Demo 10

Promise rejection

Rejected promises skip success handlers and go to rejection handlers.

fully simulatedcurated scenariodoes not execute real code

Short answer

The key output order is bad. A rejection bypasses `.then` success callbacks until a rejection handler catches it.

Real-world bug this helps solve

A later .then receives the wrong value, an error appears in a surprising place, or a batch fails differently than expected. Centralized `.catch` handlers are common for fetch and async workflows.

Common wrong assumption

Expecting every `.then` in a chain to run.

Fix direction

Return or await the promise chain so values and failures move through the flow intentionally.

Fixed code pattern

return doWork()
  .then((value) => nextStep(value))
  .catch(handleError);

Visual proof

The console lane proves the visible order: bad. The active line and queues show why source order changed.

How to verify in a real app

Write a focused unit test that asserts the resolved value or rejected path, then verify every branch returns or awaits its work.

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Related cases

Continue with nearby confusion while the mental model is fresh.

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