Why Flow cannot call ReactDOM.render with document.getElementById(…)

Question

I was getting this error below in Flow type checking.

Cannot call ReactDOM.render with document.getElementById(...) bound to container because null [1] is
incomp         


        

Answer 1

Aleksey L. got this first in the comments, I wanted to bring this info up to the answer level for easier visual scanning.

Flow is letting you know that the call document.getElementById("root"); can return null in which case the app would completely crash. So let's guard against that:

const root = document.getElementById('root')

if (root !== null) {
  ReactDOM.render( , root)
}

Granted, this can feel a little annoying given that in all likelihood you will be controlling the HTML you are rendering into.