Today I Learned

TIL, 2021-09-23

Getting this TIL to work on new machines

  • Updated TIL Ruby version to 3.0.1, might as well.
  • Add pry-byebug to Gemfile, as previously we were loading the system gem.
  • FFI error: Reference
  • Ultisnips Error - this is cause got no Python. Just brew install python3 and brew install vim and link your Homebrew Vim instead of your system vim.
  • If Snippets are missing Reference, then check ~/.vim/bundle/vim-snippets/UltiSnips/ directory.
To enable snippet on Markdown:
snippet [r

Understanding RxJS Observables and why you need them


  • Reactive extension to JS with better performance, modularity, better debuggable call stacks while staying mostly backwards compatible.
  • Stream: A sequence of data values over time. Observables = a function that can return a stream of values to an observer over time.
  • Observers and subscriptions.
    • Observables are data source wrappers, then the observer executes some instructions when there is a new value or a change in data values.
    • Creating an observable: Observable.create((observer: any) => {'Hello World') }.
    • You have to subscribe to it, too.
  • Error value: With an error, the observer sends a JS exception. If an error is found in the Observable, nothing else can be delivered to the Observable.
  • Complete - nothing else can be delivered to the observable.
  • To destroy an Observable is to essentially remove it from the DOM by unsubscribing to it.
const haha = Observable.create((observer: any) => {"Hello World");"Hello World 2");
  setInterval(() => {                          <= This would be a normal `setInterval`.'Random Async log message'); <= If the observable is complete already, then this would not execute
  }, 2000);

  console.log('yolo');"Hello World 3");"Hello World 4");
  • Observables:
    • Emitting multiple values asynchronously is very easily handled with Observables.
    • Error handlers can also easily be done inside Observables.
    • Observables are considered lazy, so in case of no subscription, there will be no emission of data values.
    • Observables can be resolved multiple times.
Other method signature

  next(x: any):void { console.log('value', x); },
  error(err: any):void { console.error('error') },
  complete():void { console.log('done') }

RxJS Documentation


  • Function - Lazily evaluated computation that synchronously returns a single value on invocation.
  • Generator - a lazily evaluated computation that synchronously returns zero to potentially infinite values on iteration.
  • Promise - a computation that may eventually return a single value.
  • Observable - a lazily evaluated computation that can synchronously or asynchronously return zero to potentially infinite values from the time it’s invoked onwards.


function foo() {
  return 42;

const x =; // same as foo()
const y =; // same as foo()

Is similar to this:

import { Observable } from 'rxjs';

const foo = new Observable(subscriber => {

foo.subscribe(x => {
foo.subscribe(y => {
  • Both functions and Observables are lazy computations - if you don’t call the function, console.log wouldn’t happen.
  • Calling or subscribing trigger two separate side effects. As opposed to EventEmitters which share the side effects and have eager execution regardless of the existence of subscribers.
  • Observables, if no timeout/setInterval, are like functions, and are actually synchronous. However, it can return values asynchronously (unlike functions). means “give me one value synchronously”, observable.subscribe means “give me any amount of values, either sync or async”.
  • Creating observables:
    • Has only one argument, the subscribe function.
    • new Observable. But more commonly, of, from, interval.
    • Being subscribed to - note that the Observable does not even have a list of attached Observers.
  • Subscriber:
    • Is a subscription.
    • When you subscribe, you get back a Subscription, which represents the ongoing execution. Just call unsubscribe() to cancel the execution.

const yolo2 = haha.subscribe((x) => {

console.log(yolo2); Subscriber closed: false
console.log(yolo2); Subscriber closed: true
  • Just a way to know that the subscription closed. BTW when a subscription is closed, then new things emitted by next would not be received.
setTimeout(() => {
  yolo2.unsubscribe(); <= If the observable that yolo2 is subscribed to emits something, yolo2 doesn't care since it's closed now.
}, 3000);
  • Safe unsubscribe:
    • WTF was this guy saying lol.

This project is maintained by daryllxd