Determine the type used in parametric type


How can one programmatically determine the type that was used for a parametric type?

For example, if I have a type:

type MyMixableType<G, H> {
  myMethod: function(): G js server

and then a type which mixes in that type:

type MySpecificType mixes MyMixableType<TypeOne, TypeTwo>

If I wanted to implement myMethod() in MyMixableType.js by accessing whatever type was used for H, how could I go about this?

// MyMixableType.js
function myMethod() {
   // something here to effectively call...
   // var result = TypeTwo.fetch();

I thought I saw that post before but couldn’t find it - many thanks!