Difficult JS Interview Question

Tricky-JS-Chaining1.js

(function () {

   function Arithmetic(number) {
      this.value = number;
   };

   Arithmetic.val = function () {
      return this.value;
   };

   Arithmetic.add = function (number) {
      this.value += number;
   };

   Arithmetic.subtract = function (number) {
       console.log("this is this.value", this.value);
      this.value = this.value - number;
   };

   window.Arithmetic = function (number) {
      return new Arithmetic(number);
   };

    
})();


var workingNumber = Arithmetic(10).subtract(5).add(7).add(1).val(); // Should return 13
var otherWorkingNumber = Arithmetic(100).add(1).subtract(99).val(); // Should return 2