mock function from another module for testing

Question

I'm using Chai, Mocha for testing in my project. I want to test the createTreefromFolder function from a module tree.js:

export function createTreefromFolder(path: string): string[] {
  const files = listFilesFromFolder(path);
  const tree: string[] = [createFirstBranch(path)];
  files.forEach((file) => {
    tree.push(Prefix.VERTICAL.concat(file));
  });
  return tree;
}

My goal is to mock listFilesFromFolder function in my test in order to return a specific value.

In the tests folder I have:

  it("creation of a unique folder tree", () => {
    const expectedArray: string[] = ["🗃️ empty_folder"];
    mock("src.tree.listFilesFromFolder").return_value = ["empty_folder"] // mock here
    expect(createTreefromFolder(empty_test_folder_path)).to.eql(expectedArray);
  });

Is there a way to do that as python do that with unitest.mock.patch ?

Answer 1

One way is to pass listFilesFromFolder as a parameter for createTreefromFolder.

Another one -- to use some king of dependency injection, and mock listFilesFromFolder dependency inside your registry.

There's no way to patch function source code from outside as far as I know. So functionality you wanna mock should be initially imported from a mockable place, dependency container (IoC container) of some kind.