flow-is-amazing.js

export function prop<O: { +[string]: mixed }, P: $Keys<O>>(
  p: P
): (o: O) => $ElementType<O, P> {
  return function(o) {
    return o[p];
  };
}


const f = prop('hi');

const a = {
  hi: 'hi'
};

const b = {
  hi: 2
};


(f(a): string); // this is happy, as a.hi is in fact a string
(f(b): string); // flow catches this, because b.hi is a number