Force TypeScript to generate export/imports with the ".js" extension; running Node 16?

You cannot omit the file extension anymore in ESM module imports. The extension should be always .js/.jsx, not .ts/.tsx for a typescript file. So, in the index.ts you should add the extension to spig export like the following and every other file imported/exported if using ESM modules.:

index.ts

export { Spig } from './spig.js'; 

Also, moduleResolution should be set to Node16 or NodeNext so ESM modules work as expected.

As stated in the docs (enphasis by me):

Relative import paths need full extensions (e.g we have to write import "./foo.js" instead of import "./foo").

When a .ts file is compiled as an ES module, ECMAScript import/export syntax is left alone in the .js output; when it’s compiled as a CommonJS module, it will produce the same output you get today under module: commonjs.

This also means paths resolve differently between .ts files that are ES modules and ones that are CJS modules. For example, let’s say you have the following code today:

// ./foo.ts
export function helper() {
    // ...
}

// ./bar.ts
import { helper } from "./foo"; // only works in CJS
helper();

This code works in CommonJS modules, but will fail in ES modules because relative import paths need to use extensions. As a result, it will have to be rewritten to use the extension of the output of foo.ts - so bar.ts will instead have to import from ./foo.js.

// ./bar.ts
import { helper } from "./foo.js"; // works in ESM & CJS
helper();

This might feel a bit cumbersome at first, but TypeScript tooling like auto-imports and path completion will typically just do this for you.