Koa is a new web framework designed by the team behind Express, which aims to be a smaller, more expressive, and more robust foundation for web applications and APIs. Koa uses async functions to eliminate callback hell and simplify error handling. It does not bundle any middleware within its core, and it provides an elegant suite of methods that make writing servers fast and enjoyable.
HTTP Server
Koa can be used to create an HTTP server that listens on a given port. The example shows a basic server that responds with 'Hello World' to every request.
const Koa = require('koa');
const app = new Koa();
app.use(async ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);Middleware
Koa is known for its middleware stack that allows for more control over the request/response cycle. The example demonstrates a simple timing middleware that records how long a request takes to process.
const Koa = require('koa');
const app = new Koa();
app.use(async (ctx, next) => {
const start = Date.now();
await next();
const ms = Date.now() - start;
ctx.set('X-Response-Time', `${ms}ms`);
});
app.use(async ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);Error Handling
Koa provides a structured way to handle errors. In this example, middleware is used to catch and handle errors that may occur during request processing.
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 = err.message;
ctx.app.emit('error', err, ctx);
}
});
app.on('error', (err, ctx) => {
console.error('server error', err, ctx);
});
app.listen(3000);Context
Koa provides a context object encapsulating the Node's request and response objects into a single object which provides many helpful methods for writing web applications and APIs.
const Koa = require('koa');
const app = new Koa();
app.use(async ctx => {
ctx.body = `Request Type: ${ctx.method}`;
});
app.listen(3000);Express is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It is one of the most popular Node.js frameworks and has a large ecosystem of middleware available. Compared to Koa, Express is more established but relies on callback functions rather than async/await for handling asynchronous operations.
Hapi is a rich framework for building applications and services that allows developers to focus on writing reusable application logic instead of spending time building infrastructure. It is known for its powerful plugin system. Hapi is more configuration-driven compared to Koa's minimalistic and middleware-centric approach.
Fastify is a fast and low overhead web framework for Node.js. It is inspired by Hapi and Express and aspires to be faster than them. Fastify emphasizes performance and provides a robust plugin architecture. Unlike Koa, which is minimalist by design, Fastify comes with more built-in features.
Sails.js is a MVC framework for Node.js that is built on top of Express. It is designed to emulate the familiar MVC pattern of frameworks like Ruby on Rails, but with support for the requirements of modern apps: data-driven APIs with scalable, service-oriented architecture. Sails is more opinionated and includes more built-in features compared to Koa's minimalistic approach.
Expressive HTTP middleware framework for node.js to make web applications and APIs more enjoyable to write. Koa's middleware stack flows in a stack-like manner, allowing you to perform actions downstream then filter and manipulate the response upstream.
Only methods that are common to nearly all HTTP servers are integrated directly into Koa's small ~570 SLOC codebase. This includes things like content negotiation, normalization of node inconsistencies, redirection, and a few others.
Koa is not bundled with any middleware.
Koa requires node v7.6.0 or higher for ES2015 and async function support.
$ npm install koa
const Koa = require('koa');
const app = new Koa();
// response
app.use(ctx => {
ctx.body = 'Hello Koa';
});
app.listen(3000);
Koa is a middleware framework that can take 3 different kinds of functions as middleware:
Here is an example of logger middleware with each of the different functions:
app.use(async (ctx, next) => {
const start = new Date();
await next();
const ms = new Date() - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});
// Middleware normally takes two parameters (ctx, next), ctx is the context for one request,
// next is a function that is invoked to execute the downstream middleware. It returns a Promise with a then function for running code after completion.
app.use((ctx, next) => {
const start = new Date();
return next().then(() => {
const ms = new Date() - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});
});
To use generator functions, you must use a wrapper such as co that is no longer supplied with Koa.
app.use(co.wrap(function *(ctx, next) {
const start = new Date();
yield next();
const ms = new Date() - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
}));
Old signature middleware (v1.x) support will be removed in v3
Koa v2.x will try to convert legacy signature, generator middleware on app.use, using koa-convert.
It is however recommended that you choose to migrate all v1.x middleware as soon as possible.
// Koa will convert
app.use(function *(next) {
const start = new Date();
yield next;
const ms = new Date() - start;
console.log(`${this.method} ${this.url} - ${ms}ms`);
});
You could do it manually as well, in which case Koa will not convert.
const convert = require('koa-convert');
app.use(convert(function *(next) {
const start = new Date();
yield next;
const ms = new Date() - start;
console.log(`${this.method} ${this.url} - ${ms}ms`);
}));
For Node 4.0+ and Babel 6.0 you can setup like this:
$ npm install babel-register babel-plugin-transform-async-to-generator --save
// set babel in entry file
require('babel-register')({
plugins: ['transform-async-to-generator']
});
Check out an example in koa's test.
$ make test
See AUTHORS.
MIT
FAQs
Koa web app framework
The npm package koa receives a total of 2,384,581 weekly downloads. As such, koa popularity was classified as popular.
We found that koa demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 11 open source maintainers collaborating on the project.
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