JAVASCRIPT

Private slots are new and can be created via Instance private fields

// ES2022
class InstPrivateClass {
  #privateField1 = 'private field 1'; // (A)
  #privateField2; // (B) required!
  constructor(value) {
    this.#privateField2 = value; // (C)
  }
  /**
   * Private fields are not accessible outside the class body.
   */
  checkPrivateValues() {
  console.log(this.#privateField1); // output -> 'private field 1'
  console.log(this.#privateField2); // output -> 'constructor argument'

  }
}

const inst = new InstPrivateClass('constructor argument');
  inst.checkPrivateValues();


console.log("inst", Object.keys(inst).length === 0) //output -> inst, true

Private slots are new and can be created via Private slot checks

class C1 {
  #priv() {}
  static check(obj) {
    return #priv in obj;
  }
}

console.log(C1.check(new C1())) // output true