privatizer.js

privatizer.js

/*
given goals:
https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#why-is-encapsulation-a-goal-of-this-proposal
& given example:
https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#how-can-you-provide-hidden-but-not-encapsulated-properties-using-symbols

Hidden & encapsulated values can be shared with same instances via WeakMap & Proxies
Undesirable access via callbacks passed in & whatnot just need to be defended against.
One could imagine wanting to allow callbacks read access to privates but not write, doable w/a proxy & setup condition.

*/

function privatizer(){
    const store = new WeakMap
	return {
		privates:{
            get:p=>store.get(p),
            set(target,nu){
				Object.keys(nu).forEach(k=>{
					if(nu[k].constructor===Function.constructor && nu[k].bind){
						nu[k]=nu[k].bind({})
					}
				})
				return store.set(target,nu)
			}
		}
	}
}

const Person = (function() {
    let ids = 0;
	const {privates} = privatizer()
    return class Person {
        constructor(name,greeting=(other)=>`howdy ${other}`) {
            this.name = name;
            privates.set(this,{id: ids++,greeting})
        }
		sayMyId(){ console.log(`${this.name}'s id is: ${privates.get(this).id}`) }
        equals(otherPerson) {
            return privates.get(this).id === privates.get(otherPerson).id
        }
		greet(other){
			console.log(`${this.name}: ${privates.get(this).greeting(other.name)}`)
		}
    }
})();
let alice = new Person('Alice',function(name){this.id=9;return `hark to ${name}!!`});//oh no, attempts to change privates via this!
let bob = new Person('Bob',function(name){this.id=9;return `salutations to ${name}!`});
let Imposter=Person.bind(alice)//?
let dillon = new Imposter('Dillon')

alice.sayMyId()    //Alice's id is: 0
bob.sayMyId()      //Bob's id is: 1
dillon.sayMyId()   //Dillon's id is: 2

alice.greet(bob)   //Alice: hark to Bob!!
bob.greet(alice)   //Bob: salutations to Alice from Bob!
bob.greet(bob)     //Bob: salutations to Bob from Bob!
dillon.greet(alice)//Dillon: howdy Alice

alice.sayMyId()    //Alice's id is: 0
bob.sayMyId()      //Bob's id is: 1
dillon.sayMyId()   //Dillon's id is: 2