Correct way to hide components in React.js

I do not think there will be any definite answer for this question.Each approach has its benefits and drawbacks.

With CSS you have:

• it might work faster
• no need to think about restoring child control states if control is shown again.

With returning null:

• the total rendered DOM might be considerably smaller. This is important if you have many such components that might be hidden.
• there will be no collisions in rendered html. Lets say you have tabular view. Each tab is its own complex form with many child controls. If you have some raw javascript/jquery/whatever working with their ids/classnames etc. - its quite hard to ensure each tab/form has unique ids, unless you do not render them.

From my point of view the decision will be based upon the structure of your control. If it have complex structure with many nested children and you do not have any means of restoring their states when switched on again - go with CSS, but I would say this is a short term solution for quite simple controls only. In all other cases I would go with not rendering a component.

If you think you would need to display the component again, during that page life, then I would recommend using css way, as the impact on DOM manipulation would be less in that case. In some other cases probably returning null would be more helpful.

For the most part, your two solutions are interchangeable. They both "work" fine.

I would warn against preemptive optimization in choosing which of these methods to choose. If you do need to eventually modify your code and try the other method, this is an absurdly simple swap to make and shouldn't be an obstacle. So don't worry about it until there's a reason to worry about it.