ADVANCED JSX

class vs className

Grammar in JSX is mostly the same as in HTML, but there are subtle differences to watch out for. Probably the most frequent of these involves the word class.

In HTML, it's common to use class as an attribute name:

<h1 class="big">Hey</h1>

In JSX, you can't use the word class! You have to use className instead:

<h1 className="big">Hey</h1>

This is because JSX gets translated into JavaScript, and class is a reserved word in JavaScript.

When JSX is rendered, JSX className attributes are automatically rendered as class attributes.

Self-Closing Tags

What's a self-closing tag?

Most HTML elements use two tags: an opening tag and a closing tag.
However, some HTML elements such as img and input use only one tag.
The tag that belongs to a single-tag element isn't an opening tag nor a closing tag: it's a self-closing tag.

When you write a self-closing tag in HTML, it is optional to include a forward-slash immediately before the final angle-bracket:

Fine in HTML with a slash:

  <br />

Also fine, without the slash:

  <br>

But!
In JSX, you have to include the slash. If you write a self-closing tag in JSX and forget the slash, you will raise an error:

Fine in JSX:

  <br />

NOT FINE AT ALL in JSX:

  <br>

Curly Braces in JSX

ReactDOM.render(
  <h1>2 + 3</h1>,
  document.getElementById('app')
);

This code doesn't behave as one might expect.
Instead of adding 2 and 3, it printed out "2 + 3" as a string of text. Why?

This happened because 2 + 3 is located in between h1 tags.

Any code in between the tags of a JSX element will be read as JSX, not as regular JavaScript!
JSX doesn't add numbers - it reads them as text, just like HTML.

You need a way to write code that says, "Even though I am located in between JSX tags, treat me like ordinary JavaScirpt and not like JSX."

You can do this by wrapping your code in curly braces.

20 Digits of Pi in JSX

const pi = (
  <div>
    <h1>
      PI, YALL!
    </h1>
    <p>
      {Math.PI.toFixed(20)}
    </p>
  </div>
);

ReactDOM.render(
	pi,
	document.getElementById('app')
);

• The code is written in a JavaScript file. By default, it will all be treated as regular JavaScript.
• Find div. From there up through div, the code will be treated as JSX.
• Find Math. From there up through (20), the code will be treated as regular JavaScript again.
• The curly braces themselves won't be treated as JSX nor as JavaScript. They are markers that signal the beginning and end of a JavaScript injection into JSX, similar to the quotation marks that signal the boundaries of a string.

Variables in JSX

When you inject JavaScript into JSX, that JavaScript is part of the same environment as the rest of the JavaScirpt in your file.

That means that you can access variables while inside of a JSX expression, even if those variables were declared on the outside.

// Declare a variable:
const name = 'Gerdo';

// Access your variable 
// from inside of a JSX expression:
const greeting = <p>Hello, {name}!</p>;

Variable Attributes in JSX

When writing JSX, it's common to use variables to set attributes.

Here's an example of how that might work:

// Use a variable to set the `height` and `width` attributes:

const sideLength = "200px";

const panda = (
  <img 
    src="images/panda.jpg" 
    alt="panda" 
    height={sideLength} 
    width={sideLength} />
);

Notice how in this example, the img's attributes each get their own line.
This can make your code more readable if you have a lot of attributes on one element.

Object properties are also often used to set attributes:

const pics = {
  panda: "http://bit.ly/1Tqltv5",
  owl: "http://bit.ly/1XGtkM3",
  owlCat: "http://bit.ly/1Upbczi"
}; 

const panda = (
  <img 
    src={pics.panda} 
    alt="Lazy Panda" />
);

const owl = (
  <img 
    src={pics.owl} 
    alt="Unimpressed Owl" />
);

const owlCat = (
  <img 
    src={pics.owlCat} 
    alt="Ghastly Abomination" />
); 

Event Listeners in JSX

JSX elements can have event listeners, just like HTML elements can. Programming in React menas constantly working with event listeners.

You create an event listener by giving a JSX element a special attribute. Here's an example:

<img onClick={myFunc} />

An event listener attribute's name should be something like onClick or onMouseOver: the word on, plus the type of event that you're listening for. You can see a list of valid event names here. https://reactjs.org/docs/events.html#supported-events

An event listener attribute's value should be a function. The above example would only work if myFunc were a valid function that had been defined elsewhere:

function myFunc() {
  alert('Make myFunc the pFunc... omg that was horrible i am so sorry');
}

<img onClick={myFunc} />

Note that in HTML, event listener names are written in all lowercase, such as onclick or onmouseover. In JSX, event listener names are written in camelCase, such as onClick or onMouseOver.

JSX Conditionals: If Statements That Don't Work

You've learned how to use curly braces to inject JavaScript into a JSX expression!

Here's a rule that you need to know: you can not inject an if statement into a JSX expression.

