feat: extend basePath() helper to accept Hono instances

[G4brym]

Summary

Extends the existing basePath() helper in hono/route to also accept a Hono instance, allowing libraries to read a Hono app's base path at setup time — without relying on the private _basePath property.

Motivation

Currently, the only way to read a Hono instance's base path outside of a request handler is by accessing the private _basePath property. This is fragile — it is not part of the public API and could break without notice.

Libraries that integrate with Hono (such as chanfana, an OpenAPI schema generator) need to detect the base path to correctly generate API documentation and route schemas. Right now, chanfana reads _basePath defensively with runtime type guards (see PR), but a proper public API would be much better.

Changes

• src/helper/route/index.ts — Added an overload to basePath() that accepts a HonoBase instance and returns its base path directly
• src/helper/route/index.test.ts — Added 4 tests for the new overload:
  • Returns "/" for a fresh instance
  • Returns the base path after basePath()
  • Returns the merged path after chained basePath() calls
  • Original instance is unaffected after basePath() creates a clone

Usage

import { basePath } from 'hono/route'

const app = new Hono().basePath('/api')
basePath(app) // '/api'

const v1 = app.basePath('/v1')
basePath(v1)  // '/api/v1'
basePath(app) // '/api' (unchanged)

The existing basePath(c: Context) handler behaviour is fully preserved.

[yusukebe]

Hey @G4brym

If you want to get a base path in a handler, you can use the basePath() in the Route Helper: https://hono.dev/docs/helpers/route#basepath

Have you checked it?

[G4brym]

Hey @yusukebe, thanks for the pointer!
I checked that, but it requires a Context (c) and only works inside a request handler
my use case is different, I need the base path at setup time, outside of a request

for example:

const app = new Hono().basePath('/api')                                                                                                                                                                                                                                                                              
const router = fromHono(app)  // ← need to know "/api" here, no `c` exists yet 

[G4brym]

or maybe the basePath helper function could be updated to also work with Hono apps, like:

import {
  basePath,
} from 'hono/route'

const subApp = new Hono()
const path = basePath(subApp) // '/api'

app.route('/:sub', subApp)

i can update the pr if you agree with this change

[G4brym]

Hey there, just updated the pr to match the comment above

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.89%. Comparing base (b1df304) to head (7a32c2d).
⚠️ Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4766      +/-   ##
==========================================
+ Coverage   91.48%   92.89%   +1.41%     
==========================================
  Files         177      177              
  Lines       11551    11800     +249     
  Branches     3353     3517     +164     
==========================================
+ Hits        10567    10962     +395     
+ Misses        983      837     -146     
  Partials        1        1              

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[yusukebe]

Hi @G4brym

Sorry for being late. I think this is good.

We have to discuss whether we should use overload. We can also use such as basePathFromApp.

Hey @usualoma What do you think of this PR?