React is an interesting mix of functional and OO. It's OO, in that the primary approach for defining components is class-based, and components have state and lifecycles. It's functional, in that the render methods are expected to be pure functions based on component state and props, and simply output a description of what the UI should look like as a result. Also, as a whole, React definitely pushes you to view the system in terms of composition, state transformations, and pure functions, rather than imperative "toggle this, add that, update the other thing".