How do resolves and rejects matchers work?

These matchers unwrap a promise so you can assert on the settled result without writing a try/catch. resolves follows the fulfilled path, and rejects follows the rejected path. Both return a promise, which is why the await is not optional.

test("resolves with the parsed body", async () => {
  await expect(loadUser("u1")).resolves.toEqual({ id: "u1" });
});

test("rejects with a readable error", async () => {
  await expect(loadUser("missing")).rejects.toThrow("not found");
});

What each path expects

• resolves fails if the promise rejects
• rejects fails if the promise fulfils
• rejects.toThrow matches an error's message or constructor
• rejects.toMatchObject checks fields on an error object
• resolves.toBeUndefined is valid for a void async function

A common mistake is asserting rejects on a function that throws synchronously. If the function is not async and throws before returning a promise, the call throws immediately and the assertion never sees a rejected promise. Wrap the call in an async function or use toThrow in that case.