How do you test errors and failure paths in Jest?

Failure paths are part of the contract. A function that throws a typed error, a UI that shows a retry message, and a retry that gives up after three attempts all deserve tests. Matching only that something threw is weak because any unrelated bug also throws.

test("rejects an invalid total", () => {
  expect(() => createInvoice({ total: -1 })).toThrow(ValidationError);
  expect(() => createInvoice({ total: -1 })).toThrow("total must be positive");
});

test("shows a retry message when the API fails", async () => {
  server.use(
    http.get("/api/items", () => new HttpResponse(null, { status: 500 })),
  );

  render(<ItemList />);

  expect(await screen.findByRole("alert")).toHaveTextContent("Try again");
});

What to assert on an error

• The error type, using the constructor when you have a custom class
• A stable part of the message, not the full stack
• The user-visible error state and any recovery action
• That state was not partially mutated after the failure
• That a retry or fallback ran the expected number of times

For async code, rejects.toThrow handles a rejected promise. If the function throws synchronously, use toThrow with a wrapper function instead, because there is no promise to await. Passing a function to toThrow is required; expect(fn()).toThrow() throws before the matcher runs.