Kaj za vraga je JSX in zakaj bi ga morali uporabljati za izdelavo svojih React aplikacij

Kot razvijalci za lažje delo uporabljamo različna orodja in odprtokodne pakete. Nekateri so tako široko uporabljeni v celotni skupnosti, da se zdijo izvorni za JavaScript. Vendar niso in lahko bistveno spremenijo način vsakodnevnega pisanja kode .

Ena od teh tehnologij, ki jo verjetno že uporabljate, je JSX - XML podobna sintaksna razširitev za JavaScript . Ustvarila ga je ekipa na Facebooku in naj bi poenostavila izkušnjo razvijalcev. Kot piše v specifikaciji, je bila utemeljitev za ustvarjanje JSX:

"... določiti jedrnato in znano sintakso za definiranje drevesnih struktur z atributi." ~ JSX Spec

Zdaj si verjetno rečete: "Hej, Ryan, to se sliši čudovito, ampak že pojdi na kodo ", zato je tu naš prvi primer.

const helloWorld = 

Hello, World!

;

In to je to!

Zgornji delček kode se zdi znan, a ste se kdaj ustavili, da bi pomislili na njegovo moč? JSX omogoča, da lahko prehodimo drevesne strukture, sestavljene iz elementov HTML ali React, kot da gre za standardne vrednosti JavaScript.

Čeprav vam ni treba uporabljati JSX pri pisanju React-a (ali uporabite React, da poskusite JSX), ni zanikati, da je pomemben del ekosistema React, zato se potopimo in poglejmo, kaj se dogaja pod pokrovom.

Uvod v JSX

Prva stvar, ki jo je treba opozoriti pri uporabi sintakse JSX, je, da mora biti React v obsegu. To je posledica tega, kako se prevede. Vzemimo za primer to komponento:

function Hello() { return 

Hello, World!

}

V ozadju se vsak element, ki ga Hellokomponenta upodobi, prevede v React.createElementklic.

V tem primeru:

function Hello() { return React.createElement("h1", {}, "Hello, World!")}

Enako velja za ugnezdene elemente. Spodnja dva primera bi navsezadnje imela enak pribitek.

// Example 1: Using JSX syntaxfunction Nav() { return ( 
    
  • Home
  • About
  • Portfolio
  • Contact
);}
// Example 2: Not using JSX syntaxfunction Nav() { return ( React.createElement( "ul", {}, React.createElement("li", null, "Home"), React.createElement("li", null, "About"), React.createElement("li", null, "Portfolio"), React.createElement("li", null, "Contact") ) );}

React.createElement

Ko React ustvari elemente, pokliče to metodo, ki ima tri argumente.

  1. Ime elementa
  2. Predmet, ki predstavlja rekvizite elementa
  3. Niz podrejenih elementov

Tu je treba omeniti, da React razlaga male elemente kot elemente HTML in Pascal (npr. ThisIsPascalCase) kot komponente po meri. Zaradi tega bi si naslednje primere razlagali drugače.

// 1. HTML elementReact.createElement("div", null, "Some content text here")
// 2. React elementReact.createElement(Div, null, "Some content text here")

Prvi primer bi ustvaril iv> with the s tring "Some content text here" as its child. However, the second version would throw an error (unless, of course, a custom comp onent <Div /> was in sco pe) because is undefined.

Props in JSX

When working in React, your components often render children and need to pass them data in order for the children to render properly. These are called props.

I like to think of React components as a group of friends. And what do friends do? They give each other props. Thankfully, JSX offers us a number of ways to do that.

// 1. Props defaulted to true
// 2. String literals
// 3. JavaScript expressions
// 4. Spread attributes

But beware! You cannot pass if statements or for loops as props because they are statements, not expressions.

Children in JSX

As you are building your app, you eventually start having components render children. And then those components sometimes have to render children. And so on and so forth.

Since JSX is meant to make it easy for us to reason about tree-like structures of elements, it makes all of this very easy. Basically, whatever elements a component returns become its children.

There are four ways to render child elements using JSX:

Strings

This is the simplest example of JSX children. In the case below, React creates a <h1> HTML element with one child. The child, however, is not another HTML element, just a simple string.

function AlertBanner() { return ( 

Your bill is due in 2 days

)}

JSX Elements

This is probably the use case that new React developers would be most familiar with. In the component below, we’re returning an HTML child (the er>), which has two children of it s own &lt;Na v /> and &lt;ProfilePic /> both of which are custom defined JSX elements.

function Header(props) { return ( )}

Expressions

Expressions allow us to easily render elements in our UI that are the result of a JavaScript computation. A simple example of this would be basic addition.

Say we have a component called /> that renders information about a bill or receipt. Let’s assume it takes one prop c alled total that represents the pre-tax cost and another prop taxRate, which represents the applicable tax rate.

Using expressions, we can easily render out some useful information for our users!

function BillFooter(props) { return ( 
Tax: {props.total * props.taxRate}
Total: {props.total + props.total * props.taxRate}
);}

Functions

With functions, we can programmatically create elements and structures, which React will then render for us. This makes it easy to create multiple instances of a component or render repeated UI elements.

As an example, let’s use JavaScript’s .map() function to create a navigation bar.

// Array of page informationconst pages = [ { id: 1, text: "Home", link: "/" }, { id: 2, text: "Portfolio", link: "/portfolio" }, { id: 3, text: "Contact", link: "/contact" }];// Renders a 
    
    with programmatically created
  • childrenfunction Nav() { return (
      {pages.map(page => { return (
    • {page.text}
    • ); })}
    );}

Now, if we want to add a new page to our site, all we need to do is add a new object to the pages array and React will take care of the rest!

Take note of the key prop. Our function returns an array of sibling elements, in this case <li>s, and React needs a way to keep track of which mounts, unmounts or updates. To do that, it relies on this unique identifier for each element.

Use the tools!

Sure, you can write React applications without JSX, but I’m not really sure why you’d want to.

The ability JSX gives us to pass around elements in JavaScript like they were first-class citizen lends itself well to working with the rest of the React ecosystem. So well, in fact, you may have been writing it every day and not even known it.

Bottom line: just use JSX. You’ll be happy you did.