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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, () => {
    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 already 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.path 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 or undefined

As of v0.5.0, this is an alias of the @polka/url module. For nearly all cases, you'll notice no changes.

But, for whatever reason, you can quickly swap in parseurl again:

const app = polka();
app.parse = require('parseurl');
//=> Done!

Returns: Polka

Boots (or creates) the underlying http.Server for the first time. All arguments are passed to server.listen directly with no changes.

As of v0.5.0, this method no longer returns a Promise. Instead, the current Polka instance is returned directly, allowing for chained operations.

// Could not do this before 0.5.0
const { server, handler } = polka().listen();

// Or this!
const app = polka().listen(PORT, onAppStart);

app.use('users', require('./users'))
  .get('/', (req, res) => {
    res.end('Pretty cool!');
  });

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/1.1 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/1.1 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.path, 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 organized into tiers.

Unlike Express, Polka middleware are tiered into "global", "filtered", and "route-specific" groups.

• Global middleware are defined via .use('/', ...fn) or .use(...fn), which are synonymous.
  Because every request's pathname begins with a /, this tier is always triggered.

• Sub-group or "filtered" middleware are defined with a base pathname that's more specific than '/'. For example, defining .use('/users', ...fn) will run on any /users/**/* request.
  These functions will execute after "global" middleware but before the route-specific handler.

• Route handlers match specific paths and execute last in the chain. They must also match the method action.

Once the chain of middleware handler(s) has been composed, Polka will iterate through them sequentially until all functions have run, until a chain member has terminated the response early, or until a chain member has thrown an error.

Contrast this with Express, which does not tier your middleware and instead iterates through your entire application in the sequence that you composed it.

// Express
express()
  .get('/', get)
  .use(foo)
  .get('/users/123', user)
  .use('/users', users)

// Polka
Polka()
  .get('/', get)
  .use(foo)
  .get('/users/123', user)
  .use('/users', users)

$ curl {APP}/
# Express :: [get]
# Polka   :: [foo, get]

$ curl {APP}/users/123
# Express :: [foo, user]
# Polka   :: [foo, users, user]

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 https://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     1.96ms  119.06us   5.33ms   92.57%
        Req/Sec    12.78k   287.46    13.13k    90.00%
      508694 requests in 10.00s, 50.45MB read
    Requests/sec:  50867.22
    Transfer/sec:      5.05MB

Polka
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     1.98ms  119.26us   4.45ms   92.87%
        Req/Sec    12.68k   287.74    13.05k    94.06%
      509817 requests in 10.10s, 50.56MB read
    Requests/sec:  50475.67
    Transfer/sec:      5.01MB

Rayo
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     2.02ms  116.55us   6.66ms   92.55%
        Req/Sec    12.43k   262.32    12.81k    91.58%
      499795 requests in 10.10s, 49.57MB read
    Requests/sec:  49481.55
    Transfer/sec:      4.91MB

Fastify
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     2.10ms  138.04us   5.46ms   91.50%
        Req/Sec    11.96k   414.14    15.82k    95.04%
      479518 requests in 10.10s, 66.31MB read
    Requests/sec:  47476.75
    Transfer/sec:      6.57MB

Koa
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     2.95ms  247.10us   6.91ms   72.18%
        Req/Sec     8.52k   277.12     9.09k    70.30%
      342518 requests in 10.10s, 47.36MB read
    Requests/sec:  33909.82
    Transfer/sec:      4.69MB

Express
    Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     4.91ms  484.52us  10.65ms   89.71%
        Req/Sec     5.11k   350.75     9.69k    98.51%
      204520 requests in 10.10s, 40.57MB read
    Requests/sec:  20249.80
    Transfer/sec:      4.02MB

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. Polka uses a tiered middleware system.

   Express maintains the sequence of your route & middleware declarations during its runtime, which can pose a problem when composing sub-applications. Typically, this forces you to duplicate groups of logic.

   Please see Middleware Sequence for an example and additional details.

2. Any built-in view/rendering engines.

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

3. 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?');
   }

4. 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)

5. 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/*', _ => {});

6. Polka instances are not (directly) the request handler.

   Most packages in the Express ecosystem expect you to pass your app directly into the package. This is because express() returns a middleware signature directly.

   In the Polka-sphere, this functionality lives in your application's handler instead.

   Here's an example with supertest, a popular testing utility for Express apps.

   const request = require('supertest');
   const send = require('@polka/send-type');
   
   const express = require('express')();
   const polka = require('polka')();
   
   express.get('/user', (req, res) => {
     res.status(200).json({ name: 'john' });
   });
   
   polka.get('/user', (req, res) => {
     send(res, 200, { name: 'john' });
   });
   
   function isExpected(app) {
     request(app)
       .get('/user')
       .expect('Content-Type', /json/)
       .expect('Content-Length', '15')
       .expect(200);
   }
   
   // Express: Pass in the entire application directly
   isExpected(express);
   
   // Polka: Pass in the application `handler` instead
   isExpected(polka.handler);

MIT © Luke Edwards