Much Fun with RxJS-Autorun

Hi! 👋

Today I'll show you some tricks using rxjs and rxjs-autorun that we reviewed in the previous post. Let's quickly recap what it does:

you pass it an expression that uses some streams
whenever those streams emit, autorun runs the expression with new values
you get a stream of those evaluations results

Now let's go fast explore what we can do with it!

Mapping a single stream: 🌱 🌿 🌳

The most basic usage: we have a stream t of growing integer values: 0, 1, 2, etc (RxJS timer will create us one). And we want to substitute that integer with a string: "even" or "odd":

const t = timer(0, 1_000);
computed(() => $(t) % 2 ? 'odd' : 'even');
// Result:
// > even > odd > even > odd …

Explanation: $(t) will wait until stream t emits a value. Then with every value emission, computed will re-evaluate its expression, putting the result into the output stream.

Combinations: 📦 + 🎀 = 🎁

By tracking multiple streams inside expressions we can combine emissions from these streams:

import { timer, of } from 'rxjs';
import { computed, $ } from 'rxjs-autorun';

const a = timer(0, 1_000);
const b = of('🐇');
computed(() => $(a) + ' - ' + $(b))
// > 0 - 🐇 > 1 - 🐇 > 1 - 🐇 …

Similarly to combining into strings, we can combine into Arrays:

computed(() => [ $(a), $(b) ])
// > [ 0, 🐇 ] > [ 1, 🐇 ] …

Objects:

computed(() => ({ a: $(a), b: $(b) }))
// > { a: 0, b: 🐇 } > { a: 1, b: 🐇 } …

or even call functions!

function toString(count, animal) {
  return (count < 3 ? 'some' : 'many') + animal;
}
computed(() => toString( $(a), $(b) ))
// > some 🐇 > some 🐇 … > many 🐇 …

Switching: 🌤 🌧

Now, imagine we have a function getWeather that returns an Observable with the weather forecast for a given city. Using the same ?: from the first example we could create a widget to show weather for two cities:

const t = timer(0, 1_000);
const nice = getWeather('Nice'); // => Observable<'Nice: 🌤'>
const cannes = getWeather('Cannes'); // => Observable<'Cannes: 🌧'>
computed(() => $(t) % 2 ? $(nice) : $(cannes));
// > Cannes: 🌧 > Nice: 🌤 > Cannes: 🌧 > Nice: 🌤 …

Tricks: 🐇 🎩

Now, remember I said that $(…) will wait for the stream to emit its first value before continuing evaluation. Let's do some tricks exploiting this rxjs-autorun feature!

Filtering

computed will wait for every observable to emit in order to produce a value. But what if we pass it a never emitting Observable: NEVER? Correct, this will pause emission until other tracked Observable emits:

const a = timer(0, 1_000);
computed(() => $(a) % 2 ? $(NEVER) : $(a) + ' even' );
// > 0 even > 2 even > 4 even …

Take while

Similarly to NEVER, we can exploit awaiting by passing it an Observable that completes never ever emitting a value: EMPTY. If an expression relies on a stream that we know won't emit a value — then surely the resulting Observable won't emit either! So computed completes:

const a = timer(0, 1_000);
const c = computed(() => $(a) < 5 ? $(a) : $(EMPTY));
// > 0 > 1 > 2 > 3 > 4 > COMPLETE!

NOTE: this feature is in beta-testing now and will be available with the next release

Bonus: 🙂

I showed you two functions: computed to run expressions and $ to track values. But $ has a sibling! A _ function, similarly to $, will read the current value of a stream but it won't trigger expression re-evaluation if that stream updates!

Now, imagine the examples above if you mix $ and _. Got you thinking, eh? Then imagine that all the computed streams can be used in other computed expressions!

🙂 → 😲 → 🤯

Outro: 🛸💨

You'll find the docs and the library here: github.com/kosich/rxjs-autorun

I hope you had fun! If you enjoyed reading — please, indicate that with ❤️ 🦄 📘 buttons — it helps a lot!

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Thank you for reading this article! Stay reactive and have a nice day 🙂

Cya! 👋

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