Learn Koa-js - 10 Code Examples & CST Typing Practice Test

Koa.js is a modern, minimalist web framework for Node.js, created by the same team behind Express.js. It leverages async/await for clean middleware handling and provides a lightweight foundation for building APIs and web applications.

View all 10 Koa-js code examples →

Koa.js Simple Counter API Koa.js Hello World API Koa.js JSON Echo Koa.js Query Parameter Example Koa.js Route Parameter Example Koa.js Middleware Logging Koa.js Async Handler Example Koa.js 404 Middleware Example Koa.js Error Handling Example Koa.js Combined Middleware Example

Learn KOA-JS with Real Code Examples

Updated Nov 25, 2025

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Code Sample Descriptions

Koa.js Simple Counter API

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.

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Koa.js Hello World API

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'.

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Koa.js JSON Echo

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.

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Koa.js Query Parameter Example

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.

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Koa.js Route Parameter Example

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.

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Koa.js Middleware Logging

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.

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Koa.js Async Handler Example

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.

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Koa.js 404 Middleware Example

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.

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Koa.js Error Handling Example

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.

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Koa.js Combined Middleware Example

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.

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Frequently Asked Questions about Koa-js

What is Koa-js?

What are the primary use cases for Koa-js?

Building RESTful APIs with async/await middleware. Small to medium web application backends. Microservices requiring modular architecture. Integration with custom routing and authentication solutions. Prototyping fast and lightweight Node.js servers

What are the strengths of Koa-js?

Clean async/await syntax for middleware. Lightweight and modular. Flexibility to choose libraries as needed. Good performance for small-to-medium workloads. Strong community backing from Express creators

What are the limitations of Koa-js?

No built-in routing or utilities (requires external packages like koa-router). Requires manual setup for common web tasks. Smaller ecosystem compared to Express. Not ideal for large-scale opinionated frameworks. Less beginner-friendly due to minimal abstractions

How can I practice Koa-js typing speed?

CodeSpeedTest offers 10+ real Koa-js code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.

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Code Sample Descriptions

Koa.js Simple Counter API

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.

Let’s Try →

Koa.js Hello World API

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'.

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Koa.js JSON Echo

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.

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Koa.js Query Parameter Example

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.

Let’s Try →

Koa.js Route Parameter Example

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.

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Koa.js Middleware Logging

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.

Let’s Try →

Koa.js Async Handler Example

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.

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Koa.js 404 Middleware Example

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.

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Koa.js Error Handling Example

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.

Let’s Try →

Koa.js Combined Middleware Example

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.

Let’s Try →

Frequently Asked Questions about Koa-js

What is Koa-js?

What are the primary use cases for Koa-js?

What are the strengths of Koa-js?

What are the limitations of Koa-js?

How can I practice Koa-js typing speed?

CodeSpeedTest offers 10+ real Koa-js code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.