React - JSX

JSX is an React expression that compiles to the native javascript (ie createElement).

The first motivation of JSX is that it's easier to read and write.

JSX:

• is easier to read
• can be used as a template because it can embedded javascript expression
• Prevents Injection Attacks

const title = response.potentiallyMaliciousInput;
// This is safe:
const element = <h1>{title}</h1>;

Babel transpile (ie convert) JSX down to Javascript with React.createElement() calls. ¹⁾

Example that you can run on the Babel online compiler playground

const element = (
  <h1 className="greeting">
    Hello, world!
  </h1>
);

const element = React.createElement(
  'h1',
  {className: 'greeting'},
  'Hello, world!'
);

Multiple JSX lines should be wrap in parentheses to avoid automatic semicolon insertion

Because Jsx is an expression, you can:

• use JSX inside if statements
• use JSX inside for loops,
• assign it to variables,
• accept it as arguments,

• With a For loop, you feed an array that you put inside the JSX return expression

............
const items = []
for (const [index, value] of elements.entries()) {
  items.push(<input value={value} label={index} />)
}

return (
<div>
    {items}
</div>
);
....

• Map

return (
<div>
    {elements.map((value, index) => {
        return <input value={value} label={index} />
    })}
</div>
);
....

function getGreeting(user) {
  if (user) {
    return <h1>Hello, {formatName(user)}!</h1>;
  }
  return <h1>Hello, Stranger.</h1>;
}

Any valid JavaScript expression can be written inside curly braces in JSX

<ReactElementTagName>
   Hello, world! 
   <ChildReactElement/>
   { console.log("Javascript code is between {}") } 
</ReactElementTagName>;

where:

• React - Element

• Espace: {“\u00A0”}

All Jsx attribute have a camelCase name in order to make a difference with html attribute:

• class becomes className
• tabindex becomes tabIndex

You should either use:

• quotes (for string values)
• or curly braces (for javascript expressions),

but not both in the same attribute.

const element = <div tabIndex="0"></div>;
const element = <img src={user.avatarUrl}></img>;

You declare the type with the JSX.Element

let inputElements: JSX.Element;

See How to render Jsx code in the browser

See How to create JSX dynamically from Json

• https://facebook.github.io/react/docs/introducing-jsx.html