React Hooks für Anfänger - Eine gehirnfreundliche Anleitung zu useState und useEffect

"Was zum Teufel sind Haken?"

Ich stellte fest, dass ich dies fragte, gerade als ich dachte, ich hätte alle Grundlagen von React abgedeckt. So ist das Leben eines Frontend-Entwicklers, das Spiel ändert sich ständig. Geben Sie Hooks ein.

Es ist immer schön, etwas Neues zu lernen, oder? Natürlich! Aber manchmal müssen wir uns fragen: "Warum? Was ist der Sinn dieser neuen Sache? Muss ich sie lernen?"

Bei Haken lautet die Antwort "nicht sofort". Wenn Sie React gelernt haben und bisher klassenbasierte Komponenten verwendet haben, besteht keine Eile, zu Hooks zu wechseln. Haken sind optional und können zusammen mit Ihren vorhandenen Komponenten verwendet werden. Hassen Sie es nicht, wenn Sie Ihre gesamte Codebasis neu schreiben müssen, damit etwas Neues funktioniert?

Wie auch immer, hier sind einige Gründe, warum Haken überhaupt eingeführt wurden und warum ich Anfängern empfehle, sie zu lernen.

Verwenden des Zustands in Funktionskomponenten

Vor Hooks konnten wir den Status in Funktionskomponenten nicht verwenden. Das heißt, wenn Sie eine gut gestaltete und getestete Funktionskomponente haben, die plötzlich den Status speichern muss, stehen Sie vor der schmerzhaften Aufgabe, Ihre Funktionskomponente in eine Klassenkomponente umzuwandeln.

Hurra! Wenn Sie den Status innerhalb der Funktionskomponenten zulassen, müssen Sie unsere Präsentationskomponenten nicht umgestalten. Weitere Informationen finden Sie in diesem Artikel.

Klassenkomponenten sind klobig

Seien wir ehrlich, Klassenkomponenten werden mit viel Boilerplate geliefert. Konstruktoren, die "dies" überall binden. Durch die Verwendung von Funktionskomponenten wird ein Großteil davon entfernt, sodass unser Code leichter zu befolgen und zu warten ist.

Weitere Informationen hierzu finden Sie in den React-Dokumenten:

Besser lesbarer Code

Da wir mit Hooks Funktionskomponenten verwenden können, bedeutet dies, dass im Vergleich zu Klassenkomponenten weniger Code vorhanden ist. Dies macht unseren Code besser lesbar. Nun, das ist sowieso die Idee.

Wir müssen uns keine Sorgen um die Bindung unserer Funktionen machen oder uns daran erinnern, was "dies" auch bedeutet, und so weiter. Wir können uns stattdessen darum kümmern, unseren Code zu schreiben.

Wenn Sie gerade erst mit React anfangen, habe ich eine Reihe von ersten Beiträgen in meinem Blog, die Ihnen helfen könnten! Schau es dir hier an:

Reaktionshaken reagieren

Ah, Staat. Ein Eckpfeiler des React-Ökosystems. Lassen Sie uns unsere Füße mit Haken nass machen, indem wir den häufigsten Haken einführen, mit dem Sie arbeiten werden - useState().

Schauen wir uns eine Klassenkomponente mit Status an.

 import React, { Component } from 'react'; import './styles.css'; class Counter extends Component { state = { count: this.props.initialValue, }; setCount = () => { this.setState({ count: this.state.count + 1 }); }; render() { return ( 

This is a counter using a class

{this.state.count}

Click to Increment ); } } export default Counter;

Mit React Hooks können wir diese Komponente neu schreiben und viele Dinge entfernen, um das Verständnis zu erleichtern:

 import React, { useState } from 'react'; function CounterWithHooks(props) { const [count, setCount] = useState(props.initialValue); return ( 

This is a counter using hooks

{count}

setCount(count + 1)}>Click to Increment ); } export default CounterWithHooks;

Auf den ersten Blick gibt es weniger Code, aber was ist los?

Reaktionszustandssyntax

Also haben wir unseren ersten Haken gesehen! Hurra!

 const [count, setCount] = useState(); 

Grundsätzlich wird hierfür die Destrukturierungszuweisung für Arrays verwendet. Die useState()Funktion gibt uns 2 Dinge:

  • eine Variable , die den Zustandswert zu halten , in diesem Fall nennt man das count- eine Funktion , um den Wert zu ändern , in diesem Fall, es heißt setCount.

Sie können diese beliebig benennen:

 const [myCount, setCount] = useState(0); 

Und Sie können sie im gesamten Code wie normale Variablen / Funktionen verwenden:

 function CounterWithHooks() { const [count, setCount] = useState(); return ( 

This is a counter using hooks

{count}

setCount(count + 1)}>Click to Increment ); }

Beachten Sie den useStateHaken oben. Wir erklären / zerstören 2 Dinge:

  • counter: Ein Wert, der unseren Zustandswert enthält
  • setCounter: eine Funktion, die unsere counterVariable ändert

As we continue through the code, you'll see this line:

{count}

This is an example of how we can use a state hook variable. Within our JSX, we place our count variable within {} to execute it as JavaScript, and in turn the count value gets rendered on the page.

Comparing this to the old "class-based" way of using a state variable:

{this.state.count}

You'll notice we no longer need to worry about using this, which makes our life a lot easier - for example, the VS Code editor will give us a warning if {count} is not defined, allowing us to catch errors early. Whereas it won't know if {this.state.count} is undefined until the code is run.

