# Callbacks & Callback Hell — JavaScript Fundamentals: From Syntax to Async

Source: https://www.skillbyai.com/en/javascript/js-callbacks

> Handle asynchronous work with callbacks and see why nesting them leads to callback hell.

## What is a callback

A callback is a function passed into another function, to be run once some work finishes.

```js
function fetchData(callback) {
  setTimeout(() => callback("data loaded"), 1000);
}
fetchData((result) => console.log(result));
```

## Callback hell

Nesting async steps inside each other's callbacks quickly forms a hard-to-read pyramid.

```js
getUser(1, (user) => {
  getPosts(user.id, (posts) => {
    getComments(posts[0].id, (comments) => {
      console.log(comments);
    });
  });
});
```

## Why it's a problem

Deeply nested callbacks are hard to read and to handle errors in — this pyramid shape is exactly what Promises were built to fix.