Main logic

Now it's working.
You don't need this `change` function I think. What you can is, always using your object properties (`this.value`, `this.operator`, `this.result`) and having a method `runOperation` like

```javascript
Calculator.property.runOperation = function () {
  switch(this.operator) {
    case '+':
      this.result = add(this.value, this.result);
      break;
    // ....
  }
}
```

By having such a function, you just need to call `self.runOperation` and your properties will be automatically updated.