Compare callbacks, Promises and async/await.

Question

Accepted Answer

These are three generations of handling **asynchronous** operations in Node — each improving readability over the last, but all built on the same event-loop foundation.

## 1. Callbacks — the original (and "callback hell")

```js
fs.readFile("a.txt", (err, a) => {
  if (err) return handle(err);            // error-first callback convention
  fs.readFile("b.txt", (err, b) => {       // nesting deepens with each step...
    if (err) return handle(err);
    db.save(a + b, (err) => {              // "pyramid of doom"
      if (err) return handle(err);
    });
  });
});
```

Node's convention is **error-first callbacks** (`(err, result)`). They work but nest deeply and scatter error handling — hard to read and maintain.

## 2. Promises — chainable, flatter

```js
readFile("a.txt")
  .then(a => readFile("b.txt").then(b => a + b)) // return a promise → chain waits
  .then(combined => db.save(combined))
  .catch(handle);                                 // ONE place for all errors
```

A Promise represents a future value; `.then` chains steps and `.catch` handles any error in the chain — flatter than nested callbacks.

## 3. async/await — synchronous-looking (the modern default)

```js
async function combine() {
  try {
    const a = await readFile("a.txt");   // pause until resolved
    const b = await readFile("b.txt");
    await db.save(a + b);
  } catch (err) {
    handle(err);                          // normal try/catch works
  }
}
```

`async/await` is syntactic sugar over Promises — it reads top-to-bottom like sync code, uses ordinary `try/catch`, and is the recommended modern style.

## Don't serialize independent work

```js
// ❌ sequential — slow
const a = await getA(); const b = await getB();
// ✅ parallel — both at once
const [a, b] = await Promise.all([getA(), getB()]);
```

## The relationship

```text
Callbacks → Promises → async/await
All use the event loop. Promises wrap callbacks; async/await wraps Promises.
Modern code: async/await, with Promise.all to parallelize independent ops.
```

## Why it matters

Async handling is unavoidable in Node (all I/O is async).

Understanding the progression — error-first callbacks (still common in older APIs/libraries), Promises (chainable), and async/await (readable, the default) — lets you read any codebase and write clean async logic.

Knowing they're all the same event-loop mechanism underneath, and remembering to parallelize independent operations with `Promise.all`, is key to correct and performant async Node code.