What steps can be taken to ensure that VS Code correctly identifies TypeScript declarations within mono-repo packages?

I'm currently working on a mono-repo project setup with an isolated library package in TypeScript and another web UI package that combines TypeScript and React.

To import the compiled library package into the consumer (web UI) package, I'm using yarn to link the packages and parcel to generate distribution files for the library package. Parcel automatically generates a d.ts file in the library package's dist folder.

When I open the consumer package's file in VS Code that imports and uses the library, the IDE doesn't recognize the types declared in the library package's d.ts file.

The structure of the packages is as follows:

rootPackage
|- library
|- web-ui

In the library package, there's a types.ts file along with an index.ts file where only one type is being exported:

export type ParamType = "a" | "b" | "c";

I use parcel watch in this package to automatically refresh the dist files whenever changes are made. Parcel successfully generates the main.d.ts file which is referenced by the package.json's types attribute.

However, when trying to use the ParamType type in the web-ui package's code, I encounter an error in the IDE:

Cannot find name 'ParamType'.ts(2304)

Although running parcel in the web-ui package compiles without issues and loads in the browser fine, it seems to be a problem specific to Visual Studio Code that I'm struggling to resolve.


Edit 1

To showcase this issue, I've set up a public repository on GitHub. If you have any solutions or suggestions for how to fix this issue, please feel free to create a pull request - any help would be greatly appreciated.

Answer №1

After examining your sample repository, it appears that the structure of your library package starts from here:

/shared/lib/src/index.ts

import { ParamType } from "./types";

const Library = {
  typedParamMethod(param: ParamType) {
    console.log(param);
  },
};

export default Library;

When you utilize the library package in your web-ui package, this is how it should be done:

/ui/web/src/App.tsx

import React from 'react';
import Library from 'library';

function App() {
  React.useEffect(() => {
    const typedParam: ParamType = 'b'; // Both VSCode and tsc display the error message "Cannot find name 'ParamType'."
    Library.typedParamMethod(typedParam);
  }, []);

  return (
    <div className="web-ui">Web UI</div>
  );
}

export default App;

The issue lies in the fact that you have not exported ParamType from the root of your library package nor imported it into your web-ui package. It's important to understand that modern JavaScript and Typescript modules are isolated entities - the line

import { ParamType } from "./types"
in the root of your library package only makes ParamType accessible within that specific file, without impacting other parts of code.

To resolve this problem, include this line in the root of your library package (e.g., /shared/lib/src/index.ts):

export { ParamType } from "./types.ts";

Then, adjust the import statement in your web-ui package (in /ui/web/src/App.tsx) as follows:

import Library, { ParamType } from 'library';

(This issue and its solution are not specific to VSCode - any tool used for typechecking the web-ui package will result in the same error unless these modifications are made)

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