Object.prototype.valueOf()

function MyNumberType(n) {
  this.number = n;
}

MyNumberType.prototype.valueOf = function () {
  return this.number;
};

const object = new MyNumberType(4);

console.log(object + 3);
// : 7

valueOf()

const obj = { foo: 1 };
console.log(obj.valueOf() === obj); // true

console.log(Object.prototype.valueOf.call("primitive"));
// [String: 'primitive'] (a wrapper object)

class Box {
  #value;
  constructor(value) {
    this.#value = value;
  }
  valueOf() {
    return this.#value;
  }
}

const box = new Box(123);
console.log(box + 456); // 579
console.log(box == 123); // true

box.valueOf();

+new Date(); //  new Date().getTime() 
+{}; // NaN toString()  "[object Object]" 
+[]; // 0 toString() 
+[1]; // 1 toString()  "1" 
+[1, 2]; // NaN toString()  "1,2" 
+new Set([1]); // NaN toString()  "[object Set]" 
+{ valueOf: () => 42 }; // 42