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Advanced JSX

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

Here's an example of how that might work:

```js
// 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 for one element.

*Object properties* are also often used to set attributes: 

```js
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" />
); 
```

Variable Attributes in JSX

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

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

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

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

Variables in JSX

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

One option is to write an `if` statement and *not* inject it into JSX.

Look at **if.js**. Follow the `if` statement, all the way from line 8 down to line 20. 

**if.js** works because the words `if` and `else` are *not* injected in between JSX tags.  The `if` statement is on the outside, and no JavaScript injection is necessary.

This is a common way to express conditionals in JSX.

JSX Conditionals: If Statements That Do Work

The code in the last exercise didn'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>` and `</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 JavaScript and not like JSX."

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

Curly Braces in JSX

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:

```js
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 `<h1>Buy Drink</h1>`. Otherwise, `headline` will equal `<h1>Do Teen Stuff</h1>`.

JSX Conditionals:  The Ternary Operator

The `.map()` array method 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 using `.map()` is often the most efficient way. It can look odd at first:

```js
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 returns a new array of `<li>`s.

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.

```js
// 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>
``` 

.map in JSX

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

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

```js
<img onClick={clickAlert} />
```

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. Look through the [common components list in React docs](https://react.dev/reference/react-dom/components/common#) to browse supported event names.

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

```js
function clickAlert() {
  alert('You clicked this image!');
}

<img onClick={clickAlert} />
```

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`. 

Event Listeners in JSX

So far, we've focused on writing JSX expressions. It's similar to writing bits of HTML, but inside of a JavaScript file.

In this lesson, we're going to add something new: regular JavaScript, written inside of a JSX expression, written inside of a JavaScript file.

JavaScript In Your JSX In Your JavaScript

Congratulations! You have learned a wide variety of JSX concepts. If you don't feel like you've mastered them all yet, that's okay! These concepts will come up again and again throughout your React learning journey.
 
You are now ready to put your JSX knowledge to use!

Review

Great 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:
```js
(
  <h1>
    {
      if (purchase.complete) {
        'Thank you for placing an order!'
      }
    }
  </h1>
)
```
 
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 Statements That Don't Work

Another common JSX error involves *self-closing tags*.
 
What's a self-closing tag?

Most HTML elements use two tags: an *opening tag* (`<div>`), and a *closing tag* (`</div>`). 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 or 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:

```html
// 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:

```html
// Fine in JSX:
<br />

// NOT FINE AT ALL in JSX:
<br>
```

Self-Closing Tags

This lesson will cover more advanced JSX. You'll learn some powerful tricks and some common errors to avoid.

Grammar in JSX is mostly the same as in HTML, but there are subtle differences to watch out for. The most frequent of these involves the word `class`.
 
In HTML, it's common to use `class` as an attribute name:

```html
<h1 class="big">Title</h1>
```
 
In JSX, you can't use the word `class`! You have to use `className` instead:

```js
<h1 className="big">Title</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.

class vs className

You can now inject regular JavaScript into JSX expressions! This will be extremely useful.

In the code editor, you can see a JSX expression that displays the first twenty digits of pi.
 
Study the expression and notice the following:  
- The code is written in a JavaScript file. By default, it will all be treated as regular JavaScript.  
- Find `<div>` on line 5. 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 or 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.

20 Digits of Pi in JSX

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

```js
<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 visible! 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 lists need to have `keys`. A list needs `keys` if either of the following is true:

1. 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.
2. 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 is true, then you don't have to worry about `keys`. If you aren't sure, then it never hurts to use them!

Keys

You can write React code without using JSX at all!
 
The majority of React programmers do use JSX, but you should understand that it is possible to write React code without it.

The following JSX expression:

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

can be rewritten without JSX, like this:

```js
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 check out [the React documentation on `createElement()`](https://react.dev/reference/react/createElement) to learn more.

React.createElement

We're going to cover one final way of writing conditionals in React: the `&&` operator.

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

In the last two exercises, you wrote statements that would sometimes render a kitty and other times render a doggy. `&&` would *not* have been the best choice for that code.

`&&` works best for conditionals that will sometimes do an action but other times do *nothing at all*. 
 
Here's an example: 

```js
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.