Flow : destructuring. Missing annotation in React/Preact

Question

Just getting started with Flow but can't seem to understand what it wants me to do with adding types for destructuring objects like i.e. props

i.e.

render({ count }, { displayHelp }) {

Throws an error like

 16:   render({ count }, { displayHelp }) {
              ^^^^^^^^^ destructuring. Missing annotation

and when I add annotation it still complains

 17:   render({ count: number }, { displayHelp }) {
              ^^^^^^^^^^^^^^^^^ destructuring. Missing annotation

I am obviously missing something very simple here if anybody could point out ?

Answer 1

The problem with doing { count: number } is that this clashes with ES6 syntax for destructuring assignment, where you can use { a: b } = c in order to take the value with key a from c and have it be named b, ie:

const c = { a: 1 }
const { a: b } = c
//b is now a constant with value 1

There isn't really a good workaround for this in Flow right now, but this seems to work (although it is ugly):

render({...}: { count: number }, { displayHelp }) {

The best way right now seems to be to create a custom type which captures your props:

type propsForThisComponent = {
  propA: someType
  propB: anotherType
  ...
}

and then do:

render(props: propsForThisComponent) {

This typechecks, although it forces you to access all your props as props.propName.