This is where I get to our first of many quotes that I will leave you with. To demonstrate, let's check out a class version of our canonical “document title effect” and see the difference between how we used to write something like this side by side with an example using an npm-installed Hook that does the same thing. Let's add this side effect to our existing counter example and again look at the old way of doing this with Classes vs working with Hooks. This article was last updated in June of 2020.

If you look at this demo's main.jsx page which I have shown below, there are several target areas we can identify that will change when using a functional component and React Hooks. Join me as we explore the ins and outs of React Hooks!

useEffect(() => { That should do it!

This creates additional clutter in our JSX. We will look for instances where the demo is using this.state and this.setState because when we convert the component to functional we will not have to use the this keyword anymore, we will not need to use a constructor or call setState. const [calcNum, setCalcNum] = useState(0); const SomeComp = ({ someFunc }) => {

I wanted to add them one by one for purposes of this demo. You'll probably want to check out the CodeSandbox demo so you can see for yourself. Let's take a look at one more reducer example to help us understand them better. React Select demo with event hooks for custom integration and functionality. Just like the sum reducer, we store a value each time, but instead of adding up the values, we will store the highest value so far. Or would you rather just consume that data from any point within a component tree? // In this scenatio, someFunc will change on every render, so this useEffect will run. A sum function is the simplest example, but inside that reducer, you can do any work you want iteratively between the curly braces. So if I don't need useEffect, I can omit that from my component imports. With a better understanding of Hooks, let's take what we know to be a simple piece of code, our document title update, and create a simple custom Hook. This is where we remove a todo from the list using the array;s .filter() method and return the old state minus the deleted Todo with matching id. To prevent our function from being recreated and change pointers on each render round, we can use useCallback . Each iteration we will compare the next item in the array with the highest value so far, if it's larger it will replace it and if not, we simply continue without updating the highest value. } And this is a huge advancement for React developers. This action gets handled by a Reducer function. Others prefer to keep non-critical or UI state, such as “is this drop-down currently open”, inside a component's internal state. In our example below we have an expensive function called computeLetterCount (for demo purposes we make it slow by including a large and completely unnecessary loop). Let’s start with the first problem and see how we can prevent evaluating functions unnecessarily. Functional components are great. You will see that I left the old call to useEffect commented out, above it is how we use the custom Hook for this instead. The styling and HTML have already been taken care of, so we just need to make them work. If a function is created inside the component is passed to a child component, it will be recreated, meaning the pointer will change, causing the child to re-render or call the function unnecessarily (depending on the situation). Think about the information that might be displayed on a profile. We can use this for seeding some todo items to test that our list renders, and that the shape of our data conforms to our HTML. If you feel that you have sufficient knowledge of basic Hooks, you can skip directly to creating custom Hooks. I have forked the original StackBlitz demo from the Dialog Overview page, and that will be our starting point. It's a pretty simple demo that contains a button inside of a Counter component.

We can assume this profile component needs two components as its children: and .

So let's stop talking and start learning about React Hooks.

Hooks are a new addition in React that lets you use state and other React features without writing a class.