Here’s where immutability comes in: if you’re passing props into a PureComponent, you have to make sure that those props are updated in an immutable way. That means, if they’re objects or arrays, you’ve gotta replace the entire value with a new (modified) object or array. Just like with Bob – kill it off and replace it with a clone. If you modify the internals of an object or array – by changing a property, or pushing a new item, or even modifying an item inside an array – then the object or array is referentially equal to its old self, and a PureComponent will not notice that it has changed, and will not re-render. Weird rendering bugs will ensue.

An easy way to optimize a React component for performance is to make it a class, and make it extend React.PureComponent instead of React.Component. This way, the component will only re-render if its state is changed or if its props have changed. It will no longer mindlessly re-render every single time its parent re-renders; it will ONLY re-render if one of its props has changed since the last render.