medium

UnhandledPromiseRejectionWarning

A Promise was rejected but no .catch() handler or try/catch block caught the error. This can cause flaky tests or missed failures.

Common Causes

  1. 1Missing try/catch around async code
  2. 2Forgotten .catch() on a Promise chain
  3. 3Mocked function rejecting unexpectedly
  4. 4Test not awaiting a rejected Promise

How to Fix

Use rejects matcher for expected rejections

it('throws on invalid input', async () => {
  await expect(fetchUser(-1)).rejects.toThrow('Invalid ID');
});

Use .rejects to test that a Promise rejects with the expected error.

How ObserveOne Helps

ObserveOne catches unhandled rejections in production and test environments, preventing silent failures.

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Related Errors

Timeout — Async callback was not invoked within 5000ms

An async test did not complete within the default 5-second timeout. The async operation may be hanging, or done() was never called.

Cannot find module './Component'

Jest could not resolve a module import. The file may not exist, the path may be wrong, or the module resolution config may be misconfigured.

SyntaxError: Unexpected token

Jest encountered syntax it cannot parse. This usually means a file needs to be transformed (e.g., TypeScript, JSX, ESM) but the transformer is not configured.

Snapshot mismatch — expected value to match snapshot

The rendered output does not match the stored snapshot. The component output changed since the snapshot was last updated.

Jest mock not being called / not returning expected value

A jest.mock() or jest.fn() is not behaving as expected — either not being called, not returning the mocked value, or not intercepting the real implementation.