What is callback hell and how do you fix it?
Callback hell is deeply nested callbacks (pyramid of doom) when multiple async operations depend on each other. Fix it by using Promises (.then chains) or async/await (makes async code look synchronous with try-catch). Extract logic into named functions for readability.
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More FAQs in Node.js Async Programming Interview Questions Promises, Async/Await, and Callbacks
Promise.all: all must succeed, or all fail (fast fail). allSettled: all complete regardless of success/failure (returns status and value/reason for each). race: first to complete wins (success or failure). Use all for parallel independent operations, allSettled when you want all results, race for timeouts.
Use try-catch blocks around await calls. Catch the error, log it, and either re-throw it (for the caller to handle) or return a default value. For unhandled rejections, add process.on('unhandledRejection', handler) as a safety net.
Use Promise.all: const [a, b, c] = await Promise.all([fetchA(), fetchB(), fetchC()]). All three start immediately and run in parallel. Total time is the longest operation, not the sum. Compare to sequential (await each one separately) which takes the sum of all times.
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