# Extra operators and factories

The operators and factories listed above are the core functions. `xstream` has plenty of extra operators, [documented here](./EXTRA_DOCS.md).

# FAQ

**Q: Why does `imitate()` support a Stream but not a MemoryStream?**

A: MemoryStreams are meant for representing "values over time" (your age), while Streams represent simply events (your birthdays). MemoryStreams are usually initialized with a value, and `imitate()` is meant for creating circular dependencies of streams. If we would attempt to imitate a MemoryStream in a circular dependency, we would either get a race condition (where the symptom would be "nothing happens") or an infinite cyclic emission of values.

If you find yourself wanting to use `imitate()` with a MemoryStream, you should rework your code around `imitate()` to use a Stream instead. Look for the stream in the circular dependency that represents an event stream, and that would be a candidate for creating a MimicStream which then imitates the real event stream.

**Q: What's the difference between xstream and RxJS?**

A: Read this [blog post](http://staltz.com/why-we-built-xstream.html) on the topic.

**Q: What is the equivalent of [`withLatestFrom`](http://reactivex.io/rxjs/class/es6/Observable.js~Observable.html#instance-method-withLatestFrom) in xstream?**

<!-- skip-example -->
```js
A.withLatestFrom(B, (a, b) => a + b)
```

can be achieved in *xstream* with

```js
B.map(b => A.map(a => a + b)).flatten()
```

And can be interpreted/read as "when a `B` event happens, remember it and map it to all the subsequent events of `A` mapped to `a + b`".

-----

# Misc.

## Acknowledgements

*xstream* is built by [staltz](https://github.com/staltz) and [TylorS](https://github.com/tylors).

## CHANGELOG

Read the [CHANGELOG](https://github.com/staltz/xstream/blob/master/CHANGELOG.md) for release notes of all versions of *xstream*.

## License

[MIT](https://github.com/staltz/xstream/blob/master/LICENSE)