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Hooks are an upcoming feature that lets you use state and other React features without writing a class component - functions FTW.
Hooks is the latest pattern and a new feature that's supposedly better than sliced bread or ketchup, you decide ;). Everyone used to go nuts over Render props but now it's all hooks.
Hooks are now available in React 16.8 so you might need to upgrade your React project
Problems Hooks are trying to address
Every time something new comes out we get excited. It's ketchup, it's the best thing since sliced bread and so on. We hope that this will finally be the solution to all our problems, so we use it, again and again, and again. We've all been guilty of doing this at one time or another, abusing a pattern or paradigm and yes there has always been some truth to it that the used pattern has been limited.
Below I will try to lay out all the different pain points that make us see Hooks as this new great thing. A word of caution though, even Hooks will have drawbacks, so use it where it makes sense. But now back to some bashing and raving how the way we used to build React apps were horrible;)
There are many problems Hooks are trying to address and solve. Here is a list of offenders:
- wrapper hell, we all know the so-called wrapper hell. Components are surrounded by layers of providers, consumers, higher-order components, render props, and other abstractions, exhausted yet? ;)
Like the whole wrapping itself wasn't bad enough we need to restructure our components which is tedious, but most of all we lose track over how the data flows.
- increasing complexity, something that starts out small becomes large and complex over time, especially as we add lifecycle methods
-
life cycle methods does too many things, components might perform some data fetching in
componentDidMountandcomponentDidUpdate. SamecomponentDidMountmethod might also contain some unrelated logic that sets up event listeners, with cleanup performed incomponentWillUnmount
Just create smaller components?
In many cases, it's not possible because:
- difficult to test, stateful logic is all over the place, thus making it difficult to test
- classes confuse both people and machines, you have to understand how this works in JavaScript, you have to bind them to event handlers etc. The distinction between function and class components in React and when to use each one leads to disagreements and well all know how we can be when we fight for our opinion, spaces vs tabs anyone :)?.
- minify issues, classes present issues for today's tools, too. For example, classes don't minify very well, and they make hot reloading flaky and unreliable. Some of you might love classes and some of you might think that functions is the only way. Regardless of which we can only use certain features in React with classes and if it causes these minify issues, we must find a better way.
The selling point of Hooks
Hooks let you use more of React's features without classes. Not only that, we are able to create Hooks that will allow you to:
- extract stateful logic from a component, so it can be tested independently and reused.
- reuse stateful logic, without changing your component hierarchy. This makes it easy to share Hooks among many components or with the community.
What is a hook?
Hooks let you split one component into smaller functions based on what pieces are related (such as setting up a subscription or fetching data), rather than forcing a split based on lifecycle methods.
Let's have an overview of the different Hooks available to use. Hooks are divided into Basic Hooks and Additional Hooks. Let's list the Basic Hooks first and mention briefly what their role is:
Basic Hooks
- useState, this is a Hook that allows you to use state inside of function component
- useEffect, this is a Hook that allows you to perform side effect in such a way that it replaces several life cycle methods
- useContext, accepts a context object (the value returned from React.createContext) and returns the current context value, as given by the nearest context provider for the given context. When the provider updates, this Hook will trigger a re-render with the latest context value.
We will focus on useState and useEffect in this article.
Additional Hooks
We will not be covering Additional Hooks at all as this article would be way too long but you are encouraged to read more about them on Additional Hooks
- useReducer, alternative to useState, it accepts a reducer and returns a pair with the current state and a dispatch function
- useCallback, will return a memoized version of the callback that only changes if one of the inputs has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders
- useMemo, passes a create function and an array of inputs. useMemo will only recompute the memoized value when one of the inputs has changed. This optimization helps to avoid expensive calculations on every render.
