[React] Call setState with null to Avoid Triggering an Update in React 16

Sometimes it’s desired to decide within an updater function if an update to re-render should be triggered. Calling 

.setState

 with 

null

 no longer triggers an update in React 16. This means we can decided if the state gets updated within our 

.setState

 method itself!

selectCity = evt => {
    const newValue = evt.target.value;
    this.setState(state => {
      if (state.city === newValue) {
        return null;
      }
      return {
        city: newValue
      };
    });
  };