feat(mount)!: introduce Mount Middleware and obsolete app.mount

[yusukebe]

I realized that app.mount() doesn't need to be placed in hono-base.ts. The codebase of app.mount() is a little bit fat compared to other methods in hono-base.ts, though the lines are very few in the real world. Anyway, the unnecessary should be removed from hono-base.ts.

So, we can make it a middleware called Mount Middleware. The usage is below:

import { Router as IttyRouter } from 'itty-router'
import { Hono } from 'hono'
import { mount } from 'hono/mount'

const ittyRouter = IttyRouter()
ittyRouter.get('/hello', () => new Response('Hello from itty-router'))

const app = new Hono()
app.all('/itty-router/*', mount(ittyRouter.handle))

I think the API syntax is very Hono-like and love it.

As a result, the bundle size will be 501B smaller (vs current main):

This PR will obsolete app.mount() completely, so this is a breaking change. Should be introduced in the next major release v5.

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[github-actions]

Bundle size check

	main (26de731)	#5221 (7bf1a2b)	+/-
Bundle Size (B)	19,000B	18,499B	-501B
Bundle Size (KB)	18.55K	18.07K	-0.48K

Compiler Diagnostics (tsc)

	main (26de731)	#5221 (7bf1a2b)	+/-
Files	83	83	0
Lines	59,440	59,400	-40
Identifiers	60,621	60,569	-52
Symbols	106,532	106,201	-331
Types	163,091	163,074	-17
Instantiations	414,023	413,935	-88
Memory used	168,508K	169,101K	593K
I/O read	0.02s	0.01s	-0.01s
I/O write	0s	0s	0s
Parse time	0.33s	0.33s	0s
Bind time	0.13s	0.15s	0.02s
Check time	1.37s	1.32s	-0.05s
Emit time	0s	0s	0s
Total time	1.83s	1.81s	-0.02s

Compiler Diagnostics (typescript-go)

	main (26de731)	#5221 (7bf1a2b)	+/-
Files	215	215	0
Lines	147,088	147,048	-40
Identifiers	121,974	121,922	-52
Symbols	189,100	188,569	-531
Types	262,348	262,330	-18
Instantiations	621,588	621,500	-88
Memory used	161,795K	161,172K	-623K
Memory allocs	1,781,183	1,780,945	-238
Config time	0.001s	0.001s	0s
BuildInfo read time	0.001s	0s	-0.001s
Parse time	0.084s	0.093s	0.009s
Bind time	0s	0s	0s
Check time	0.428s	0.431s	0.003s
Emit time	0.005s	0.005s	0s
Changes compute time	0.023s	0.028s	0.005s
Total time	0.571s	0.591s	0.02s

---

Reported by octocov

[github-actions]

HTTP Performance Benchmark

Framework	Runtime	Average	Ping	Query	Body
hono (origin/main)	bun	36,812.83	49,950.15	34,020.43	26,467.90
hono (current)	bun	36,039.57	49,446.31	32,514.94	26,157.47
Change		-2.10%	-1.01%	-4.43%	-1.17%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.92%. Comparing base (26de731) to head (ef48b56).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5221      +/-   ##
==========================================
+ Coverage   79.77%   79.92%   +0.15%     
==========================================
  Files         155      156       +1     
  Lines       10930    10906      -24     
  Branches     2290     2290              
==========================================
- Hits         8719     8717       -2     
+ Misses       2211     2189      -22     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[yusukebe]

Hey @usualoma !

What do you think of moving the mount feature from hono-base.ts to middleware? I want your opinion on the direction only (no implementation yet)!

[usualoma]

@yusukebe
That's a great change!

I think mount is a feature that symbolizes Hono, but I don't think it's a “characteristic of the Hono instance itself,” so I think it makes sense for it to be implemented as middleware (which will also make the Hono instance smaller).