- useRef, returns a mutable ref object whose .current property is initialized to the passed argument (initialValue). The returned object will persist for the full lifetime of the component
- useImperativeHandle, customizes the instance value that is exposed to parent components when using ref
- useLayoutEffect, the signature is identical to useEffect, but it fires synchronously after all DOM mutations. Use this to read layout from the DOM and synchronously re-render
- useDebugValue, can be used to display a label for custom Hooks in React DevTools
As you can see above I've pretty much borrowed the explanation for each of these Additional Hooks from the documentation. The aim was merely to describe what exists, give a one-liner on each of them and urge you to explore the documentation once you feel you've mastered the Basic Hooks.
useState Hook
This Hook lets us use state inside of a function component. Yep, I got your attention now right? Usually, that's not possible and we need to use a class for that. Not anymore. Let's show what using useState hook looks like. We need to do two things to get started with hooks:
- scaffold a project using Create React App
-
upgrade
reactandreact-dom, this steps necessary if you are on a react version before 16.8
The first one we will solve by typing:
npx create-react-app hooks-demo
next, we need to upgrade react and react-dom so they are using the experimental version of React where hooks are included:
yarn add react@next react-dom@next
Now we are good to go.
Our first Hook
Let's create our first hook using useState and focus on just understanding how to use it. Let's see some code:
import React, { useState } from 'react';
const Counter = () => {
const [counter, setCounter] = useState(0);
return (
<div> {counter}
<button onClick={() => setCounter(counter +1)}>
Increment
</button>
</div>
)
}
export default Counter;
Ok, we see that we use the Hook useState by invoking it and we invoke it like so:
useState(0)
This means we give it an initial value of 0. What happens next is when we invoke useState we get an array back that we do a destructuring on. Let's examine that closer:
const [counter, setCounter] = useState(0);
Ok, we name the first value in the array counter and the second value setCounter. The first value is the actual value that we can showcase in our render method. The second value setCounter() is a function that we can invoke and thereby change the value of counter. So in a sense, setCounter(3) is equivalent to writing:
this.setState({ counter: 3 })
A second Hook example - using a cart
Just to ensure we understand how to use it fully let's create a few more states:
import React, { useState } from 'react';
const ProductList = () => {
const [products] = useState([{ id: 1, name: 'Fortnite' }]);
const [cart, setCart] = useState([]);
const addToCart = (p) => {
const newCartItem = { ...p };
setCart([...cart, newCartItem]);
}
return (
<div>
<h2>Cart items</h2>
{cart.map(item => <div>{item.name}</div>)}
<h2>Products</h2>
{products.map(p => <div onClick={() => addToCart(p)}>{p.name}</div>)}
</div>
)
}
export default ProductList;
Above we are creating the states products and cart and we also get change function setCart(). We can see in the markup we invoke the method addToCart() if clicking on any of the items in our products list. This leads to the invocation of setCart(), which leads to the selected product ot be added as a cart item in our cart state.
This is a simple example but it really showcases the usage of setState Hook.
Introducing the Effect Hook
The Effect Hook is meant to be used to perform side effects like for example HTTP calls. It performs the same task as life cycle methods componentDidMount, componentDidUpdate, and componentWillUnmount.
Here is how we can use it:
import React, { useEffect, useState } from 'react';
const products = [
{ id: 1, name: "Fortnite" },
{ id: 2, name: "Doom" }
];
const api = {
getProducts: () => { return Promise.resolve(products);},
getProduct: (id) => { return Promise.resolve(
products.find(p => p.id === id));
}
}
const ProductList = () => {
const [products, setProducts] = useState([]);
const [product, setProduct] = useState('');
const [selected, setSelected] = useState(2);
async function fetchData() {
const products = await api.getProducts();
setProducts(products);
}
async function fetchProduct(productId) {
const p = await api.getProduct(productId);
setProduct(p.name);
}
useEffect(() => {
console.log('use effect');
fetchData();
fetchProduct(selected);
}, [selected]);
return (
<React.Fragment>
<h1>Async shop</h1>
<h2>Products</h2>
{products.map(p => <div>{p.name}</div>)}
<h3>Selected product</h3> {product}
<button onClick={() => setSelected(1)}>Change selected</button
</React.Fragment>
);
}
export default ProductList;
Ok, a lot of interesting things was happening here. Let's start by looking at our usage of useEffect:
useEffect(() => {
console.log('use effect');
fetchData();
fetchProduct(selected);
}, [selected]);
What we are seeing above is us calling fetchData() and fetchProduct(). Both these methods calls methods marked by async. Why can't we just make the calling function in useEffect async? Well that's a limitation of Hooks, unfortunately.
