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Polka

A micro web server so fast, it'll make you dance! 👯

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

Install

$ npm install --save polka

Usage

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`);
  });

API

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

polka(options)

Returns an instance of Polka~!

options.onError

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!

options.onNoMatch

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.

use(base, ...fn)

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.

base

Type: String
Default: undefined

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

fn

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.

parse(req)

Returns: Object

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

listen(port, hostname)

Returns: Promise

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

handler(req, res, parsed)

The main Polka ClientRequest 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.

req

Type: ClientRequest

res

Type: ServerResponse

parsed

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.

Routing

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.

Basics

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}!`);
});

Patterns

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/*)

Parameters

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

Methods

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.

Handlers

Request handlers accept the incoming ClientRequest 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.

Async Handlers

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.getHeader('authorization');
  if (!token) return app.send(res, 401);
  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

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.getHeader('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();
}

Middleware Errors

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 😹

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) 💩

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

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"}

Benchmarks

A round of Polka-vs-Express benchmarks across varying 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.

Node 8.9.0

Native
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     2.32ms  187.31us   7.59ms   91.40%
        Req/Sec     5.19k   310.24     8.57k    96.40%
      415726 requests in 10.10s, 41.23MB read
    Requests/sec:  41154.58
    Transfer/sec:      4.08MB

Polka
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     2.33ms  171.65us   6.72ms   89.72%
        Req/Sec     5.18k   271.94     8.17k    97.02%
      414739 requests in 10.10s, 41.13MB read
    Requests/sec:  41053.98
    Transfer/sec:      4.07MB

Express
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     5.26ms  504.27us   9.29ms   84.42%
        Req/Sec     2.29k   163.51     4.44k    98.51%
      183462 requests in 10.10s, 36.39MB read
    Requests/sec:  18157.81
    Transfer/sec:      3.60MB

Fastify
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     2.75ms  226.61us  11.96ms   85.59%
        Req/Sec     4.37k   192.11     5.90k    95.53%
      350903 requests in 10.10s, 43.50MB read
    Requests/sec:  34734.29
    Transfer/sec:      4.31MB

Koa
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     3.58ms  445.92us  12.66ms   86.06%
        Req/Sec     3.37k   231.62     4.53k    84.99%
      269997 requests in 10.10s, 37.34MB read
    Requests/sec:  26721.65
    Transfer/sec:      3.70MB

Comparisons

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:

Any built-in view/rendering engines.

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

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?');
}

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)

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/*', _ => {});

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