JavaScript recently added private class fields, named with a hash prefix.
(https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields)
I'm looking for a clear answer on whether this form of privacy is 'per class' or 'per instance.'
Say a Person class is defined with a private field named '#secret'.
I use 'new' to construct two instances of Person class: person1 and person2.
Does person1 have access to inspect and modify #secret field of person2?
class Person {
#secret;
constructor(secret) {
this.#secret = secret;
}
getSomebodyElsesSecret(somebody) {
return somebody.#secret;
}
setSomebodyElsesSecret(somebody, value) {
somebody.#secret = value;
}
}
They're per class - an instance can view and modify another instance.
class Person {
#secret;
constructor(secret) {
this.#secret = secret;
}
getSomebodyElsesSecret(somebody) {
return somebody.#secret;
}
setSomebodyElsesSecret(somebody, value) {
somebody.#secret = value;
}
}
const a = new Person('a');
const b = new Person('b');
console.log(a.getSomebodyElsesSecret(b))
a.setSomebodyElsesSecret(b, 'c');
console.log(a.getSomebodyElsesSecret(b))One way to look at it is that private fields are similar to variables scoped only to the class - they can be referenced however you want by anything inside the class's {}, and can't be referenced outside.
In terms of scope, it's a bit like a WeakMap that defines the private variables and an IIFE that returns the class:
const Person = (() => {
const secrets = new WeakMap();
return class Person {
constructor(secret) {
secrets.set(this, secret);
}
getSomebodyElsesSecret(somebody) {
return secrets.get(somebody);
}
setSomebodyElsesSecret(somebody, value) {
secrets.set(somebody, value);
}
}
})();
const a = new Person('a');
const b = new Person('b');
console.log(a.getSomebodyElsesSecret(b))
a.setSomebodyElsesSecret(b, 'c');
console.log(a.getSomebodyElsesSecret(b))No. If it was public, you can access it as person2.secret but it is private now which means you cannot directly access it outside the class.