Comment by kuramitropolis

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.