"TS6133: xxx is declared but its value is never read." behaviour

[WesleyYue]

TypeScript Version: 3.8.3

Search Terms:
"is declared but its value is never read."
Code

interface A {
  one: string;
  two: string;
}

class B {
  private _aa: A;
  constructor() {
    this._aa = { one: "one", two: "two" };
    on(Callback);

    function on(cb: ()=>void) {
      cb();
    }
    function Callback() {
      console.log(this._aa.one);
    }
  }
}

new B();

Expected behavior:
I would expect to not get the error below since the _aa variable is used implicitly by reading _aa.one
Actual behavior:
When compiling, I get the following error: TS6133: '_aa' is declared but its value is never read.

Related Issues:
#19700 which leads to https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#write-only-references-are-unused but this is not the same issue. The variable in question should be read via reading one of its subfields.

[jcalz]

Could you include a Playground link that reproduces this, as requested by the bug report template? If I drop that code into the Playground like this I get errors but not the one you're referring to.

[WesleyYue]

I'm new to typescript/javascript so I actually need a bit of help putting this example together that is stripped from the actual code I'm working with. I stripped the example code furthermore and I get a different error from my actual code. Could my issue be that the this keyword is not captured in the context of Callback()? If so, is there a way to make it so that it is captured? I realize that one solution could be to change Callback() to a member function of the class B, but just for educational purposes is it possible for Callback() to capture this from the constructor scope as is?

[WesleyYue]

Figured it out: the callbacks were indeed capturing an incorrect this context which I solved by using arrow functions type callbacks instead of what I had.