On to the next line!

  setCount(count + 1)}>Click to Increment 

Here, we're using the setCount function (remember we destructured/declared this from the useState() hook) to change the count variable.

When the button is clicked, we update the count variable by 1. Since this is a change of state this triggers a rerender, and React updates the view with the new count value for us. Sweet!

How can I set the initial state?

You can set the initial state by passing an argument to the useState() syntax. This can be a hardcoded value:

 const [count, setCount] = useState(0); 

Or can be taken from the props:

 const [count, setCount] = useState(props.initialValue); 

This would set the count value to whatever the props.initialValue is.

That sums up useState(). The beauty of it is that you can use state variables/functions like any other variable/function you would write yourself.

How do I handle multiple state variables?

This is another cool thing about hooks. We can have as many as we like in a component:

 const [count, setCount] = useState(props.initialValue); const [title, setTitle] = useState("This is my title"); const [age, setAge] = useState(25); 

As you can see, we have 3 seperate state objects. If we wanted to update the age for example, we just call the setAge() function. The same with count and title. We no longer are tied to the old clunky class component way where we have one massive state object stored using setState():

 this.setState({ count: props.initialValue, title: "This is my title", age: 25 }) 

So, what about updating things when props or state changes?

When using hooks and functional components, we no longer have access to React lifecycle methods like componentDidMount, componentDidUpdate, and so on. Oh, dear! Do not panic my friend, React has given us another hook we can use:

  • Drum Roll *

Enter useEffect!

The Effect hook (useEffect()) is where we put "side effects".

Eh, side effects? What? Let's go off-track for a minute and discuss what a side effect actually is. This will help us understand what useEffect() does, and why it's useful.

A boring computer-y explanation would be.

"In programming, a side effect is when a procedure changes a variable from outside its scope"

In React-y terms, this means "when a component's variables or state changes based on some outside thing". For example, this could be:

  • When a component receives new props that change its state
  • When a component makes an API call and does something with the response (e.g, changes the state)

So why is it called a side effect? Well, we cannot be sure what the result of the action will be. We can never be 100% certain what props we are going to receive, or what the response from an API call would be. And, we cannot be sure how this will affect our component.

Sure we can write code to validate, and handle errors, and so on, but ultimately we cannot be sure what the side effects of said things are.

So for example, when we change state, based on some outside thing this is know as a side effect.

With that out of the way, let's get back to React and the useEffect Hook!

When using functional components we no longer have access to life cycle methods like componentDidMount(), componentDidUpdate() etc. So, in effect (pun intended), the useEffect hooks replace the current React Life Cycle hooks.

Let's compare a class-based component with how we use the useEffect hook:

import React, { Component } from 'react'; class App extends Component { componentDidMount() { console.log('I have just mounted!'); } render() { return Insert JSX here ; } } 

And now using useEffect():

function App() { useEffect(() => { console.log('I have just mounted!'); }); return Insert JSX here ; } 

Before we continue, it's important to know that, by default, the useEffect hook runs on every render and re-render. So whenever the state changes in your component or your component receives new props, it will rerender and cause the useEffect hook to run again.

Running an effect once (componentDidMount)

So, if hooks run every time a component renders, how do we ensure a hook only runs once when the component mounts? For example, if a component fetches data from an API, we don't want this happening every time the component re-renders!

The useEffect() hook takes a second parameter, an array, containing the list of things that will cause the useEffect hook to run. When changed, it will trigger the effect hook. The key to running an effect once is to pass in an empty array:

useEffect(() => { console.log('This only runs once'); }, []); 

So this means the useEffect hook will run on the first render as normal. However, when your component rerenders, the useEffect will think "well, I've already run, there's nothing in the array, so I won't have to run again. Back to sleep for me!" and simply does nothing.

In summary, empty array = useEffect hook runs once on mount

Using effects when things change (componentDidUpdate)

We've covered how to make sure a useEffect hook only runs once, but what about when our component receives a new prop? Or we want to run some code when the state changes? Hooks let us do this as well!

 useEffect(() => { console.log("The name props has changed!") }, [props.name]); 

Notice how we are passing stuff to the useEffect array this time, namely props.name.

In this scenario, the useEffect hook will run on the first load as always. Whenever your component receives a new name prop from its parent, the useEffect hook will be triggered, and the code within it will run.

We can do the same thing with state variables:

const [name, setName] = useState("Chris"); useEffect(() => { console.log("The name state variable has changed!"); }, [name]); 

Whenever the name variable changes, the component rerenders and the useEffect hook will run and output the message. Since this is an array, we can add multiple things to it:

const [name, setName] = useState("Chris"); useEffect(() => { console.log("Something has changed!"); }, [name, props.name]); 

This time, when the name state variable changes, or the name prop changes, the useEffect hook will run and display the console message.

Can we use componentWillUnmount()?

To run a hook as the component is about to unmount, we just have to return a function from the useEffect hook:

useEffect(() => { console.log('running effect'); return () => { console.log('unmounting'); }; }); 

Can I use different hooks together?

Yes! You can use as many hooks as you want in a component, and mix and match as you like:

function App = () => { const [name, setName] = useState(); const [age, setAge] = useState(); useEffect(()=>{ console.log("component has changed"); }, [name, age]) return( Some jsx here... ) } 

Conclusion - What Next?

There you have it. Hooks allow us to use good old fashioned JavaScript functions to create simplier React components, and reduce alot of boilerplate code.

Now, run off into the world of Reac hooks and try building stuff yourself! Speaking of building stuff yourself...