asynciterator
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An asynchronous iterator library for advanced object pipelines.
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# Asynchronous iterators for JavaScript
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**AsyncIterator is a lightweight JavaScript implementation of demand-driven object streams,**
and an alternative to the two-way flow controlled [Node.js `Stream`](https://nodejs.org/api/stream.html).
As opposed to `Stream`, you cannot _push_ anything into an `AsyncIterator`;
instead, an iterator _pulls_ things from another iterator.
This eliminates the need for expensive, complex flow control.
[Read the full API documentation.](http://rubenverborgh.github.io/AsyncIterator/docs/)
## Data streams that only generate what you need
`AsyncIterator` allows functions to
**return multiple _asynchronously_ and _lazily_ created values**.
This adds a missing piece to JavaScript,
which natively supports returning a single value synchronously
and asynchronously (through [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)),
but multiple values only synchronously (through [`Iterable`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols)):
<table>
<tr>
<td> </td>
<th>single value</th>
<th>multiple values</th>
</tr>
<tr>
<th>synchronous</th>
<td><code>T getValue()</code></td>
<td><code>Iterable<T> getValues()</code></td>
</tr>
<tr>
<th>asynchronous</th>
<td><code>Promise<T> getValue()</code></td>
<td><strong><code>AsyncIterator<T> getValues()</code></strong></td>
</tr>
</table>
Like `Iterable`, **an `AsyncIterator` only generates items when you ask it to**.
This contrast with patterns such as [`Observable`](http://reactivex.io/intro.html),
which are data-driven and don't wait for consumers to process items.
### The asynchronous iterator interface
An asynchronous iterator is an object that exposes a series of data items by:
- implementing [`EventEmitter`](https://nodejs.org/api/events.html#events_class_eventemitter)
- returning an item when you call `iterator.read` (yielding `null` when none is available at the moment)
- informing when new items might be available through `iterator.on('readable', callback)`
- informing when no more items will become available through `iterator.on('end', callback)`
- streaming all of its items when you register through `iterator.on('data', callback)`
Any object that conforms to the above conditions can be used with the AsyncIterator library
(this includes [Node.js Streams](https://nodejs.org/api/stream.html)).
The `AsyncIterator` interface additionally exposes
[several other methods and properties](http://rubenverborgh.github.io/AsyncIterator/docs/AsyncIterator.html).
## Example: fetching Wikipedia links related to natural numbers
In the example below, we create an iterator of links found on Wikipedia pages for natural numbers.
```JavaScript
import https from 'https';
import { resolve } from 'url';
import { IntegerIterator } from 'asynciterator';
// Iterate over the natural numbers
const numbers = new IntegerIterator({ start: 0, end: Infinity });
// Transform these numbers into Wikipedia URLs
const urls = numbers.map(n => `https://en.wikipedia.org/wiki/${n}`);
// Fetch each corresponding Wikipedia page
const pages = urls.transform((url, done, push) => {
https.get(url, response => {
let page = '';
response.on('data', data => { page += data; });
response.on('end', () => { push(page); done(); });
});
});
// Extract the links from each page
const links = pages.transform((page, done, push) => {
let search = /href="([^"]+)"/g, match;
while (match = search.exec(page))
push(resolve('https://en.wikipedia.org/', match[1]));
done();
});
```
We could display a link every 0.1 seconds:
```JavaScript
setInterval(() => {
const link = links.read();
if (link)
console.log(link);
}, 100);
```
Or we can get the first 30 links and display them:
```JavaScript
links.take(30).on('data', console.log);
```
In both cases, pages from Wikipedia will only be fetched when needed—the data consumer is in control.
This is what makes `AsyncIterator` [lazy](https://en.wikipedia.org/wiki/Lazy_evaluation).
If we had implemented this using the `Observable` pattern,
an entire flow of unnecessary pages would be fetched,
because it is controlled by the data publisher instead.
## Usage
`AsyncIterator` implements the [`EventEmitter`](https://nodejs.org/api/events.html#events_class_eventemitter) interface
and a superset of the [`Stream`](https://nodejs.org/api/stream.html) interface.
### Consuming an AsyncIterator in on-demand mode
By default, an AsyncIterator is in _on-demand_ mode,
meaning it only generates items when asked to.
The [`read` method](http://rubenverborgh.github.io/AsyncIterator/docs/AsyncIterator.html#read) returns the next item,
or `null` when no item is available.
```JavaScript
const numbers = new IntegerIterator({ start: 1, end: 2 });
console.log(numbers.read()); // 1
console.log(numbers.read()); // 2
console.log(numbers.read()); // null
```
If you receive `null`,
you should wait until the next [`readable` event](http://rubenverborgh.github.io/AsyncIterator/docs/AsyncIterator.html#.event:readable) before reading again.
This event is not a guarantee that an item _will_ be available.
```JavaScript
links.on('readable', () => {
let link;
while (link = links.read())
console.log(link);
});
```
The [`end` event](http://rubenverborgh.github.io/AsyncIterator/docs/AsyncIterator.html#.event:end) is emitted after you have read the last item from the iterator.
### Consuming an AsyncIterator in flow mode
An AsyncIterator can be switched to _flow_ mode by listening to the [`data` event](http://rubenverborgh.github.io/AsyncIterator/docs/AsyncIterator.html#.event:data).
In flow mode, iterators generate items as fast as possible.
```JavaScript
const numbers = new IntegerIterator({ start: 1, end: 100 });
numbers.on('data', number => console.log('number', number));
numbers.on('end', () => console.log('all done!'));
```
To switch back to on-demand mode, simply remove all `data` listeners.
### Setting and reading properties
An AsyncIterator can have custom properties assigned to it,
which are preserved when the iterator is cloned.
This is useful to pass around metadata about the iterator.
```JavaScript
const numbers = new IntegerIterator();
numbers.setProperty('rate', 1234);
console.log(numbers.getProperty('rate')); // 1234
const clone = numbers.clone();
console.log(clone.getProperty('rate')); // 1234
numbers.setProperty('rate', 4567);
console.log(clone.getProperty('rate')); // 4567
```
You can also attach a callback
that will be called as soon as the property is set:
```JavaScript
const numbers = new IntegerIterator();
numbers.getProperty('later', console.log);
numbers.setProperty('later', 'value');
// 'value'
```
### Consuming an AsyncIterator as EcmaScript-AsyncIterator
Due to the syntactical sugar [EcmaScript's AsyncIterator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator)
provides, our iterators can also be consumed as such.
If high performance over large iterators is required, this method of consumption not recommended.
```JavaScript
const numbers = new IntegerIterator({ start: 1, end: 100 });
for await (const number of numbers)
console.log('number', number);
console.log('all done!');
```
Error events emitted within the iterator can be caught by wrapping the for-await-block in a try-catch.
In cases where the returned EcmaScript AsyncIterator will not be fully consumed,
it is recommended to manually listen for error events on the main AsyncIterator
to avoid uncaught error messages.
## License
The asynciterator library is copyrighted by [Ruben Verborgh](http://ruben.verborgh.org/)
and released under the [MIT License](http://opensource.org/licenses/MIT).