[Bug]: Unhandled promise rejections leading to unexpected jest output (with workaround)

[vovkasm]

Version

30.2.0

Steps to reproduce

1. Clone demo repo at https://github.com/vovkasm/jest-bug-demonstration.git
2. cd jest-bug-demonstration
3. npm ci
4. npm run problem

Expected behavior

Not sure, but with workaround (npm run workaround) it can behave more or less as expected. I think most users expects to get clean assertion error.

> NODE_OPTIONS=--unhandled-rejections=warn jest demo1

(node:55520) UnhandledPromiseRejectionWarning
(Use `node --trace-warnings ...` to show where the warning was created)
(node:55520) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
 FAIL  ./demo1.spec.ts
  ✕ Async. Should fail with correct assertion error (3 != 2) (3 ms)

  ● Async. Should fail with correct assertion error (3 != 2)

    expect(received).toBe(expected) // Object.is equality

    Expected: 2
    Received: 3

      2 |   const res = Promise.reject(new Error("Task failed"));
      3 |
    > 4 |   expect(3).toBe(2);
        |             ^
      5 |
      6 |   await res.catch(() => {});
      7 | });

      at Object.toBe (demo1.spec.ts:4:13)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        0.252 s, estimated 1 s

Actual behavior

When running tests with Jest, unhandled promise rejections cause unexpected behavior. Currently, when an unhandled promise rejection occurs (due to Promise.reject() without proper handling), Jest's output is disrupted by the Node.js error message, making it difficult to discern the actual test failures.

> jest demo1


 RUNS  ./demo1.spec.ts
/Users/vovkasm/tmp/test1/demo1.spec.ts:4
  const res = Promise.reject(new Error("Task failed"));
                             ^

[Error: Task failed]

Node.js v24.11.0

Additional context

Minimal test code to reproduce:

it("Async. Should fail with correct assertion error (3 != 2)", async () => {
  const res = Promise.reject(new Error("Task failed"));

  expect(3).toBe(2);

  await res.catch(() => {});
});

In real life it was much more complex scenario.

Environment

System:
    OS: macOS 15.7.1
    CPU: (8) arm64 Apple M1 Pro
  Binaries:
    Node: 24.11.0 - /Users/vovkasm/.local/state/fnm_multishells/54801_1762875355874/bin/node
    npm: 11.6.1 - /Users/vovkasm/.local/state/fnm_multishells/54801_1762875355874/bin/npm
    bun: 1.3.2 - /opt/homebrew/bin/bun
    Deno: 2.5.6 - /opt/homebrew/bin/deno
  npmPackages:
    jest: ^30.2.0 => 30.2.0

[vovkasm]

Also, another workaround would be to place this snippet in setup.js:

// Without this handler, promise rejections from test code will overwrite real test assertions.
// We just log rejections for now.
process.on("unhandledRejection", (reason) => {
  console.log("Unhandled Rejection with reason:", reason);
});

When this handler is installed, Node.js no longer looks at the --unhandled-rejections flag, and instead calls the custom handler in case of unhandled promise rejections.

[Hesowcharov]

one more thing to the context - also, the reproducer can be run without false positives if apply --waitForUnhandledRejections cli-option:

npx jest demo1 --waitForUnhandledRejections

 FAIL  ./demo1.spec.ts
  ✕ Async. Should fail with correct assertion error (3 != 2) (5 ms)

  ● Async. Should fail with correct assertion error (3 != 2)

    expect(received).toBe(expected) // Object.is equality

    Expected: 2
    Received: 3

      2 |   const res = Promise.reject(new Error("Task failed"));
      3 |
    > 4 |   expect(3).toBe(2);
        |             ^
      5 |
      6 |   res.catch(() => {});
      7 | });

      at Object.toBe (demo1.spec.ts:4:13)

  ● Async. Should fail with correct assertion error (3 != 2)

    Task failed

      1 | it("Async. Should fail with correct assertion error (3 != 2)", async () => {
    > 2 |   const res = Promise.reject(new Error("Task failed"));
        |                              ^
      3 |
      4 |   expect(3).toBe(2);
      5 |

      at Object.<anonymous> (demo1.spec.ts:2:30)

Also, we noticed that the documentation says:

Jest 30 now waits an extra event loop turn to confirm that a promise rejection remains unhandled before failing a test.
This flag, when disabled, can restore the previous behavior (not waiting) if absolutely needed. Most users should not need to change this – the default now favors correctness by preventing false failures.

It looks like waitForUnhandledRejections option should be enabled by default for jest v30.