Fascia

Give your Express controllers TypeScript superpowers.

Installing

Fascia has not yet released a stable version. You can get the current alpha with

npm install fascia@next

Quickstart

import express from 'express';
import { withConnection } from 'fascia';

const app = express();

app.get(
  '/healthcheck',
  withConnection(conn => {
    return {
      body: 'all is well',
      status_code: 200,
      headers: {
        'content-type': 'text/plain',
      },
    };
  }),
);

What is it?

Express' typescript bindings leave much to be desired. Looking at the signature for this function, what does it do?

import express from 'express';

async function requiresLogin(
  req: express.Request,
  res: express.Response,
  next: express.NextFunction,
): void;

From the name, it looks like it screens out unauthenticated users. When things go well, does it take on a user property to the request? Maybe. When something goes wrong, does it

• call next(err), to be caught by an error handler registered later?
• respond to the request with an error page when something goes wrong?
• throw an error (probably a mistake)?

The point is that you can't tell from looking at the signature. What's more, the type-checker can't tell.

Fascia enables you to encode more information about your controllers and middleware into the type system, so that you can catch errors earlier.

Patterns

Attaching user information to a request

import { Connection } from 'fascia';
import { Forbidden } from 'fascia/errors';

interface AuthorizedConn extends Connection {
  user: User;
}

async function requiresLogin(conn: Connection): Promise<AuthorizedConn> {
  if (!conn.headers.authorization) throw new Forbidden();
  const user = await verifyAuthHeader(conn.headers.authorization);
  return {
    ...conn,
    user,
  };
}

type-checking the body of a request

We lean on io-ts to to some of the heavy lifting here.

import { withConnection, decodeBody, TypedBody } from 'fascia';
import * as t from 'io-ts';

const LoginInfoV = t.type({
  username: t.string,
  password: t.string,
});

type LoginInfo = t.TypeOf<typeof LoginInfoV>;
// 👆equivalent to writing out
//   interface LoginInfo { username: string; password: string }
// but avoids the duplication.

interface WithUser {
  user: { username: string; id: string };
}

async function verifyLogin<C extends TypedBody<LoginInfo>>(conn: C>): Promise<C & WithUser> {
  const user = await lookupUser(conn.body.username);
  if (!user || !checkPassword(user.password, conn.body.password)) {
    throw new Forbidden();
  }
  return {
    ...conn,
    user,
  };
}

app.post(
  '/login',
  withConnection(conn =>
    decodeBody(LoginInfoV)
      .then(verifyLogin)
      .then(successfulLogin),
  ),
);