In componentWillUnmount, set a var componentUnmounted true. In componentDidMount in the callback for the fetch, check for componentUnmounted before setting state for an error or whatever you want to do.

Set a variable... set a property in the component state? Or set a static variable on the class?

I use a static var on the class.

  componentDidMount() {
   if (!this.unmounted) ...
  }

  componentWillUnmount() {
   this.unmounted = true
  }

That's not a static car on the class. It's on the instance! A static class property would entail something like `Foo.unmounted = true` (a bad idea).