Observer pattern

Observer.js

const makeObservable = (fn) => {
    const observers = new Map();
    const register = (observer) => observers.set(observer, observer);
    const deRegister = (observer) => observers.delete(observer)
    const notify = (value) => observers.forEach(observer => observer && observer(value));
    
    fn(notify);
    
    return { register, deRegister };
}

/*
const messageObservable = makeObservable((notify) => {
    setInterval(() => notify(Math.floor(Math.random() * 100)), 1000)
});

messageObservable.register((value) => console.log("from first", value))

messageObservable.register((value) => console.log("from first", value))

const fn1 = (s) => console.log("hello", s);

messageObservable.register(fn1)

*/

“The observer pattern is a software design pattern in which an object, called the subject(observable), maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.” — Wikipedia definition