You can now write and run unit tests in tgui.
It's quite simple: create a file ending in .test.ts or .spec.ts (usually with the same filename as the file you're testing), and create a test case:
test('something', () => {
expect('a').toBe('a');
});Refer to README to learn how to run tests.
There is an example test in packages/common/react.spec.ts.
You can read more about Jest here: https://jestjs.io/docs/en/getting-started
Note, that there is still no real solution to test UIs for now, even though a lot of the support is here (jest + jsdom). That will come later.