TypeScript has property decorators that you can apply to your class’s data properties like so:

class MyClass {
  @autocastValuesTo(MyPropType)
  private myProp: MyPropType;
}

However, these decorators do not work exactly as you might expect. To quote the TypeScript handbook:

… there is currently no mechanism to describe an instance property when defining members of a prototype, and no way to observe or modify the initializer for a property.

What this means is that your decorator will be applied the moment the class is loaded, and your decorator will only have access to the class itself and the name of the property. By the time any instance of your class is created, you’ll have no connection to that instance from the decorator.

So what do you do if you want to change the behaviour of a property so that every instance of the class is affected? I spent a few hours one afternoon hacking at this problem.

The first solution I came up with was to alter the class prototype, so that each instance inherits the altered behaviour. This will work fine for some cases. The problem is that, since the property is now defined on the prototype and inherited, it is no longer an own property of any instance. That means it won’t be iterated over in for ... of loops or listed with e.g. Object.keys() or Object.values(). For private properties this is not a big problem, but if you’re decorating a public property of a data representation, then it is a big drawback.

Here is a more involved solution that will ensure the altered behaviour is installed as an own property of the instance itself: put a property on the prototype with a getter and setter that will install a property of the same name but with the right behaviour on the called instance automatically the first time it is used. In other words: the prototype property overwrites itself during the first call.

Here is a gist that shows how that would work. The assumption here is that you want to apply some kind of mapping to any value assigned to the decorated property. It installs a setter on instances that maps values and stores them in a map keyed to the called instance, and a getter that reads from that map. No getter is defined for the prototype, so the instance property remains unknown if it is read before it is initialized.

P.S.: Ironically I came upon this answer on StackOverflow only a short while after I spent hours on this problem. Seems I wasn’t the first person to come up with this solution.