feat: add createElysia factory helper to preserve prefix literal types

[shisashi]

Summary

• Add createElysia<const P>() — a type-safe factory function that preserves the literal prefix type via const generic inference
• Add type-level test verifying that factory-created sub-apps produce independent response types when composed via .group() + .use()

Closes #1725

Motivation

When multiple Elysia sub-apps are created via a plain (non-generic) factory function and composed under .group() via .use(), the BasePath generic widens to string. This causes CreateEden to produce index signatures ({ [x: string]: ... }) instead of literal keys, and TypeScript's & intersection merges all response types together.

createElysia is a thin wrapper that preserves the literal type:

import { createElysia, t } from 'elysia'

// ✅ Prefix literal '/sessions' is preserved — no type intersection
const sessionsApp = createElysia({ prefix: '/sessions' })
  .get('/', () => ({ sessions: [] }), {
    response: t.Object({ sessions: t.Array(t.String()) })
  })

See issue #1725 for the full root cause analysis.

Test plan

• tsc --project tsconfig.test.json passes (including the new type test)
• Verify the new export is available: import { createElysia } from 'elysia'

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Introduced a factory function that simplifies creation of Elysia sub-applications with improved type safety. Using const generics, it preserves literal base path type information, enabling type-safe composition of multiple sub-apps while maintaining response type independence.

• Tests

  • Added comprehensive tests verifying type safety and response type independence when composing factory-created sub-applications with different base paths.

[coderabbitai]

Walkthrough

A new type-safe factory function createElysia is introduced that uses a const generic to preserve literal BasePath types, enabling safer composition of sub-apps. The factory instantiates Elysia with the provided configuration while maintaining type precision during nested app composition.

Changes

Cohort / File(s)	Summary
Factory Function Export src/index.ts	Adds new public createElysia factory function with const generic BasePath extends string = '' that instantiates and returns a type-safe Elysia instance, preserving literal path types for safer sub-app composition.
Type Safety Tests test/types/index.ts	Adds test scenarios for the new createElysia factory, verifying that independently created sub-apps with prefixes maintain independent response types when composed under a parent route without intersection.

Possibly related issues

• Factory-created sub-apps cause response type intersection in ~Routes #1725: The const generic BasePath extends string = '' in the new createElysia factory directly addresses the issue of factory-widening prefix types to generic string, enabling type-safe preservation of literal paths during sub-app composition.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

Hop into type-safe composition, friend! 🐰
With const generics, the types never bend,
Sub-apps compose without fear of intersection,
The factory ensures prefect perfection! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: adding a factory helper function to preserve prefix literal types, which is the core objective of the PR.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

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

---

No actionable comments were generated in the recent review. 🎉

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}