You can directly import any valid JS module using TypeScript if you have “allowJS”: true in the compiler options of your tsconfig.json file. The comparison to FFI doesn’t make any sense. FFI requires compiling a special library that explicitly exports the C types. This is different than TS. Using TS, you can import any valid JS module without any special preparation. Also, you can’t import all compiled languages into each other using FFI. You can only import and export C-based types. There is no way to export a Go struct for consumption via FFI, for instance. All valid JS modules will work with TypeScript.

>There is no way to export a Go struct for consumption via FFI, for instance.

I hope not.

>Using TS, you can import any valid JS module without any special preparation

Why does the DTS ecosystem exist then (https://www.npmjs.com/package/@types/node etc)

d.ts files allow you to add types to imported JS. You’re free to import raw JS as an any type and the compiler won’t complain.

Okay...

    $ echo "export class Foo { a = 1 }" > test.mjs

    $ npx ts-node
    > import('./test.mjs').then(console.log)
      error TS7016: Could not find a declaration file for module './test.mjs'. '/home/user/Lab/test.mjs' implicitly has an 'any' type.

    $ npx ts-node -O '{"allowJs":true}'
    > import('./test.mjs').then(console.log)
      Promise { <pending > }
      Error [ERR_REQUIRE_ESM]: require() of ES Module /home/user/Lab/test.mjs not supported.

    $ echo "module.exports.Foo = class Foo { a = 1 }" > test.cjs

    $ npx ts-node -O '{"allowJs":true}'
    > const { Foo } = require('./test.cjs')
    > Foo
      [class Foo]
    > new Foo
      Foo { a = 1 }
    > function bar (foo: Foo) {}
      TS2749: 'Foo' refers to a value, but is being used as a type here. Did you mean 'typeof Foo'?

No I did not, TypeScript, and that's about as much as it seems to interoperate. I guess in some cases practical migration might be viable - but as for the general case, the out-of-the-box experience seems to speak otherwise.

It's not even viable to write in JS and manually write a DTS - no way to check em against each other. Except idk library in JS, test suite in TS anyway. So not a lot of lateral movement possible in practice.

ts-node isn't an official part of the TypeScript project. It has notoriously bad module support.

Try this:

    // tsconfig.json
    {
      "compilerOptions": {
        "outDir": "dist",
        "module": "es2020",
        "allowJs": true
      },
      "files": ["index.ts"]
    }

    // index.ts
    import('./test.mjs').then(console.log)

    > npx tsc && node ./dist/index.js
    [Module: null prototype] { Foo: [Function: Foo] }

I'm not even talking about the module support. (It just seems to have the default support of TypeScript.)

Yes, it imports the module. No, it doesn't do even basic type inference (knowing that Foo is a class and consequently allowing it to be used as a type name) - which it would, if it was a superset of JS. Instead it seems to import everything as "any", which is... a start, I guess?