Support importing TypeScript files from TypeSpec

[timotheeguerin]

Problem Statement

Currently, the TypeSpec compiler only supports importing .tsp (TypeSpec) and .js/.mjs (JavaScript) files. With Node.js 22.11+ supporting native TypeScript execution without any flag, users should be able to import "./my-decorators.ts"; directly in their .tsp files without a separate build step.

Proposed Solution

The change is minimal — add .ts as a recognized extension in the compiler:

Changes Required

File	Change
packages/compiler/src/core/source-file.ts	Add .ts → "js" in getSourceFileKindFromExt
Tests	Add tests for .ts file imports mirroring existing .js tests
Documentation (imports.md)	Mention .ts support

How it works

1. Once getSourceFileKindFromExt returns "js" for .ts, the source-loader routes it to importJsFile()
2. importJsFile() → loadJsFile() → host.getJsImport(path) → import(pathToFileURL(path).href)
3. Node.js 22.11+ handles the .ts import natively via strip-types

What does NOT need to change

• module-resolver.ts loadAsFile extensions — only used for extensionless resolution which TypeSpec doesn't use
• directoryIndexFiles — not adding index.ts since we're only supporting explicit extensions
• node-host.ts getJsImport — import() already handles .ts in supported Node versions
• SourceFileKind type — .ts maps to existing "js" kind
• Browser host — no special handling

Design Decisions

• Node version: Keep minimum at Node 22.0.0. If TS import fails on older Node (<22.11), the natural import() error is clear enough ("Unknown file extension .ts for /path/to/file.ts").
• Extensions: Only .ts for now (skip .mts/.cts).
• Extensionless resolution: Not applicable — TypeSpec requires explicit extensions in imports.
• Browser host: Let it try and fail naturally (no special handling).
• tspMain: No change needed — it's for .tsp files only.
• exports interaction: Works naturally — module resolver doesn't filter by extension, just needs getSourceFileKindFromExt to recognize .ts.
• Convention for libraries: No special convention — just document that .ts works and let users handle their own dev/publish workflow.

Node.js Compatibility

• Node 22.0–22.5: No TS support
• Node 22.6–22.10: Requires --experimental-strip-types flag
• Node 22.11+: Unflagged, just works (stable)
• Node 23.6+: Also unflagged

[github-actions]

🎯 Issue Triage

Feature request to allow TypeSpec (.tsp) files to directly import "./my-decorators.ts" TypeScript files, leveraging Node.js 22.11+ native TypeScript execution (strip-types) without a separate build step.

Category: Feature Request
Area: compiler:core

🔍 Analysis

• The core change is minimal: add .ts → "js" mapping in getSourceFileKindFromExt in packages/compiler/src/core/source-file.ts
• Implementation piggybacks on the existing importJsFile() → loadJsFile() → host.getJsImport() → import() pipeline; Node.js 22.11+ handles the .ts import natively from there
• Node compatibility breakdown: no TS support before 22.6; requires --experimental-strip-types on 22.6–22.10; unflagged stable on 22.11+ and 23.6+
• Older Node fails naturally with a clear "Unknown file extension .ts" error — no special error handling needed
• Issue author (a repo member) has already analyzed the full scope and documented design decisions thoroughly
• Documentation update to imports.md is included in the proposal
• No browser host changes needed; .mts/.cts intentionally excluded for now

📝 Suggested next steps

• Add .ts → "js" mapping in getSourceFileKindFromExt (packages/compiler/src/core/source-file.ts)
• Add tests mirroring existing .js import tests
• Update imports.md to document .ts support
• Consider whether to add a diagnostic/warning when a .ts import is encountered on Node < 22.11

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