Polka is an extremely minimal, highly performant Express.js alternative. Yes, you're right, Express is already super fast & not that big 🤔 — but Polka shows that there was (somehow) room for improvement!

Essentially, Polka is just a native HTTP server with added support for routing, middleware, and sub-applications. That's it! 🎉

And, of course, in mandatory bullet-point format:

• 33-50% faster than Express for simple applications
• Middleware support, including Express middleware you already know & love
• Nearly identical application API & route pattern definitions
• ~90 LOC for Polka, 120 including its router

$ npm install --save polka

const polka = require('polka');

function one(req, res, next) {
  req.hello = 'world';
  next();
}

function two(req, res, next) {
  req.foo = '...needs better demo 😔';
  next();
}

polka()
  .use(one, two)
  .get('/users/:id', (req, res) => {
    console.log(`~> Hello, ${req.hello}`);
    res.end(`User: ${req.params.id}`);
  })
  .listen(3000).then(_ => {
    console.log(`> Running on localhost:3000`);
  });

Polka extends Trouter which means it inherits its API, too!

Returns an instance of Polka~!

Type: Server

A custom, instantiated server that the Polka instance should attach its handler to. This is useful if you have initialized a server elsewhere in your application and want Polka to use it instead of creating a new http.Server.

Polka only updates the server when polka.listen is called. At this time, Polka will create a http.Server if a server was not provided via options.server.

Important: The server key will be undefined until polka.listen is invoked, unless a server was provided.

Type: Function

A catch-all error handler; executed whenever a middleware throws an error. Change this if you don't like default behavior.

Its signature is (err, req, res, next), where err is the String or Error thrown by your middleware.

Caution: Use next() to bypass certain errors at your own risk!
You must be certain that the exception will be handled elsewhere or can be safely ignored.
Otherwise your response will never terminate!

Type: Function

A handler when no route definitions were matched. Change this if you don't like default behavior, which sends a 404 status & Not found response.

Its signature is (req, res) and requires that you terminate the response.

Attach middleware(s) and/or sub-application(s) to the server. These will execute before your routes' handlers.

Important: If a base pathname is provided, all functions within the same use() block will only execute when the req.pathname matches the base path.

Type: String
Default: undefined

The base path on which the following middleware(s) or sub-application should be mounted.

Type: Function|Array

You may pass one or more functions at a time. Each function must have the standardized (req, res, next) signature.

You may also pass a sub-application, which must be accompanied by a base pathname.

Please see Middleware and Express' middleware examples for more info.

Returns: Object

This is an alias of the awesome parseurl module. There are no Polka-specific changes.

Returns: Promise

Wraps the native server.listen with a Promise, rejecting on any error.

The main Polka IncomingMessage handler. It receives all requests and tries to match the incoming URL against known routes.

If the req.url is not immediately matched, Polka will look for sub-applications or middleware groups matching the req.url's base path. If any are found, they are appended to the loop, running after any global middleware.

Note: Any middleware defined within a sub-application is run after the main app's (aka, global) middleware and before the sub-application's route handler.

At the end of the loop, if a middleware hasn't yet terminated the response (or thrown an error), the route handler will be executed, if found — otherwise a (404) Not found response is returned, configurable via options.onNoMatch.

Type: IncomingMessage

Type: ServerResponse

Type: Object

Optionally provide a parsed URL object. Useful if you've already parsed the incoming path. Otherwise, app.parse (aka parseurl) will run by default.

Routes are used to define how an application responds to varying HTTP methods and endpoints.

If you're coming from Express, there's nothing new here!
However, do check out Comparisons for some pattern changes.

Each route is comprised of a path pattern, a HTTP method, and a handler (aka, what you want to do).

In code, this looks like:

app.METHOD(pattern, handler);

wherein:

• app is an instance of polka
• METHOD is any valid HTTP method, lowercased
• pattern is a routing pattern (string)
• handler is the function to execute when pattern is matched

Also, a single pathname (or pattern) may be reused with multiple METHODs.

The following example demonstrates some simple routes.

const app = polka();

app.get('/', (req, res) => {
  res.end('Hello world!');
});

app.get('/users', (req, res) => {
  res.end('Get all users!');
});

app.post('/users', (req, res) => {
  res.end('Create a new User!');
});

app.put('/users/:id', (req, res) => {
  res.end(`Update User with ID of ${req.params.id}`);
});

app.delete('/users/:id', (req, res) => {
  res.end(`CY@ User ${req.params.id}!`);
});

Unlike the very popular path-to-regexp, Polka uses string comparison to locate route matches. While faster & more memory efficient, this does also prevent complex pattern matching.

