Hvad er JSX, og hvorfor skal du bruge det til at opbygge dine React-apps

Som udviklere bruger vi en række værktøjer og open source-pakker for at gøre vores job lettere. Nogle af dem er så udbredt i hele samfundet, at de synes at være hjemmehørende i JavaScript. Men det er de ikke, og de kan fundamentalt ændre, hvordan du skriver kode dagligt.

En af disse teknologier, som du sandsynligvis allerede bruger, er JSX - en XML-lignende syntaksudvidelse til JavaScript . Oprettet af teamet på Facebook er det beregnet til at forenkle udvikleroplevelsen. Som specifikationen siger, var begrundelsen for oprettelse af JSX:

"... at definere en kortfattet og velkendt syntaks til definition af træstrukturer med attributter." ~ JSX Spec

Nu siger du sandsynligvis til dig selv: "Hej, Ryan, det lyder godt, men kom allerede til koden ", så her er vores første eksempel.

const helloWorld = 

Hello, World!

;

Og det er det!

Kodestykket ovenfor ser velkendt ud, men er du nogensinde stoppet for at tænke på dens magt? JSX gør det, så vi kan passere træstrukturer, der består af HTML eller React-elementer, som om de var standard JavaScript-værdier.

Mens du ikke behøver at bruge JSX, når du skriver React (eller brug React for at prøve JSX), kan det ikke benægtes, at det er en vigtig del af React-økosystemet, så lad os dykke ind og se, hvad der foregår under emhætten.

Kom godt i gang med JSX

Den første ting at bemærke, når du bruger JSX-syntaks, er at React skal være inden for rækkevidde. Dette skyldes, hvordan det bliver samlet. Tag denne komponent for eksempel:

function Hello() { return 

Hello, World!

}

Bag kulisserne Hellotransponeres hvert element, der gengives af komponenten, til et React.createElementopkald.

I dette tilfælde:

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

Det samme gælder for indlejrede elementer. De to eksempler nedenfor giver i sidste ende den samme markering.

// 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

Når React opretter elementer, kalder den denne metode, som tager tre argumenter.

  1. Elementets navn
  2. Et objekt, der repræsenterer elementets rekvisitter
  3. En række af elementets børn

En ting at bemærke her er, at React fortolker små bogstaver som HTML- og Pascal-sager (f.eks. ThisIsPascalCase) -elementer som brugerdefinerede komponenter. På grund af dette vil følgende eksempler blive fortolket forskelligt.

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

Det første eksempel ville generere en 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.