Edit: I misinterpreted the question at first. I guess in TS you declare a class' properties explicitly while in javascript it is usually done in the constructor. The feature must be related to these dynamic properties. Original answer below.
This[0] page says this problem can be also seen in Typescript. But TS has the arrow function as a class method feature that in Javascript is only at stage 2 in it's way to standardization, so it's easier to solve there.