However, have no fear! 💥 All the basic and most commonly used patterns are supported. You probably only ever used these patterns in the first place. 😉

See Comparisons for the list of RegExp-based patterns that Polka does not support.

The supported pattern types are:

• static (/users)
• named parameters (/users/:id)
• nested parameters (/users/:id/books/:title)
• optional parameters (/users/:id?/books/:title?)
• any match / wildcards (/users/*)

Any named parameters included within your route pattern will be automatically added to your incoming req object. All parameters will be found within req.params under the same name they were given.

Important: Your parameter names should be unique, as shared names will overwrite each other!

app.get('/users/:id/books/:title', (req, res) => {
  let { id, title } = req.params;
  res.end(`User: ${id} && Book: ${title}`);
});

$ curl /users/123/books/Narnia
#=> User: 123 && Book: Narnia

Any valid HTTP method is supported! However, only the most common methods are used throughout this documentation for demo purposes.

Note: For a full list of valid METHODs, please see this list.

Request handlers accept the incoming IncomingMessage and the formulating ServerResponse.

Every route definition must contain a valid handler function, or else an error will be thrown at runtime.

Important: You must always terminate a ServerResponse!

It's a very good practice to always terminate your response (res.end) inside a handler, even if you expect a middleware to do it for you. In the event a response is/was not terminated, the server will hang & eventually exit with a TIMEOUT error.

Note: This is a native http behavior.

If using Node 7.4 or later, you may leverage native async and await syntax! 😻

No special preparation is needed — simply add the appropriate keywords.

const app = polka();

const sleep = ms => new Promise(r => setTimeout(r, ms));

async function authenticate(req, res, next) {
  let token = req.headers['authorization'];
  if (!token) return (res.statusCode=401,res.end('No token!'));
  req.user = await Users.find(token); // <== fake
  next(); // done, woot!
}

app
  .use(authenticate)
  .get('/', async (req, res) => {
    // log middleware's findings
    console.log('~> current user', req.user);
    // force sleep, because we can~!
    await sleep(500);
    // send greeting
    res.end(`Hello, ${req.user.name}`);
  });

Middleware are functions that run in between (hence "middle") receiving the request & executing your route's handler response.

Coming from Express? Use any middleware you already know & love! 🎉

The middleware signature receives the request (req), the response (res), and a callback (next).

These can apply mutations to the req and res objects, and unlike Express, have access to req.params, req.pathname, req.search, and req.query!

Most importantly, a middleware must either call next() or terminate the response (res.end). Failure to do this will result in a never-ending response, which will eventually crash the http.Server.

// Log every request
function logger(req, res, next) {
  console.log(`~> Received ${req.method} on ${req.url}`);
  next(); // move on
}

function authorize(req, res, next) {
  // mutate req; available later
  req.token = req.headers['authorization'];
  req.token ? next() : ((res.statusCode=401) && res.end('No token!'));
}

polka().use(logger, authorize).get('*', (req, res) => {
  console.log(`~> user token: ${req.token}`);
  res.end('Hello, valid user');
});

$ curl /
# ~> Received GET on /
#=> (401) No token!

$ curl -H "authorization: secret" /foobar
# ~> Received GET on /foobar
# ~> user token: secret
#=> (200) Hello, valid user

In Polka, middleware functions are mounted globally, which means that they'll run on every request (see Comparisons). Instead, you'll have to apply internal filters to determine when & where your middleware should run.

Note: This might change in Polka 1.0 🤔

function foobar(req, res, next) {
  if (req.pathname.startsWith('/users')) {
    // do something magical
  }
  next();
}

If an error arises within a middleware, the loop will be exited. This means that no other middleware will execute & neither will the route handler.

Similarly, regardless of statusCode, an early response termination will also exit the loop & prevent the route handler from running.

There are three ways to "throw" an error from within a middleware function.

Hint: None of them use throw 😹

1. Pass any string to next()

   This will exit the loop & send a 500 status code, with your error string as the response body.

   polka()
     .use((req, res, next) => next('💩'))
     .get('*', (req, res) => res.end('wont run'));

   $ curl /
   #=> (500) 💩

2. Pass an Error to next()

   This is similar to the above option, but gives you a window in changing the statusCode to something other than the 500 default.

   function oopsies(req, res, next) {
     let err = new Error('Try again');
     err.code = 422;
     next(err);
   }

   $ curl /
   #=> (422) Try again

3. Terminate the response early

   Once the response has been ended, there's no reason to continue the loop!

