setState.is.Asynchronous.js

setState is Asynchronous

I didn’t notice this when I started out with React. The setState function is asynchronous!

If you call setState and immediately inspect this.state, chances are it will not be updated yet.

If you need to set the state and immediately act on that change, you can pass in a callback function like this:

this.setState({name: 'Joe'}, function() {
	// called after state has been updated
	// and the component has been re-rendered
});

Another alternative is to use the componentWillUpdate or componentDidUpdate lifecycle hooks, which will be called 
immediately before and after rendering in response to your state change. 
They’ll also get called whenever props change – so if you absolutely want to respond only to state changes, 
use the callback approach.

from:

https://daveceddia.com/react-gotchas/?utm_campaign=0630gotchas