In Angular6, i am getting a variable value to client from Parent using @Input()

```
@Input() orderdata: OrderData;

```

and binded to a field using [ngModel] and (ngModelChange) function.

In the child component when user changes this variable value (binded) I need to do a validation comparing the new value and old value. How to do this?

I tried assigning orderdata to a new variable 

```
oldorderdata = orderdata; 

```

and then later perform the comparison. But when ever user changes the orderdata at the moment itself oldorderdata is also changed.