What is the difference between callback and promise in Node.js?

Question

Accepted Answer

**Callback** — function passed as argument. **Promise** — object representing async operation.

**Callback:**
- Function passed as argument.
- Called when operation completes.
- Can lead to callback hell.

```javascript
// Callback pattern
fs.readFile("file.txt", "utf8", (err, data) => {
    if (err) {
        console.error(err);
        return;
    }
    console.log(data);
});
```

**Promise:**
- Object representing future value.
- Three states: pending, fulfilled, rejected.
- Chainable with .then().

```javascript
// Promise pattern
const promise = new Promise((resolve, reject) => {
    fs.readFile("file.txt", "utf8", (err, data) => {
        if (err) reject(err);
        else resolve(data);
    });
});

promise
    .then(data => console.log(data))
    .catch(err => console.error(err));
```

**Key differences:**
- **Callback** — function, callback hell.
- **Promise** — object, chainable.

Node.js: What is the difference between callback and promise in Node.js?