📘 doc: document specialized response mapping optimization

[DenisCDev]

Summary

Adds documentation for the specialized response mapping optimization to the JIT Compiler internals page.

This covers the new AOT compilation path introduced in elysiajs/elysia#1799, where the compiler detects TypeBox response schema kinds at compile time and generates specialized inline response code that bypasses the generic mapResponse dispatch chain.

What's documented

• Why the generic mapResponse is slow (10+ type checks, megamorphic call sites)
• How the compiler detects schema Kind and generates type-guarded fast paths
• Code examples showing before/after generated code
• Per-adapter differences (Bun Response.json() vs Web Standard JSON.stringify())
• Safety guarantees (type guard fallback, conditions that disable specialization)

Placed as a new subsection under Compiler Optimizations, right after the existing mapResponse/mapCompactResponse example — it's the natural continuation of that topic.

Summary by CodeRabbit

• Documentation
  • Updated internal documentation with expanded explanation of response mapping optimization techniques and JIT compiler performance improvements, including code examples and platform-specific considerations.

[coderabbitai]

Walkthrough

Documentation update to the JIT compiler guide adding a new section on specialized response mapping. Explains how Elysia generates optimized fast paths for routes with known response schemas, bypassing generic type checks, including code examples and platform-specific considerations.

Changes

Cohort / File(s)	Summary
JIT Compiler Documentation docs/internal/jit-compiler.md	Added comprehensive section on specialized response mapping optimization, including code comparisons (generic vs. specialized paths), type-guard mechanisms, platform-specific implications for Bun and Web Standard APIs, and conditions that disable specialization.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

A rabbit hops through docs so bright,
Explaining compilers that take flight,
Fast paths and guards, oh what a sight!
Response mapping done just right, 🐰✨
Knowledge shared with pure delight!

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately describes the main change: documenting a specialized response mapping optimization. It is specific, concise, and directly reflects the content added to the documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

docs/internal/jit-compiler.md (1)

54-55: ⚠️ Potential issue | 🟡 Minor

Fix incorrect variable reference in code example.

Line 55 references req.params.id, but the handler parameter is destructured as ({ params }) on line 54, so req is undefined. This should be params.id.

📝 Proposed fix

 .patch('/user/:id', ({ params }) => {
-	return { id: req.params.id }
+	return { id: params.id }
 })

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@docs/internal/jit-compiler.md` around lines 54 - 55, The example handler
destructures its argument as ({ params }) but returns req.params.id, which is
undefined; update the return to use params.id instead so the function
(.patch('/user/:id', ({ params }) => { ... }) / the handler body) references the
destructured params variable rather than req.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@docs/internal/jit-compiler.md`:
- Around line 54-55: The example handler destructures its argument as ({ params
}) but returns req.params.id, which is undefined; update the return to use
params.id instead so the function (.patch('/user/:id', ({ params }) => { ... })
/ the handler body) references the destructured params variable rather than req.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: d62031d8-db9c-4ffc-b12f-db781a380e03

📥 Commits

Reviewing files that changed from the base of the PR and between 1da49a4 and 6ee3ce4.

📒 Files selected for processing (1)

• docs/internal/jit-compiler.md