Editable promises - beginner

Promise Chain Return

Toggle return and add handlers to see where undefined appears.

partially simulatedcurated scenariodoes not execute real code

Short answer

The key output order is undefined. A block-bodied arrow function needs an explicit `return`.

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. Missing returns commonly break request chains and test setup.

Common wrong assumption

Confusing expression arrows with block arrows.

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: undefined. 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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Data pipelines often parse, map, and render through promise chains.

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