Path mapping in yarn workspaces

[andrew8er]

I have a problem in which path mapping is not flexible enough for different project scenarios.

I have the following project layout: I have separate main and tests directories which use different versions of the protobufjs type definitions.

Project layout

/src
+-- tsconfig.base.json
+-- /main
    +-- (sources…)
    +-- tsconfig.json
+-- /tests
    +-- (sources…)
    +-- tsconfig.json
/node_modules
    +-- /@types
        +-- /protobufjs
            +-- index.d.ts

The main sources should use the newer definitions that come included with the npm package, the tests use generated code that refers to an older and incompatible version of protobufjs that uses the definitions from node_module/@types. I therefore use the following path mapping, which works as intended:

/src/tsconfig.base.json

{
	"compilerOptions": {
		"rootDir": "./",
		"baseUrl": "./",
	}
}

/src/main/tsconfig.base.json

{
	"extends": "../tsconfig.base.json",
	"include": [
		"**/*.ts"
	]
}

/src/test/tsconfig.base.json

{
	"extends": "../tsconfig.base.json",
	"compilerOptions": {
		"paths": {
			"protobufjs": ["../node_modules/@types/protobufjs"],
		}
	},
	"include": [
		"**/*.ts"
	]
}

The problem arises when I include this package in a yarn workspace. In this situation the node_modules folder gets hoisted up one level, and since the path mappings are interpreted as being relative to baseUrl, they do no longer resolve to a valid directory.

Quoting from the yarn documentation this seems an obvious problem:

The package layout will be different between your workspace and what your users will get (the workspace dependencies will be hoisted higher into the filesystem hierarchy). Making assumptions about this layout was already hazardous since the hoisting process is not standardized, so theoretically nothing new here.

I see two ways to resolve this problem:

1. Require paths to be interpreted as being relative to baseUrl to have a leading ./. Interpret path mappings without it as relative paths subject to node module resolution logic. This would require small fixes for many projects.

2. Introduce a special leading signifier (e.g ^) or add a special attribute in compilerOptions (e.g. nodePaths) for path mappings subject to node module resolution logic.

A path mapping can list multiple paths to try. Could you try this with:

"protonbufjs": ["../node_modules/@types/protobufjs", "../../node_modules/@types/protonbufjs"]

That way should work in both situations.

[andrew8er]

Yes, I thought of that too, but this is only a stopgap solution given the fact that packages in yarn workspaces can be arbitrarily nested.

Would it be possible to give tests its own package.json so that @types/protonbufjs is installed locally? That way you don't need path mapping at all.

Path mappings aren't supposed to go through module resolution -- since it's the exception to normal module resolution. It's an unusual situation to want different parts of your code to resolve the same package to two different locations in the same node_modules directory, since presumably there will only be one real package at runtime.

[andrew8er]

Mh, I see your point. Having an extra package just for the tests seems a bit overkill for me. I will see how I can work around it. Feel free to close this issue.

OK. You could always just include as many ../../../ path mappings as are sufficient for your needs.