Unable to reuse nested routers (router mutation)

[MarcusCemes]

I wanted to reopen an issue that never got fixed in the old unmaintained koa-router. https://github.com/ZijianHe/koa-router/issues/244#issuecomment-184696207

Tested with

koa-router version: 8.0.0

koa version: 2.7.0

Code sample:

sharedRouter.get('/hello', (ctx) => {
    ctx.body = 'Hello World!';
});

rootRouter.use(sharedRouter.routes());
rootRouter.use('/foo', sharedRouter.routes());
rootRouter.use('/bar', sharedRouter.routes());

app.use(rootRouter.routes());

Expected Behavior:

The following paths should return a Hello World! body:

• GET /hello
• GET /foo/hello
• GET /bar/hello

Actual Behavior:

Each route returns a 404, as koa is unable to match a route.

Each time the shared router is mounted (.routes() is called), the shared router is mutated, with the new mounted path being prefixed to the shared router. The resulting route for the shared router is /bar/foo/hello, which I believe is then tested three times.

Use case

While this can easily be avoided, it is an inconvenience and unpredictable behaviour.

A mutation is not predictable. My understanding is that routers should behave like self-contained packages that can be mounted dynamically in several places, providing the same route tree.

[niftylettuce]

https://github.com/koajs/router/pull/24#issuecomment-563359492