React Function Components with hooks vs Class Components
Officially it sounds like hooks will completely replace classes?? maybe one day, but think about it; hooks have been around for 3 years (as of Mar 2021), and there are pros and cons in adopting them (More pros than cons... don't get me wrong)
I have plenty more experience myself with state management/classes and after doing a lot of research and testing, I found out that we need to know both classes and hooks very well. Hooks require a fraction of the code for simple components and seem excellent for optimizing HOCs. Meanwhile classes seem better with routing, container components and asynchronous programming for example.
I'm sure there are plenty more cases where each technology is better, but my point is that programmers need know both hooks and classes very well specially when working on projects with 100,000+ lines of code and millions of users. Read more here: https://stackoverflow.com/a/60134353/11239755
The idea behind introducing Hooks and other features like React.memo and React.lazy is to help reduce the code that one has to write and also aggregate similar actions together.
The docs mention few really good reason to make use of Hooks instead of classes
It’s hard to reuse stateful logic between components Generally when you use HOC or renderProps you have to restructure your App with multiple hierarchies when you try to see it in DevTools, Hooks avoid such scenarios and help in clearer code
Complex components become hard to understand Often with classes Mutually unrelated code often ends up together or related code tends to be split apart, it becomes more and more difficult to maintain. An example of such a case is event listeners, where you add listeners in componentDidMount and remove them in componentWillUnmount . Hooks let you combine these two
Classes confuse both people and machines With classes you need to understand binding and the context in which functions are called, which often becomes confusion.
function components with hooks can't help in perf as class components does. They can't skip re-renders as they don't have shouldComponentUpdate implemented.
Function component can be memoized in a similar way as React.PureComponent with Classes by making use of React.memo and you can pass in a comparator function as the second argument to React.memo that lets you implement a custom comparator
The idea is to be able write the code that you can write using React class component using function component with the help of Hooks and other utilities. Hooks can cover all use cases for classes while providing more flexibility in extracting, testing, and reusing code.
Since hooks is not yet fully shipped, its advised to not use hooks for critical components and start with relatively small component, and yes you can completely replace classes with function components
However one reason that you should still go for Class components over the function components with hooks until Suspense is out for data fetching. Data fetching with useEffect hooks isn't as intuitive as it is with lifecycle methods.
Also @DanAbramov in one of his tweets mentioned that hooks are designed to work with Suspense and until suspense is out it's better to use Class
Hooks greatly reduce the amount of code you need to write and increase its readability.
It is worth noting though that there are hidden processes going on behind (Just like component did mount etc.) that mean if you don't understand what is going on it can be difficult to troubleshoot. It is best to experiment with them and read through the docs fully before implementing on a live project.
Also there is still limited support/documentation for testing hooks compared to classes. https://dev.to/theactualgivens/testing-react-hook-state-changes-2oga
Update 28/08/2020 Use the react hooks testing library with custom hooks for testing https://github.com/testing-library/react-hooks-testing-library