Guide

This guide covers Koa topics are not directly API related, such as best practices for writing middleware, application structure suggestions.

Writing Middleware

Koa middleware are simple functions which return a GeneratorFunction, and accept another. When the middleware is run by an "upstream" middleware, it must manually yield to 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:

function *responseTime(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  this.set('X-Response-Time', ms + 'ms');
}

app.use(responseTime);

Here's another way to write the same thing, inline:

app.use(function *(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  this.set('X-Response-Time', ms + 'ms');
});

If you're a front-end developer you can think any code before yield next; as the "capture" phase, while any code after is the "bubble" phase. This crude gif illustrates how ES6 generators allow us to properly utilize stack flow to implement request and response flows:

Create a date to track duration
Yield control to the next middleware
Create another date to track response time
Yield control to the next middleware
Yield immediately since contentLength only works with responses
Yield 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) yields to what looks to be nothing - it's actually yielding to a no-op generator 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 yield next; from the furthest "downstream" middleware everything would function appropritaely, however it would no longer conform to this behaviour.

For example this would be fine:

app.use(function *response(){
  if ('/' != this.url) return;
  this.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 function *(next){
    var str = format
      .replace(':method', this.method)
      .replace(':url', this.url);

    console.log(str);

    yield 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 function *logger(next){

  }
}

Adding multiple middleware at once

To add multiple middleware at once chain them together with .call() and then return another function that yields the chain.

function *random(next){
  if (this.path == '/random') {
    this.body = Math.floor(Math.random()*10);
  } else {
    yield next;
  }
};

function *backwords(next) {
  if (this.path == '/backwords') {
    this.body = 'sdrowkcab';
  } else {
    yield next;
  }
}

function *pi(next){
  if (this.path == '/pi') {
    this.body = String(Math.PI);
  } else {
    yield next;
  }
}

app.use(function*(next){
  yield random.call(this, backwords.call(this, pi.call(this, next)));
});

Response Middleware

Middleware that decide to respond to a request and wish to bypass downstream middleware may simply omit yield 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(function *(next){
  console.log('>> one');
  yield next;
  console.log('<< one');
});

app.use(function *(next){
  console.log('>> two');
  this.body = 'two';
  yield next;
  console.log('<< two');
});

app.use(function *(next){
  console.log('>> three');
  yield next;
  console.log('<< three');
});

The following configuration omits yield next in the second middleware, and will still respond with "two", however the third (and any other downstream middleware) will be ignored:

app.use(function *(next){
  console.log('>> one');
  yield next;
  console.log('<< one');
});

app.use(function *(next){
  console.log('>> two');
  this.body = 'two';
  console.log('<< two');
});

app.use(function *(next){
  console.log('>> three');
  yield next;
  console.log('<< three');
});

When the furthest downstream middleware executes yield next; it's really yielding to a noop function, allowing the middleware to compose correctly anywhere in the stack.

Async operations

The Co forms Koa's foundation for generator delegation, 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.

var fs = require('co-fs');

app.use(function *(){
  var paths = yield fs.readdir('docs');

  var files = yield paths.map(function(path){
    return fs.readFile('docs/' + path, 'utf8');
  });

  this.type = 'markdown';
  this.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:

var path = require('path');
var static = require('koa-static');

var publicFiles = static(path.join(__dirname, 'public'));
publicFiles._name = 'static /public';

app.use(publicFiles);

Now, instead of just seeing "static" when debugging, you will see:

  koa:application use static /public +0ms

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