Looking at the definition of these two methods it looks like the following:
async function fetchData() {
const products = await api.getProducts();
setProducts(products);
}
async function fetchProduct(productId) {
const p = await api.getProduct(productId);
setProduct(p.name);
}
We see above that we are calling getProducts() and getProduct() on our api object, which both returns a Promise. After having received the resolved Promises, using await we call setProducts() and setProduct() that are functions we get from our useState Hook. Ok, so this explains how useEffect in this case acts like componentDidMount but there is one more detail. Let's look at our useEffect function again:
useEffect(() => {
console.log('use effect');
fetchData();
fetchProduct(selected);
}, [selected]);
The interesting part above is the second argument [selected]. This is us looking at the selected variable and let ourselves be notified of changes, if a change happens to the variable selected then we will run our useEffect function.
Now, try hitting the bottom button and you will see setSelected being invoked which trigger useEffect, because we are watching it.
Life cycle
Hooks replaces the needs for many life cycle methods in general so it's important for us to understand which ones.
Let's discuss Effect Hooks in particular and their life cycle though.
The following is known about its life cycle:
- By default, React runs the effects after every render
- After changes are flushed, our effect is being run after React has flushed changes to the DOM - including the first render
Accessing the DOM tree
Let's talk about when we access the DOM tree, to perform a side effect. If we are not using Hooks we would be doing so in the methods componentDidMount and componentDidUpdate. The reason is we can't use the render method cause then it would happen to early.
Let's show how we would use life cycle methods to update the DOM:
componentDidMount() {
document.title = 'Component started';
}
componentDidUpdate() {
document.title = 'Component updated'
}
We see that we can do so using two different life cycle methods.
Accessing the DOM tree with an Effects Hook would look like the following:
const TitleHook = () => {
const [title, setTitle] = useState('no title');
useEffect(() => {
document.title = `App name ${title} times`;
})
}
As you can see above we have access to props as well as state and the DOM.
Let's remind ourselves what we know about our Effect Hook namely this:
Our effect is being run after React has flushed changes to the DOM - including the first render
That means that two life cycle methods can be replaced by one effect.
Handling set up/ tear down
Let's now look at another aspect of the useEffect Hook namely that we can, and we should clean up after ourselves. The idea for that is the following:
useEffect(() => {
// set up
// perform side effect
return () => {
// perform clean up here
}
});
Above we see that inside of our useEffect() function we perform our side effect as usual, but we can also set things up. We also see that we return a function. Said function will be invoked the last thing that happens.
What we have here is set up and tear down. So how can we use this to our advantage? Let's look at a bit of a contrived example so we get the idea:
useEffect(() => {
const id = setInterval(() => console.log('logging'));
return () => {
clearInterval(id);
}
})
The above demonstrates the whole set up and tear down scenario but as I said it is a bit contrived. You are more likely to do something else like setting up a socket connection, for example, e.g some kind of subscription, like the below:
onMessage = (message) => {
// do something with message
}
useEffect(() => {
chatRoom.subscribe('roomId', onMessage)
return () => {
chatRoom.unsubscribe('roomId');
}
})
Can I create my own Hook?
Yes you can. With useState and useEffect the world is your oyster. You can create whatever Hook you need.
Ask yourself the following questions; Will my component have a state? Will I need to do a DOM manipulation or maybe an AJAX call? Most of all, is it something usable that more than one component can benefit from? If there are several yes here you can use a Hook to create it.
Let's look at some interesting candidates and see how we can use Hooks to build them out:
You could be creating things like:
- a modal, this has a state that says whether it shows or not and we will need to manipulate the DOM to add the modal itself and it will also need to clean up after itself when the modal closes
- a feature flag, feature flag will have a state where it says whether something should be shown or not, it will need to get its state initially from somewhere like localStorage and/or over HTTP
- a cart, a cart in an e-commerce app is something that most likely follows us everywhere in our app. We can sync a cart to localStorage as well as a backend endpoint.
