Predictable, Reliable, Testable Declarative: what the UI should look like, given props React is more than that, however. The real innovation with React is its programming paradigm. The Virtual DOM is merely an implementation detail that helps to provide us with that paradigm. The really great thing about React is that it’s based on functional-reactive principles. ! We can think of each function in React as being like a pure function in functional programming. And just like pure functions, we can compose them together, with the data output of each function leading into the next, in a unidirectional data flow. ! And like pure functions, React components are very predictable, reliable and testable. ! We can declare what our UI should look like, given any props.