FlowJS hackery to filter down arrays to specific types.

flowjs-boolean-filter.js

type A = {
  a: number 
};

const myA: A = { a: 1 };

const ac = [undefined, myA];

const ba: $ReadOnlyArray<A> = ac.filter(Boolean);

const c = (justAs: $ReadOnlyArray<A>) => console.log(justAs);

c(ba);