Feature flag
Let's try to sketch up our Hook and how it should be behaving:
import React, { useState } from 'react';
function useFeatureFlag(flag) {
let flags = localStorage.getItem("flags"); flags = flags ? JSON.parse(flags) : null;
const [enabled] = useState(Boolean(flags ? flags[flag]: false));
return [enabled];
}
export default useFeatureFlag;
Above we have created a Hook called useFeatureFlag. This reads its value from localStorage and it uses useState to set up our hook state. The reason for us not destructuring out a set method in the hook is that we don't want to change this value unless we reread the whole page, at which point we will read from localStorage anew.
Testing our Hook
Now that we have created our custom Hook, let's take it for a spin. The idea is for whatever component that uses our Hook to only read from its value. How that feature flag value is stored is up to the hook. So the Hook is an abstraction over localStorage.
import React from 'react';
import useFeatureFlag from './flag';
const TestComponent = ({ flag }) => {
const [enabled] = useFeatureFlag(flag);
return (
<React.Fragment>
<div>Normal component</div>
{enabled &&
<div>Experimental</div>
}
</React.Fragment> );
};
export default TestComponent;
// using it
<TestComponent flag="experiment1">
Creating an Admin page for our Feature Flag
We said earlier that we weren't interested in changing the value exposed by useFeatureFlag. To control our feature flags, we opt for creating a specific Admin page. We count on the Admin page to be on a specific page and the component with the feature flag on another page. If that is the case, then navigating between the two pages will mean the feature flag component reads from localStorage.
Back to the Admin page, it would be neat if we could list all the flags and toggle them any way we want to. Let's write such a component. Our component should be quite simple as it should only render a list of flags. However, it will need to be able to update a flag when the user chooses to.
We will need the following:
- a simple list component, that renders all the feature flags and supports the toggling of a specific flag
-
a Hook, that is an abstraction over
localStoragebut that is also able to update its state
The code follows below:
import React, { useState } from 'react';
const useFlags = () => {
let flags = localStorage.getItem("flags"); flags = flags ? JSON.parse(flags) : {};
const [ flagsValue, setFlagsValue ] = useState(flags);
const updateFlags = (f) => {
localStorage.setItem("flags", JSON.stringify(f));
setFlagsValue(f);
}
return [flagsValue, updateFlags];
}
const FlagsPage = () => {
const [flags, setFlags] = useFlags();
const toggleFlag = (f) => {
const currentValue = Boolean(flags[f]);
setFlags({...flags, flags[f]: !currentValue});
}
return (
<React.Fragment>
<h1>Flags page</h1>
{Object
.keys(flags)
.filter(key => flags[key]).map(flag =>
<div>
<button onClick={() => toggleFlag(flag)}>{flag}</button
</div>
)
}
</React.Fragment>
)
}
export default FlagsPage;
What we are doing above is to read out the flags from localStorage and then we render them all out in the component. While rendering them out, flag by flag, we also hook-up ( I know we are talking about Hooks here but no pun intended, really :) ) a method on the onClick handler. That method is toggleFlag() that lets us change a specific flag. Inside of toggleFlag() we not only set the new flag value but we also ensure our flags have the latest updated value by us invoking setFlags on the Hook.
It should also be said that us creating useFlags Hook have made the code in FlagsPage component quite simple, so hooks are good at cleaning up a bit too.
Summary
In this article, we have tried to explain the background and the reason Hooks where created and what problems it was looking to address and hopefully fix.
We've learned that Hooks is a way to give functional components state but that they are also able to remove the need for some lifecycle methods. There are a lot of Hooks given to you out of the box like the following two Hooks:
- useState, is a Hook we can use to persist state in a functional component
- useEffect, is also a Hook but for side effects
but there are many many more that I urge you to go explore, like these:
Additional Hooks https://reactjs.org/docs/hooks-reference.html#additional-hooks
With Hooks we can create really cool and reusable functionality, so go out there, be awesome and create your own hooks.
Further reading
I welcome any comments or maybe a link to a Hook you built :)
Stay awesome out there !
Top comments (2)
They're no longer expiremental
I know.. I should update that. Thanks for the comment :)