What is callback hell in JavaScript?
Deeply nested callbacks that are hard to read and maintain. Each async operation depends on the previous one, leading to a pyramid shape. Fix with promises (chain .then calls) or async/await (flatten to synchronous-looking code).
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More FAQs in Callback Interview Questions in JavaScript
When you pass a callback to a function, you give up control of when and how it is called. The function might call it multiple times, not at all, or with unexpected arguments. Promises solve this by guaranteeing the callback is called once.
The Node.js convention: the first argument is an error. Always check if (err) first and handle it. In promises, use .catch instead. Never ignore the error argument in a callback.
No. map, filter, and reduce call their callbacks synchronously. setTimeout, event listeners, and fetch call their callbacks asynchronously. The timing depends on the calling function, not the callback itself.
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