Testing TypeScript function which is not exported

Question

I use Mocha/Chai for testing JavaScript front-end code and now we switched to TypeScript. I have several functions I want to test. But they shouldn't be exportable. Can I get an access to this functions and test it without adding export to them?

Answer 1

While it is not possible to access non-exported function directly there is still a way to export them in a "semi-hidden" way. One possible approach would be:

// In your library module declare internal functions as non-exported like normal.
function someInternalFunctionA(x: number): number {
  return x;
}

function someInternalFunctionB(x: number): number {
  return x;
}

// At the bottom, offer a public escape hatch for accessing certain functions
// you would like to be available for testing.
export const _private = {
  someInternalFunctionA,
  someInternalFunctionB,
};

On test side you can do:

import { _private } from "./myModule";

test("someInternalFunctionA", () => {
  expect(_private.someInternalFunctionA(42)).toEqual(42);
});

What I like about the approach:

• No need to mark someInternalFunctionA with export directly.
• It should still be very obvious that the stuff under _private isn't officially part of the public interface.