"Koje su, dovraga, kuke?"

Otkrio sam da to pitam baš kao što sam mislio da sam pokrio sve osnove React-a. Takav je život frontend programera, igra se uvijek mijenja. Uđite Kuke.

Uvijek je lijepo naučiti nešto novo zar ne? Naravno! Ali ponekad se moramo zapitati "Zašto? Koji je smisao ove nove stvari? Moram li je naučiti"?

Kod kuka, odgovor je "ne odmah". Ako ste učili React i do danas ste koristili komponente temeljene na nastavi, nema žurbe s prelaskom na udice. Kuke nisu dodatne i mogu raditi zajedno s vašim postojećim komponentama. Zar ne mrziš kad moraš prepisati cijelu svoju bazu kodova da bi neka nova stvar uspjela?

U svakom slučaju, evo nekoliko razloga zašto su udice uopće predstavljene i zašto preporučujem početnicima da ih uče.

Korištenje stanja u funkcionalnim komponentama

Prije kuka, nismo mogli koristiti stanje u funkcionalnim komponentama. To znači da ako imate lijepo izrađenu i testiranu funkcionalnu komponentu koja odjednom treba pohraniti stanje, zapeli ste s bolnim zadatkom refaktoriranja svoje funkcionalne komponente u komponentu klase.

Ura! Dopuštanje stanja unutar funkcionalnih komponenti znači da ne moramo refaktorirati svoje prezentacijske komponente. Pogledajte ovaj članak za više.

Komponente klase su nezgrapne

Priznajmo, komponente klase dolaze s puno pločica. Konstruktori, obvezujući, svugdje koristeći "ovo". Korištenje funkcionalnih komponenti uklanja mnogo toga, tako da naš kôd postaje lakše slijediti i održavati.

Više o tome možete pročitati na React dokumentima:

Čitljiviji kod

Budući da nam kuke dopuštaju upotrebu funkcionalnih komponenti, to znači da je manje koda u odnosu na komponente klase. To čini naš kod čitljivijim. Pa, to je ionako ideja.

Ne moramo se brinuti o vezivanju svojih funkcija ili se sjetiti na što se ovo "odnosi", itd. Možemo se brinuti da li ćemo umjesto toga napisati naš kôd.

Ako tek započinjete s Reactom, na svom blogu imam gomilu početnih postova koji bi vam mogli pomoći! Pogledajte ovdje:

React State Hook

Ah, država. Kamen temeljac ekosustava React. Nakvasimo noge udicama uvođenjem najčešće kuke s kojom ćete raditi - useState().

Pogledajmo komponentu klase koja ima stanje.

 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; 

Pomoću React Hooks-a možemo prepisati ovu komponentu i ukloniti puno stvari, što olakšava razumijevanje:

 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; 

Na prvi pogled je manje koda, ali što se događa?

Sintaksa stanja reakcije

Dakle, vidjeli smo našu prvu udicu! Ura!

 const [count, setCount] = useState(); 

U osnovi, ovo koristi dodjeljivanje destrukturiranja za nizove. useState()Funkcija daje nam 2 stvari:

• varijabla koja sadrži vrijednost stanja , u ovom se slučaju zove count- funkcija za promjenu vrijednosti , u ovom slučaju se zove setCount.

Možete ih imenovati kako god želite:

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

A možete ih koristiti u cijelom kodu poput normalnih varijabli / funkcija:

 function CounterWithHooks() { const [count, setCount] = useState(); return ( 
This is a counter using hooks
{count}
  setCount(count + 1)}>Click to Increment ); } 

Primijetite useStatekuku na vrhu. Izjavljujemo / destrukturiramo dvije stvari:

• counter: vrijednost koja će zadržati našu državnu vrijednost
• setCounter: funkcija koja će promijeniti našu countervarijablu

Kako nastavljamo s kodom, vidjet ćete ovaj redak:

{count}

Ovo je primjer kako možemo koristiti varijablu stanja kuke. Unutar našeg JSX-a smještamo našu countvarijablu {}da bi je izvršili kao JavaScript, a zauzvrat se countvrijednost prikazuje na stranici.

Uspoređujući ovo sa starim načinom korištenja varijable stanja "na temelju klase":

{this.state.count}

Primijetit ćete da se više ne moramo brinuti o korištenju this, što nam uvelike olakšava život - na primjer, uređivač VS koda upozorit će nas ako {count}nije definiran, što nam omogućuje rano hvatanje pogrešaka. Dok neće znati je li {this.state.count}nedefinirano dok se kôd ne pokrene.

Na sljedeći redak!

  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...