text: var calculator = calculator || {};
  
  (function(addition) {
      let value = 0;
  
      function add(x) {
          value += x;
          return this;
      }
  
      function substract(x) {
          value -= x;
          return this;
      }
  
      addition.add = add;
      addition.substract = substract;
  }(calculator));