Private fields demo

01-public-fields.js

// thing that exists and we already using
class TreasureChest {
    contents = ['golden coin', 'cobweb'];
}

const chest = new TreasureChest();
console.log(chest.contents);

02-underscore-private-fields.js

// how to do "private fields" before
class TreasureChest {
    _contents = ['golden coin', 'cobweb'];

    numberOfThings() {
        return this._contents.length;
    }
}

/// it is not really private
console.log(chest._contents);

// "private" variables leak in case of inheritance
class VeryImportantTreasureChest extends TreasureChest {
    _contents = new Map(); // leads to more treasure;
}

const brokenChest = new VeryImportantTreasureChest();
console.log(brokenChest.numberOfThings());

03-actually-private-fields.js

// same propsal also has a notion of private fields
class TreasureChest {
    #contents = ['golden coin', 'cobweb'];

    numberOfThings() {
        return this.#contents.length;
    }
}

const chest = new TreasureChest();
console.log(chest.#contents); // SyntaxError
           
class VeryImportantTreasureChest extends TreasureChest {
    #contents = new Map();
}

const chest = new VeryImportantTreasureChest();
console.log(chest.numberOfThings()); // no name clashes

04-access-to-other-instances.js

// we still can access private fields of other instances
// of the same class
class TreasureChest {
    #contents = ['golden coin', 'cobweb'];

    hasSameItems(otherChest) {
        if (this.#contents.length !== otherChest.#contents.length) {
            return false;
        }

        return this.#contents.every(item => otherChest.#contents.includes(item));
    }
}

const chest1 = new TreasureChest();
const chest2 = new TreasureChest();

console.log(chest1.hasSameItems(chest2));