What is callback hell and how do you avoid it?

Callback hell (also called the "pyramid of doom") occurs when multiple nested callbacks create code that is deeply indented and difficult to read, maintain, and debug. Example: reading a file, then parsing it, then making an API call, then writing results — each step nesting inside the previous callback. Solutions: (1) Named functions — extract each callback into a named function instead of nesting anonymously, flattening the structure. (2) Promises — chainable .then() handlers allow sequential async operations in a flat, readable chain. (3) async/await — syntactic sugar over Promises that makes async code look synchronous and sequential, dramatically improving readability. (4) Modularization — split complex async flows into small, single-purpose functions. Modern Node.js code primarily uses async/await, which is the recommended approach.