This guide covers Koa topics that are not directly API related, such as best practices for writing middleware and application structure suggestions. In these examples we use async functions as middleware - you can also use commonFunction or generatorFunction which will be a little different.
Koa middleware are simple functions with the signature (ctx, next) (i.e. a MiddlewareFunction). When
the middleware is run, it must manually invoke next() to run the "downstream" middleware.
For example if you wanted to track how long it takes for a request to propagate through Koa by adding an
X-Response-Time header field the middleware would look like the following:
async function responseTime(ctx, next) {
const start = Date.now();
await next();
const ms = Date.now() - start;
ctx.set('X-Response-Time', `${ms}ms`);
}
app.use(responseTime);If you're a front-end developer you can think any code before next(); as the "capture" phase,
while any code after is the "bubble" phase. This crude gif illustrates how async function allow us
to properly utilize stack flow to implement request and response flows:
- Create a date to track response time
- Await control to the next middleware
- Create another date to track duration
- Await control to the next middleware
- Set the response body to "Hello World"
- Calculate duration time
- Output log line
- Calculate response time
- Set
X-Response-Timeheader field - Hand off to Koa to handle the response
Next we'll look at the best practices for creating Koa middleware.
This section covers middleware authoring best practices, such as middleware accepting options, named middleware for debugging, among others.
When creating a public middleware, it's useful to conform to the convention of wrapping the middleware in a function that accepts options, allowing users to extend functionality. Even if your middleware accepts no options, this is still a good idea to keep things uniform.
Here our contrived logger middleware accepts a format string for customization,
and returns the middleware itself:
function logger(format) {
format = format || ':method ":url"';
return async function (ctx, next) {
const str = format
.replace(':method', ctx.method)
.replace(':url', ctx.url);
console.log(str);
await next();
};
}
app.use(logger());
app.use(logger(':method :url'));Naming middleware is optional, however it's useful for debugging purposes to assign a name.
function logger(format) {
return async function logger(ctx, next) {
};
}Sometimes you want to "compose" multiple middleware into a single middleware for easy re-use or exporting. You can use koa-compose
const compose = require('koa-compose');
async function random(ctx, next) {
if ('/random' == ctx.path) {
ctx.body = Math.floor(Math.random() * 10);
} else {
await next();
}
};
async function backwards(ctx, next) {
if ('/backwards' == ctx.path) {
ctx.body = 'sdrawkcab';
} else {
await next();
}
}
async function pi(ctx, next) {
if ('/pi' == ctx.path) {
ctx.body = String(Math.PI);
} else {
await next();
}
}
const all = compose([random, backwards, pi]);
app.use(all);Middleware that decide to respond to a request and wish to bypass downstream middleware may
simply omit next(). Typically this will be in routing middleware, but this can be performed by
any. For example the following will respond with "two", however all three are executed, giving the
downstream "three" middleware a chance to manipulate the response.
app.use(async function (ctx, next) {
console.log('>> one');
await next();
console.log('<< one');
});
app.use(async function (ctx, next) {
console.log('>> two');
ctx.body = 'two';
await next();
console.log('<< two');
});
app.use(async function (ctx, next) {
console.log('>> three');
await next();
console.log('<< three');
});The following configuration omits next() in the second middleware, and will still respond
with "two", however the third (and any other downstream middleware) will be ignored:
app.use(async function (ctx, next) {
console.log('>> one');
await next();
console.log('<< one');
});
app.use(async function (ctx, next) {
console.log('>> two');
ctx.body = 'two';
console.log('<< two');
});
app.use(async function (ctx, next) {
console.log('>> three');
await next();
console.log('<< three');
});When the furthest downstream middleware executes next();, it's really yielding to a noop
function, allowing the middleware to compose correctly anywhere in the stack.
Async function and promise forms Koa's foundation, allowing
you to write non-blocking sequential code. For example this middleware reads the filenames from ./docs,
and then reads the contents of each markdown file in parallel before assigning the body to the joint result.
const fs = require('fs').promises;
app.use(async function (ctx, next) {
const paths = await fs.readdir('docs');
const files = await Promise.all(paths.map(path => fs.readFile(`docs/${path}`, 'utf8')));
ctx.type = 'markdown';
ctx.body = files.join('');
});Koa along with many of the libraries it's built with support the DEBUG environment variable from debug which provides simple conditional logging.
For example
to see all Koa-specific debugging information just pass DEBUG=koa* and upon boot you'll see the list of middleware used, among other things.
$ DEBUG=koa* node --harmony examples/simple
koa:application use responseTime +0ms
koa:application use logger +4ms
koa:application use contentLength +0ms
koa:application use notfound +0ms
koa:application use response +0ms
koa:application listen +0ms
Since JavaScript does not allow defining function names at
runtime, you can also set a middleware's name as ._name.
This is useful when you don't have control of a middleware's name.
For example:
const path = require('path');
const serve = require('koa-static');
const publicFiles = serve(path.join(__dirname, 'public'));
publicFiles._name = 'static /public';
app.use(publicFiles);Now, instead of just seeing "serve" when debugging, you will see:
koa:application use static /public +0ms
Example of setting up an HTTP2 server with Koa using the HTTP compatibility layer:
import Koa from 'koa'
import http2 from 'node:http2'
import fs from 'node:fs'
const app = new Koa();
const onRequestHandler = app.callback();
const serverOptions = {
key: fs.readFileSync('key.pem'),
cert: fs.readFileSync('cert.pem')
}
const server = http2.createSecureServer(serverOptions, onRequestHandler);An example of using server-side events with Koa:
import PassThrough from 'stream'
import OpenAI from 'openai'
import Koa from 'koa'
const openai = new OpenAI({
apiKey: process.env.OPENAI_API_KEY,
})
const app = new Koa()
app.use(async (ctx, next) => { // TODO: use your favorite routing library
const { message } = await ctx.request.json() // this example uses koa-body-parsers
const stream = await client.chat.completions.create({
model: 'gpt-4o-mini',
messages: [{ role: 'user', content: messages }],
stream: true,
})
ctx.response.type = 'text/event-stream'
const body = ctx.body = new PassThrough()
// streaming needs to be handled in a separate event loop
;(async () => {
for await (const chunk of stream) {
const content = chunk.choices[0]?.delta?.content
if (!content) continue
body.write(`data: ${JSON.stringify({
delta: {
content: chunk.choices[0].delta.content || ''
}
})}}\n\n`)
body.end()
}
})().catch((err) => app.emit('error', err))
})