Writing unit tests is pretty straightforward but there are some things to consider.

The unit test for a particular class/file should have the same name as the class under test but end with unit.test.ts instead. This makes it get picked up automatically when our tests run.

They generally follow a pattern like so:

suite(`My new unit test', () => {
    let foo: IFoo;

    setup(() => {
       // setup mocks
       foo = mock(FooClass); // Using ts-mockito to mock
       when(foo.bar).thenReturn(true);
    });

    test(`Test baz`, async () => {
       const baz = new Baz(foo);
       assert.ok(baz.bar, `Bar is not correct');
    });
});

Mostly we use ts-mockito to generate mock objects and sinon to stub out method calls.

In unit tests we try to follow the pattern of testing the public output of a particular class or function. Embedding internal details of a function class into a unit test ends up meaning the test is a copy of the function itself and makes it hard to maintain.

When a unit test fails (or you're just starting to write one), you can debug the test.

From the debug drop down in VS code, pick this launch.json entry:

Then click on the gear icon and edit the --grep parameter to match your test.

You should be able to set a breakpoint directly in the test code.