   This approach is the most versatile as it allows to control every aspect of the outgoing res.

   function oopsies(req, res, next) {
     if (true) {
       // something bad happened~
       res.writeHead(400, {
         'Content-Type': 'application/json',
         'X-Error-Code': 'Please dont do this IRL'
       });
       let json = JSON.stringify({ error:'Missing CSRF token' });
       res.end(json);
     } else {
       next(); // never called FYI
     }
   }

   $ curl /
   #=> (400) {"error":"Missing CSRF token"}

Quick comparison between various frameworks using wrk on Node v10.4.0.
Results are taken with the following command, after one warm-up run:

$ wrk -t4 -c4 -d10s http://localhost:3000/users/123

Additional benchmarks between Polka and Express (using various Node versions) can be found here.

Important: Time is mostly spent in your application code rather than Express or Polka code!
Switching from Express to Polka will (likely) not show such drastic performance gains.

Native
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency    80.82us   37.10us   1.08ms   87.61%
        Req/Sec    12.22k     2.83k   16.70k    56.93%
      491258 requests in 10.10s, 48.72MB read
    Requests/sec:  48640.61
    Transfer/sec:      4.82MB

Polka
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency    81.03us   36.72us   1.10ms   83.89%
        Req/Sec    12.18k     3.02k   16.57k    50.00%
      489630 requests in 10.10s, 48.56MB read
    Requests/sec:  48478.90
    Transfer/sec:      4.81MB

Rayo
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency    84.09us   38.08us   1.04ms   87.26%
        Req/Sec    11.75k     3.19k   16.16k    40.59%
      472256 requests in 10.10s, 46.84MB read
    Requests/sec:  46761.11
    Transfer/sec:      4.64MB

Fastify
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency    90.56us   42.47us   1.61ms   91.22%
        Req/Sec    10.93k     3.13k   15.11k    33.42%
      439399 requests in 10.10s, 60.76MB read
    Requests/sec:  43505.05
    Transfer/sec:      6.02MB

Koa
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency   127.33us   54.31us   1.19ms   84.59%
        Req/Sec     7.80k     2.48k   10.95k    37.38%
      313672 requests in 10.10s, 43.38MB read
    Requests/sec:  31057.11
    Transfer/sec:      4.29MB

Express
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency   161.92us   70.36us   2.50ms   86.97%
        Req/Sec     6.16k     1.93k    8.81k    42.82%
      247613 requests in 10.10s, 34.00MB read
    Requests/sec:  24516.23
    Transfer/sec:      3.37MB

Polka's API aims to be very similar to Express since most Node.js developers are already familiar with it. If you know Express, you already know Polka! 💃

There are, however, a few main differences. Polka does not support or offer:

1. Any built-in view/rendering engines.

   Most templating engines can be incorporated into middleware functions or used directly within a route handler.

2. The ability to throw from within middleware.

   However, all other forms of middleware-errors are supported. (See Middleware Errors.)

   function middleware(res, res, next) {
     // pass an error message to next()
     next('uh oh');
   
     // pass an Error to next()
     next(new Error('🙀'));
   
     // send an early, customized error response
     res.statusCode = 401;
     res.end('Who are you?');
   }

3. Express-like response helpers... yet! (#14)

   Express has a nice set of response helpers. While Polka relies on the native Node.js response methods, it would be very easy/possible to attach a global middleware that contained a similar set of helpers. (TODO)

4. RegExp-based route patterns.

   Polka's router uses string comparison to match paths against patterns. It's a lot quicker & more efficient.

   The following routing patterns are not supported:

   app.get('/ab?cd', _ => {});
   app.get('/ab+cd', _ => {});
   app.get('/ab*cd', _ => {});
   app.get('/ab(cd)?e', _ => {});
   app.get(/a/, _ => {});
   app.get(/.*fly$/, _ => {});

   The following routing patterns are supported:

   app.get('/users', _ => {});
   app.get('/users/:id', _ => {});
   app.get('/users/:id?', _ => {});
   app.get('/users/:id/books/:title', _ => {});
   app.get('/users/*', _ => {});

MIT © Luke Edwards