All 15 Async Patterns questions
More in Async Patterns
What is a Promise in JavaScript, and what are its three possible states? How do you convert a Node.js callback-based function into a Promise-based one manually, without using util.promisify()? What is the purpose of the 'node:timers/promises' module, and how does it simplify working with delays in async code? What is the difference between Promise.all(), Promise.allSettled(), Promise.race(), and Promise.any()? How does async/await relate to Promises under the hood, and what does 'await' actually do to program execution? What is the difference between running async operations sequentially with multiple awaits versus concurrently with Promise.all()? How do you implement a retry-with-exponential-backoff pattern for a flaky async operation in Node.js? What is util.promisify(), and when would you still need it in modern Node.js? What is the difference between micro tasks and macro tasks in Node.js's event loop, and how does this affect Promise callback ordering? What is an async generator, and how does 'for await...of' let you consume asynchronously produced data?
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What is a callback function in Node.js, and what is 'callback hell'?

Beginner

Answer

A callback is a function passed as an argument to another function, invoked once that function's asynchronous work completes -- Node's original core APIs (fs, http, etc.) are all built around this 'error-first callback' convention. Callback hell refers to the deeply nested, hard-to-read pyramid of code that results from chaining multiple dependent asynchronous callback-based operations, one inside another, which promises and async/await were both introduced specifically to solve.
// Callback hell: deeply nested, hard to follow
fs.readFile('a.txt', (err, a) => {
  fs.readFile('b.txt', (err, b) => {
    fs.readFile('c.txt', (err, c) => {
      console.log(a, b, c);
    });
  });
});
Real-world example A legacy codebase with five levels of nested callbacks handling a user-registration flow (validate, hash password, save to database, send email, log event) is refactored to use async/await, immediately making the control flow readable top-to-bottom and making error handling consistent via a single try/catch.

Common follow-ups: What does 'error-first' mean in the error-first callback convention, and why was it adopted?;How would util.promisify() convert an existing callback-based function to return a promise instead?

Error Handling;Async Patterns
All 15 Async Patterns questions