You can just start writing JS on an existing page to output components rendered by React.<p>Fair warning though, React is focused on rendering and initializing, so it doesn&#x27;t come with a bunch of handy JS functions like YUI or jQuery. But you can use React &amp; other JS libs together, as long as you&#x27;re careful that the other JS lib doesn&#x27;t change the markup of a React component (which would take it out of sync.)