koa-router with embedded wildcards

koa-router-wildcard.js

/**
 * Adaptation of the koa-mount boilerplate example expanded to include wildcards.
 * Unfortunately, there are a number of bugs with wildcard matching and koa-router.
 * Below is a workaround which consists of using regular expressions to "match all except"
 * instead of a plain wildcard.
 *
 * Use at your own risk.
 * --Matt Warner, July 2018
 *
 * Original note:
 * This example illustrates how you may
 * implement a cascading effect using
 * several mounted applications that
 * are not aware of where they are mounted:
 *
 * GET /foo
 * GET /foo/bar
 * GET /foo/bar/baz
 */
'use strict';
const mount = require('koa-mount')
const Koa = require('koa')
const Router = require('koa-router');

const app = new Koa();
const a = new Koa();
const b = new Koa();
const c = new Koa();
const d = new Koa();

const routerA = new Router();
const routerB = new Router();
const routerC = new Router();
const routerD = new Router();

a.use(routerA.routes(), routerA.allowedMethods());
b.use(routerB.routes(), routerB.allowedMethods());
c.use(routerC.routes(), routerC.allowedMethods());
d.use(routerD.routes(), routerD.allowedMethods());

routerA.get(/^(?!\/bar).*$/, async ctx => { // everything that isn't /foo/bar gets caught here
    ctx.body = 'Hello world from A';
});

routerB.get(/^(?!\/(baz|boo)).*$/, async ctx => { // everything that isn't /foo/bar/baz or /foo/bar/boo gets caught here
    ctx.body = 'Hello world from B';
});


routerC.get(/^.*$/, async ctx => { // match everything starting with /foo/bar/baz
    ctx.body = 'Hello world from C';
});

routerD.get(/^.*$/, async ctx => { // match everything starting with /foo/bar/boo
    ctx.body = 'Hello world from D';
});

app.use(mount('/', a)) // /
a.use(mount('/bar', b)); // /bar
a.use(mount('/bar/baz', c)) // /bar/baz
a.use(mount('/bar/boo', d)) // /bar/boo

// Debug print of the routes each router has configured
console.log("A", routerA.stack.map(i => i.path));
console.log("B", routerB.stack.map(i => i.path));
console.log("C", routerC.stack.map(i => i.path));
console.log("D", routerD.stack.map(i => i.path));

app.listen(3001)
console.log('listening on port 3001')