Incorrect Types when using in a TypeScript ESM project

[Patrick Kerschbaum]

🕗 Version & Regression Information

1.2.0

⏯ Playground Link

https://stackblitz.com/edit/node-3rxg4d

1. Run npm install.
2. Run tsc compilation via npm run build. The error occurs.

💻 Code

import invariant from 'tiny-invariant';

invariant(typeof 'foo' === 'string');

🙁 Actual behavior

I tried to run the code above from a TypeScript ESM package, this means:

• "type": "module" set in package.json
• "module": "node16" set in tsconfig.json

But in such a project, the code does not compile:

❯ npm run build
$ tsc
src/test.ts:3:1 - error TS2349: This expression is not callable.
  Type 'typeof import("/home/projects/node-3rxg4d/node_modules/tiny-invariant/dist/tiny-invariant")' has no call signatures.

invariant(typeof 'foo' === 'string');

Found 1 error in src/test.ts:3

Note that the compiled output does run as expected, so the only problem is the TypeScript compilation error.

🙂 Expected behavior

The code should compile.

[Patrick Kerschbaum]

I think the root cause is the same as this: microsoft/TypeScript#50690.

[Alex Reardon]

I ran into this one today as well! What can I do to fix this?

[Patrick Kerschbaum]

Seems like we must get TypeScript into "ESM" mode when importing tiny-invariant.

I know two solutions:

1. Modify build to output dist/tiny-invariant.mjs and dist/tiny-invariant.d.mts (note the file extensions mjs and mts). And configure conditional exports (nodejs.org/api/packages.html#conditional-exports).
2. Modify build to output dist/node16/tiny-invariant.js and dist/node16/tiny-invariant.d.ts, and put a package.json there with "type": "module" set.
   And configure conditional exports.
   Because JS files are considered ESM modules when the "nearest" package.json contains this property.

Approach 2 is what safe-stable-stringify does.

[Patrick Kerschbaum]

The problem with approach 1 is that it is difficult to output dist/tiny-invariant.d.mts without creating a src/tiny-invariant.mts file.
I would go with approach 2, I will create a PR with it - have almost done it already :)