@api-ts/typed-express-router

A thin wrapper around Express's Router

Goals

• Define Express routes that are associated with routes in an api-ts apiSpec
• Augment the existing Express request with the decoded request object and api-ts route metadata
• Augment the existing Express response with a type-checked encode function
• Allow customization of what to do on decode/encode errors, per-route if desired
• Allow action to be performed after an encoded response is sent, per-route if desired
• Allow routes to define alias routes with path that is different than the one specified in the httpRoute
• Follow the express router api as closely as possible otherwise

Non-Goals

• Enforce that all routes listed in an apiSpec have an associated route handler
• Layer anything on top of the express.RequestHandler[] chain beyond the additional properties described in Goals (projects and other libraries can do this)

Usage

Creating a router

Two very similar functions are provided by this library that respectively create or wrap an Express router:

import { createRouter, wrapRouter } from '@api-ts/typed-express-router';
import express from 'express';

import { MyApi } from 'my-api-package';

const app = express();

const typedRouter = createRouter(MyApi);
app.use(typedRouter);

Adding routes

Once you have the typedRouter, you can start adding routes by the api-ts api name:

typedRouter.get('hello.world', [HelloWorldHandler]);

Here, HelloWorldHandler is a almost like an Express request handler, but req and res have an extra property. req.decoded contains the validated and decoded request. On the response side, there is an extra res.sendEncoded(status, payload) function that will enforce types on the payload and encode types appropriately (e.g. BigIntFromString will be converted to a string). The exported TypedRequestHandler type may be used to infer the parameter types for these functions.

Route aliases

If more flexibility is needed in the route path, a routeAliases function may be provided to match multiple paths. These paths may use the full Express matching syntax, but take care to preserve any path parameters or else you will likely get decode errors.

typedRouter.get('hello.world', [HelloWorldHandler], {
  routeAliases: ['/oldDeprecatedHelloWorld'],
});

Hooks and error handlers

The createRouter, wrapRouter, and individual route methods all take an optional last parameter where a post-response and error handling function may be provided. Ones specified for a specific route take precedence over the top-level ones. These may be used to customize error responses and perform other actions like metrics collection or logging.

const typedRouter = createRouter(MyApi, {
  onDecodeError: (errs, req, res) => {
    // Format `errs` however you want
    res.send(400).json({ message: 'Bad request' }).end();
  },
  onEncodeError: (err, req, res) => {
    // Ideally won't happen unless type safety is violated, so it's a 500
    res.send(500).json({ message: 'Internal server error' }).end();
  },
  afterEncodedResponseSent: (status, payload, req, res) => {
    // Perform side effects or other things, `res` should be ended by this point
    endRequestMetricsCollection(req);
  },
});

// Override the decode error handler on one route
typedRouter.get('hello.world', [HelloWorldHandler], {
  onDecodeError: customHelloDecodeErrorHandler,
});

Unchecked routes

If you need custom behavior on decode errors that is more involved than just sending an error response, then the unchecked variant of the router functions can be used. They do not fail and call onDecodeError when a request is invalid. Instead, they will still populate req.decoded, except this time it'll contain the Either<Errors, DecodedRequest> type for route handlers to inspect.

// Just a normal express route
typedRouter.getUnchecked('hello.world', (req, res) => {
  if (E.isLeft(req.decoded)) {
    console.warn('Route failed to decode! Continuing anyway');
  })

  res.send(200).end();
});

Router middleware

Middleware added with typedRouter.use() is ran just after the request is decoded but before it is validated, even on checked routes. It'll have access to req.decoded in the same way that unchecked routes do.

Other usage

Other than what is documented above, a wrapped router should behave like a regular Express one, so things like typedRouter.use() should behave the same.