This code will break:

(
  <h1>
    {
      if (purchase.complete) {
        'Thank you for placing an order!'
      }
    }
  </h1>
)

The reason why has to do with the way that JSX is compiled. You don't need to understand the mechanics of it for now, but if you're interested then you learn more here. https://reactjs.org/docs/jsx-in-depth.html

What if you want a JSX expression to render, but only under certain circumstances?
You can't inject an if statement. What can you do?

You have lots of options. In the next few lessons, we'll explore some simple ways to write conditionals(expressions that are only executed under certain conditions) in JSX.

JSX Conditionals: If Staments That Do Work

How can you write a conditional, if you can't inject an if statement into JSX?

Well, one option is to write an if statement, and not inject it into JSX.

The if statement is on the outside, and no JavaScript injection is necessary. (if and else are not injected in between JSX tags) :

if (user.age >= drinkingAge) {
  message = (
    <h1>
      Hey, check out this alcoholic beverage!
    </h1>
  );
} else {
  message = (
    <h1>
      Hey, check out these earrings I got at Claire's!
    </h1>
  );
}

JSX Conditionals: The Ternary Operator

There's a more compact way to write conditionals in JSX: the ternary operator.

The ternary operator works the same way in React as it does in regular JavaScript. However, it shows up in React surprisingly often.

Recall how it works: you write x ? y : z, where x, y, and z are all JavaScript expressions. When your code is executed, x is evaluated as either "truthy" or "falsy". If x is truthy, then the entire ternary operator returns y. If x is falsy, then the entire ternary operator returns z.

Here's how you might use the ternary operator in a JSX expression:

const headline = (
  <h1>
    { age >= drinkingAge ? 'Buy Drink' : 'Do Teen Stuff' }
  </h1>
);

In the above example, if age is greater than or equal to drinkingAge, then headline will equal Buy Drink. Otherwise, headline will equal Do Teen Stuff.

JSX Conditionals: &&

Like the ternary operator, && is not React-specific, but it shows up in React surprisingly often.

&& works best in conditionals that will sometimes do an action, but other times do nothing at all.

Here's an example:

const tasty = (
  <ul>
    <li>Applesauce</li>
    { !baby && <li>Pizza</li> }
    { age > 15 && <li>Brussels Sprouts</li> }
    { age > 20 && <li>Oysters</li> }
    { age > 25 && <li>Grappa</li> }
  </ul>
);

If the expression on the left of the && evaluates as true, then the JSX on the right of the && will be rendered.
If the first expression is false, however, then the JSX to the right of the && will be ignored and not rendered.

.map in JSX

The array method .map() comes up often in React. It's good to get in the habit of using it alongside JSX.

If you want to create a list of JSX elements, then .map() is often your best bet. If can look odd at first:

const strings = ['Home', 'Shop', 'About Me'];

const listItems = strings.map(string => <li>{string}</li>);

<ul>{listItems}</ul>

In the above example, we start out with an array of strings. We call .map() on this array of strings, and the .map() call retruns a new array of li.

On the last line of the example, note that {listItems} will evaluate to an array, because it's the returned value of .map()!
JSX li's don't have to be in an array like this, but they can be.

// This is fine in JSX, not in an explicit array:

<ul>
  <li>item 1</li>
  <li>item 2</li>
  <li>item 3</li>
</ul>

// This is also fine!

const liArray = [
  <li>item 1</li>, 
  <li>item 2<li>, 
  <li>item 3</li>
];

<ul>{liArray}</ul>

Keys

When you make a list in JSX, sometimes your list will need to include something called keys:

<ul>
  <li key="li-01">Example1</li>
  <li key="li-02">Example2</li>
  <li key="li-03">Example3</li>
</ul>

A key is a JSX attribute. The attribute's name is key. The attribute's value should be something unique, similar to an id attribute.

keys don't do anything that you can see! React uses them internally to keep track of lists. If you don't use keys when you're supposed to, React might accidentally scramble your list-items into the wrong order.

Not all list need to have keys. A list need keys if either of the following are true:

• The list-items have memory from one render to the next. For instance, when a to-do list renders, each item must "remember" whether it was checked off. The items shouldn't get amnesia when they render.
• A list's order might be shuffled. For instance, a list of search results might be shuffled from one render to the next.

If neither of these conditions are true, then you don't have to worry about keys. If you aren't sure then it never hurts to use them!

+) Each key must be a unique string that React can use to correctly pair each rendered element with its corresponding item in the array.

React.createElement

You can write React code without using JSX at all!

The majority of React programmers do use JSX, and we will use it for the remainder of this tutorial, but you should understand that it is possible to write React code without it.

The following JSX expression:

const h1 = <h1>Hello world</h1>;

can be rewritten without JSX, like this:

const h1 = React.createElement(
  "h1",
  null,
  "Hello, world"
);

When a JSX element is compiled, the compiler transforms the JSX element into the method that you see above: React.createElement(). Every JSX element is secretly a call to React.createElement().

We won't go in-depth into how React.createElement() works, but you can start with here if you'd like to learn more: https://reactjs.org/docs/react-api.html#react.createelement