# 08. Async Iteration

Asynchronous iteration works with data that is delivered asynchronously. For example, lines of text, read asynchronously from a file or an HTTP connection.

The proposal specifies a new protocol for iteration that works asynchronously:

* Async iterables are marked via `Symbol.asyncIterator` if it's defined, otherwise it will fall back to `Symbol.iterator`.
* Method `next()` of an async iterator returns `Promises` for `IteratorResults`.
* asynchronous version of the `for-of` loop: `for-await-of`:

```javascript
async function f() {
  for await (const x of createAsyncIterable(["a", "b"])) {
    console.log(x);
  }
}
// Output:
// a
// b
```

It's pretty cool that `for-await` falls back to `Symbol.iterator`. It means you can use it with regular iterables like arrays:

```javascript
async function example() {
  const arrayOfFetchPromises = [fetch("1.txt"), fetch("2.txt"), fetch("3.txt")];

  // Regular iterator:
  for (const item of arrayOfFetchPromises) {
    console.log(item); // Logs a promise
  }

  // Async iterator:
  for await (const item of arrayOfFetchPromises) {
    console.log(item); // Logs a response
  }
}
```

`for-await` will give you each value once `asyncIterator.next()` resolves. Because this involves awaiting promises, other things can happen on the main thread during iteration. `asyncIterator.next()` isn't called for the next item until your current iteration is complete. This means you'll always get the items in order, and iterations of your loop won't overlap.