Flow declaration file with CommonJS

Question

How do I write a proper Flow declaration file when using CommonJS like this? This is in a file named demo.js.

// @flow
function product(a, b) {
  return a * b;
}
exports.product = product;

Here is what I tried. This is in a file named demo.js.flow.

// @flow
declare export function product(a: number, b: number): number;

Flow still complains that the parameters a and b are missing annotations in the previous file.

Answer 1

Following the documentation, I would try with:

declare module 'demo' {
  declare module.exports: {
    product(a: number, b: number): number;
  };
}