Guide

This guide covers Koa topics that are not directly API related, such as best practices for writing middleware, application structure suggestions, here we use async function as middleware you can also you commonFunction or generatorFunction which will be a little different.

Writing Middleware

Koa middleware are simple functions which return a MiddlewareFunction with signature (ctx, next). When the middleware is run by an "upstream" middleware, 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 = new Date();
  await next();
  const ms = new Date() - 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 duration
Await control to the next middleware
Create another date to track response time
Await control to the next middleware
Await immediately since contentLength only works with responses
Await upstream to Koa's noop middleware
Ignore setting the body unless the path is "/"
Set the response to "Hello World"
Ignore setting Content-Length when no body is present
Set the field
Output log line
Set X-Response-Time header field before response
Hand off to Koa to handle the response

Note that the final middleware (step 6) await to what looks to be nothing - it's actually yielding to a no-op promise within Koa. This is so that every middleware can conform with the same API, and may be placed before or after others. If you removed next(); from the furthest "downstream" middleware everything would function appropriately, however it would no longer conform to this behaviour.

For example this would be fine:

app.use(async function response(ctx, next) {
  if ('/' != this.url) return;
  ctx.body = 'Hello World';
});

Next we'll look at the best practices for creating Koa middleware.

Middleware Best Practices

This section covers middleware authoring best practices, such as middleware accepting options, named middleware for debugging, among others.

Middleware options

When creating 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'));

Named middleware

Naming middleware is optional, however it's useful for debugging purposes to assign a name.

function logger(format) {
  return async function logger(ctx, next) {

  };
}

Combining multiple middleware with koa-compose

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' == this.path) {
    ctx.body = Math.floor(Math.random() * 10);
  } else {
    await next();
  }
};

async function backwards(ctx, next) {
  if ('/backwards' == this.path) {
    ctx.body = 'sdrawkcab';
  } else {
    await next();
  }
}

async function pi(ctx, next) {
  if ('/pi' == this.path) {
    ctx.body = String(Math.PI);
  } else {
    await next();
  }
}

const all = compose([random, backwards, pi]);

app.use(all);

Response Middleware

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 operations

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-promise');

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('');
});

Debugging Koa

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 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

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