Support path aliases for type checking (not for emitted code)

[arturenault]

🔍 Search Terms

baseUrl, paths, alias, resolution, type-checking

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I've read the docs and open issues like #10866 and #16088. I understand that the baseUrl and paths options are designed to be used by bundlers, and do not affect code emitted by tsc.

I use esbuild and have gotten it to successfully transpile my code. However, esbuild (and many other bundlers) don't perform type checking and explicitly recommend using tsc --noEmit separately to do so.

But since tsc doesn't resolve relative to baseUrl or paths, type checking fails if I use paths and/or baseUrl, since those modules cannot be resolved.

📃 Motivating Example

Users of esbuild, swc, and shouldn't have to choose between the following two options:

Without type-checking

import { context } from "context"

With type-checking

import { context } from "../../../../context"

They should have the best of both worlds.

💻 Use Cases

1. What do you want to use this for?
   To write prettier code while keeping all the benefits of type-checking and esbuild.

2. What shortcomings exist with current approaches?

• Other module bundlers exist, but they're slow and lack features. Besides, this solution would mean we'd stop having to rely on module bundlers for type-checking.
• module-alias paths are global, so it's hard to use in monorepo setups. Also only works in node.
• tsc-alias only works with emitted code.

3. What workarounds are you using in the meantime?
   Just writing ugly paths.

[RyanCavanaugh]

Could you actually give an example of what you're doing in terms of file layouts?

[arturenault]

Actually realized that what I'm asking for already works in a simple repo, but I'm working with a monorepo and it doesn't seem to be supported there.

I have a parent tsconfig.json and child tsconfig.jsons in each monorepo. In each tsconfig.json I've set baseUrl to be src. So relative paths are resolved correctly when I'm in a subdirectory containing a tsconfig.json, but not at the root (which is where I'm currently doing type-checking from). I'm learning that tsc only loads one tsconfig.json at a time, and not the one closest to the file as some other tools do.

I tried setting up projectReferences to get all the projects to get built together, but it looks like that doesn't work with noEmit as described here: #36917