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, light-weight HTTP framework for node.js to make web applications and APIs more enjoyable to write. Koa requests flow through middleware in a stack-like manner, allowing you to perform actions downstream then filter and manipulate the response upstream. Koa's use of generators also greatly increases the readability and robustness of your application.
Only methods that are common to nearly all HTTP servers are integrated directly into Koa's small ~550 SLOC codebase. This includes things like content negotiation, normalization of node inconsistencies, redirection, and a few others.
Koa is not bundled with any middleware, but there is a large collection of community middleware to choose from.
$ npm install koa
Koa is supported in node v4+ and node v0.12 with the --harmony-generators or --harmony flag.
Koa v2 is currently released with the next tag (meaning it will not be marked as latest).
You can install it with semver:
npm install koa@2
In this new version, the middleware function signature changes to adopt async/await:
// Koa application is now a class and requires the new operator.
const app = new Koa();
// uses async arrow functions
app.use(async (ctx, next) => {
try {
await next(); // next is now a function
} catch (err) {
ctx.body = { message: err.message };
ctx.status = err.status || 500;
}
});
app.use(async ctx => {
const user = await User.getById(ctx.session.userid); // await instead of yield
ctx.body = user; // ctx instead of this
});
Until async/await is natively supported in Node Koa 1.x is the official release, however you may wish to adopt 2.x early by using Babel to compile. Koa 1.x generator-based middleware may be "upgraded" to the 2.x flavour using koa-convert.
To learn more about Koa v2, follow this issue or read the docs here: https://github.com/koajs/koa/tree/v2.x/docs.
var koa = require('koa');
var app = koa();
// logger
app.use(function *(next){
var start = new Date;
yield next;
var ms = new Date - start;
console.log('%s %s - %s', this.method, this.url, ms);
});
// response
app.use(function *(){
this.body = 'Hello World';
});
app.listen(3000);
$ make test
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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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