Add type definitions for acorn.mjs

[TimvdLippe]

For TypeScript projects that use the acorn.mjs distribution bundle,
a separate .mjs.d.ts needs to be added to the folder. This was
suggested in microsoft/TypeScript#27957 (comment)

In DevTools, we use this in https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2218015
to make it compile with TypeScript

[RReverser]

Is it possible to reuse these definitions in acorn.d.ts by re-exporting from one to another (in whatever direction)? My understanding is that they should work equally well for CommonJS?

[marijnh]

Agreed, I really don't want to have two copies of the same file in the repository.

[TimvdLippe]

Should be fixed now. Initially I tried to import the acorn.d.ts in acorn.mjs.d.ts, but realized that is not possible because ESM can't import commonjs. However, the reverse is possible. So if we define all types in acorn.mjs.d.ts, then acorn.d.ts can import them.

However, this would require users to add esModuleInterop to their tsconfig.json. I locally verified on a project that would use acorn with commonjs that making this change will still make TypeScript happy. So this technically is a breaking change after all 😢 Not sure how to work around that, let me know what you think.

[RReverser]

Should be fixed now. Initially I tried to import the acorn.d.ts in acorn.mjs.d.ts, but realized that is not possible because ESM can't import commonjs.

From a quick attempt, putting this in acorn.mjs.d.ts seems to work:

import acorn from "./acorn.js";
export = acorn;

As far as I understand, it would also preserve backward compatibility - please correct me if I'm wrong and it's subject to the same issue.

[TimvdLippe]

Yes it works, but I had to set allowSyntheticDefaultImports in our tsconfig.json to make it work. I think that is an acceptable workaround for us. Thanks @RReverser for the suggestion.

[RReverser]

Ah interesting. Not sure if there's a generic way to work around that...

[RReverser]

Actually... it seems that

import * as acorn from "./acorn";
export = acorn;

works just as well even without the flag?

[TimvdLippe]

I don't know why this now works, as I tried before. But yes, this removes the need for any tsconfig change AND is not a breaking change. Awesome, thanks!

[RReverser]

Great, thanks for the PR!