flow type question mark before or after param?

Question

Can someone explain the difference between:

function foo(bar: ?string) {
  console.log(bar);
}

and:

function foo(bar?: stri         


        

Answer 1

?string (maybe type) means that bar property can be string aswell as null and void.

bar? means that this property is optional.

More info: https://flow.org/en/docs/types/primitives/