The answer
Callback hell: deeply nested callbacks are hard to read, hard to debug, and prone to error handling issues. Promises and async/await were introduced to solve this by flattening the structure.
What Are Callback Functions in JavaScript?
What is the problem with deeply nested callbacks in JavaScript?
More FAQs in What Are Callback Functions in JavaScript?
A function passed as an argument to another function, to be called later. Callbacks are used for async operations (setTimeout, event listeners, fetch) and synchronous operations (map, filter, reduce).
Because JS is single-threaded and synchronous. To handle async operations without blocking, it passes callbacks to Web APIs (setTimeout, fetch) or Node.js APIs (fs). The runtime calls the callback when the operation completes.
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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