in

const car = { make: "Honda", model: "Accord", year: 1998 };

console.log("make" in car);
// : true

delete car.make;
if ("make" in car === false) {
  car.make = "Suzuki";
}

console.log(car.make);
// : "Suzuki"

prop in object
#prop in object

class C {
  #x;
  static isC(obj) {
    try {
      obj.#x;
      return true;
    } catch {
      return false;
    }
  }
}

class C {
  #x;
  static isC(obj) {
    return #x in obj;
  }
}

class C {
  foo() {
    #x in this;
  }
}

new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class

// 
const trees = ["redwood", "bay", "cedar", "oak", "maple"];
0 in trees; // true 
3 in trees; // true 
6 in trees; // false 
"bay" in trees; // false  ()
"length" in trees; // true  (length  Array )
Symbol.iterator in trees; // true  ( ES2015 )

// 
"PI" in Math; // true 

// 
const myCar = { make: "Honda", model: "Accord", year: 1998 };
"make" in myCar; // true 
"model" in myCar; // true 

const color1 = new String("green");
"length" in color1; // true 

const color2 = "coral";
//  (color2  String )
"length" in color2;

const myCar = { make: "Honda", model: "Accord", year: 1998 };
delete myCar.make;
"make" in myCar; // false 

const trees = ["redwood", "bay", "cedar", "oak", "maple"];
delete trees[3];
3 in trees; // false 

const myCar = { make: "Honda", model: "Accord", year: 1998 };
myCar.make = undefined;
"make" in myCar; // true 

const trees = ["redwood", "bay", "cedar", "oak", "maple"];
trees[3] = undefined;
3 in trees; // true 

const empties = new Array(3);
empties[2]; // undefined 
2 in empties; // false 

const empties = new Array(3).fill(undefined);
2 in empties; // true 

const ages = { alice: 18, bob: 27 };

function hasPerson(name) {
  return name in ages;
}

hasPerson("hasOwnProperty"); // true

const ages = { alice: 18, bob: 27 };

function hasPerson(name) {
  return Object.hasOwn(ages, name);
}

hasPerson("hasOwnProperty"); // false

const ages = new Map([
  ["alice", 18],
  ["bob", 27],
]);

function hasPerson(name) {
  return ages.has(name);
}

hasPerson("hasOwnProperty"); // false

class Person {
  #age;
  constructor(age) {
    this.#age = age;
  }
  static isPerson(o) {
    return #age in o;
  }
  ageDifference(other) {
    return this.#age - other.#age;
  }
}

const p1 = new Person(20);
const p2 = new Person(30);
console.log(p1.ageDifference(p2)); // -10
console.log(Person.isPerson(p1)); // true

if (Person.isPerson(p1) && Person.isPerson(p2)) {
  console.log(p1.ageDifference(p2)); // -10
}

const p2 = {};

p1.ageDifference(p2); // TypeError: Cannot read private member #age from an object whose class did not declare it

class Person {
  #age;
  constructor(age) {
    this.#age = age;
  }
  static [Symbol.hasInstance](o) {
    // Testing `this` to prevent false-positives when
    // calling `instanceof SubclassOfPerson`
    return this === Person && #age in o;
  }
  ageDifference(other) {
    return this.#age - other.#age;
  }
}

const p1 = new Person(20);
const p2 = new Person(30);

if (p1 instanceof Person && p2 instanceof Person) {
  console.log(p1.ageDifference(p2)); // -10
}