Unhandled promise rejections: causes and fixes

Question

Explain what unhandled promise rejections are, why they happen, and how to systematically prevent them in app code.

Accepted Answer

Interview explanation:
- An unhandled rejection means a promise rejected without a rejection handler.
- In modern runtimes this is noisy and can crash processes (Node settings dependent).

Common causes:
1) Fire-and-forget promise without .catch.
2) Missing await in try/catch (error escapes scope).
3) Missing return in .then chain.

Examples:
void riskyAsync().catch(reportError) // explicit fire-and-forget handling

async function handler() {
  try {
    await riskyAsync()
  } catch (err) {
    reportError(err)
  }
}

Prevention checklist:
- Always return/await promises from functions.
- Add a terminal .catch on top-level chains.
- Wrap UI event async handlers in try/catch.
- Centralize logging for unexpected async failures.

Unhandled promise rejections: causes and fixes

Answer