What to Test

React Testing Library is built on one idea, stated by its author: the more your tests resemble the way your software is used, the more confidence they give you.

In practice that means testing what a user can observe - text on the screen, what happens when they click - rather than the internals of the component.

Test behaviour, not implementation

// Implementation: breaks the moment you rename state or restructure
expect(wrapper.state("isOpen")).toBe(true);
expect(wrapper.find("Dropdown").props().items).toHaveLength(3);

// Behaviour: survives any refactor that keeps the UI the same
await user.click(screen.getByRole("button", { name: /options/i }));
expect(screen.getByRole("listbox")).toBeVisible();
expect(screen.getAllByRole("option")).toHaveLength(3);

The second version would still pass if you swapped useState for useReducer, renamed every variable, or replaced the component with a different implementation entirely. That is the point.

What is worth testing

• The component renders the right thing for its props.
• Interactions produce the expected result.
• Conditional paths: loading, empty, error, success.
• Accessibility basics: labels, roles, focus behaviour.

What is not

• Internal state values and hook call counts.
• That a child component received a particular prop.
• Exact class names or the DOM structure.
• Third-party libraries, which have their own tests.