Are callbacks only for asynchronous code in JavaScript?
No. Callbacks are also used synchronously: map, filter, reduce, forEach, sort, find, some, every all take callbacks that run synchronously. Callbacks are just functions passed as arguments, whether sync or async.
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More FAQs in The Good and Bad Parts of Callbacks in JavaScript
Simplicity (pass a function, it gets called later), universality (work everywhere), synchronous use (map, filter, reduce), event handling (addEventListener), and closures (access outer variables).
Callback hell (deep nesting), inversion of control (trust the function to call correctly), repetitive error handling (each callback must check), hard to run in parallel (manual counter), hard to cancel, and unclear stack traces.
Use callbacks for simple one-off calls (setTimeout), event listeners, synchronous array methods (map, filter), and old codebases. Use promises/async-await for chained async operations, parallel operations (Promise.all), and complex error handling.
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