What does this question mark mean in Flow: “?() => void”

Question

In a GitHub project I recently saw this function declaration:

function configureStore(onComplete: ?() => void) {

What this question mark

Answer 1

Almost.

() => void is Flow's annotation for a function that returns nothing (undefined, aka void 0).

The leading question mark in ?MyType is Flow's way of expressing a nullable type.

So in this case configureStore accepts one argument called onComplete that must be either null or a function that returns nothing.

Flow will not add a default value for onComplete or coerce it in any way because unlike typescript, Flow does not generate any new JS code. At runtime, all Flow annotations are stripped to get vanilla JS, and that's that.