Observable classes practical examples -- case of study

Observable.js

class Observable {
    constructor(observer){
      this._observer = observer;
    }

    subscribe(observer) {
      return this._observer(observer)
    }
}

Observer.js

class Observer {
  #next;
  #error;
  #complete;

  constructor ({
    next = () => {},
    error = () => {},
    complete = () => {}
  } = {
    next ()  {},
    error ()  {},
    complete () {}
  }) {
    this.#next = next
    this.#error = error
    this.#complete = complete
  }
  
  error (err = new Error('')) {
    // Default behavior on error
    if (err === null) {
      return this.#error(new Error())
    }
    
    if (typeof err === 'string' || typeof err === 'number') {
      return this.#error(new Error(err))
    }
    
    if (err.constructor.name === 'Object' || Array.isArray(err)) {
      return this.#error(new Error('', { cause: err }))
    }

    return this.#error(err)
  }
  
  complete () {
    // Default behavior on completed
    return this.#complete()
  }
  
  next (data) {
    // Default behavior on success
    return this.#next(data)
  }
}

test.js

const myObservable = new Observable(observer => {
  setTimeout(() => {
    observer.next("got data!")
    observer.complete()
  }, 1000)
})

const myObserver = new Observer({
  error (err) {
    // Overwrited error behavior
    console.log(err)
    throw err
  }
})

myObservable.subscribe(myObserver)