ctx.params.id}` };
});

app.use(router.routes()).use(router.allowedMethods());
app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

An API showing route parameters in Koa.js.

const Koa = require('koa');
const app = new Koa();

app.use(async (ctx, next) => {
    console.log(`${ctx.method} ${ctx.url}`);
    await next();
});

app.use(ctx => {
    ctx.body = { message: 'Check logs for request details' };
});

app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

Middleware example to log all incoming requests.

const Koa = require('koa');
const Router = require('@koa/router');
const app = new Koa();
const router = new Router();

router.get('/async', async ctx => {
    const data = await new Promise(resolve => setTimeout(() => resolve('Async Response'), 500));
    ctx.body = { data };
});

app.use(router.routes()).use(router.allowedMethods());
app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

An async route handler returning data after a delay.

const Koa = require('koa');
const app = new Koa();

app.use(ctx => {
    ctx.status = 404;
    ctx.body = { error: 'Not Found' };
});

app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

Middleware example to handle 404 responses.

const Koa = require('koa');
const app = new Koa();

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

app.use(ctx => {
    ctx.body = { message: 'Hello Koa' };
});

app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

Global error handling middleware example in Koa.js.

const Koa = require('koa');
const Router = require('@koa/router');
const bodyParser = require('koa-bodyparser');

const app = new Koa();
const router = new Router();

app.use(async (ctx, next) => {
    console.log(`${ctx.method} ${ctx.url}`);
    await next();
});

app.use(bodyParser());

router.get('/', ctx => {
    ctx.body = { message: 'Welcome to Koa API' };
});

router.post('/echo', ctx => {
    ctx.body = ctx.request.body;
});

app.use(router.routes()).use(router.allowedMethods());
app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

Combines logging, body parsing, and routes in a single Koa.js app.