What is callback hell, and how do promises and async/await solve it?

Callback hell is code where async callbacks are nested inside each other, deeper and deeper. Promises and async/await turn it into flat code.

Before promises, async APIs took a callback to call when the work was done. A common form was the Node style (err, result) => {}. When each step needs the result of the step before, callbacks nest inside callbacks. The code forms a pyramid that moves to the right.

The problems are more than looks:

• You must repeat error handling at every level.
• It is easy to forget to pass an error on.
• Loops and parallel steps are hard to write.
• A third-party API might call your callback twice, or never. This is called inversion of control: you hand control of your code to someone else.

Promises fix this. The function returns an object you can chain. You get a flat list of then calls with a single catch for errors. Helpers like Promise.all handle parallel work.

async/await goes further. You write the same flow as normal top-to-bottom code, with normal try/catch and loops.