class vs className
class! You have to use className instead: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
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:
<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:
<br />
// NOT FINE AT ALL in JSX:
<br>
JavaScript In Your JSX In Your JavaScript
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.
Q) Why would we want to put JavaScript in our JSX?
Ans) We want to use JavaScript in our JSX to render logic! When we inject JS into JSX we can make this process of rendering logic (based on things like data, events, and data changing over time) more seamless by putting our markup, the HTML part of JSX, that is based on our logic, the JS part of JSX, together in the same file.
import React from 'react';
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container);
// Write code here:
root.render(<h1>2 + 3</h1>);Curly Braces in JSX
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.
import React from 'react';
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container);
// Write code here:
root.render(<h1>{2 + 3}</h1>);Variables 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.
// Declare a variable:
const name = 'Gerdo';
// Access your variable 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 for 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 means constantly working with event listeners.
You create an event listener by giving a JSX element a special attribute. Here’s an example:
<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 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:
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.
Question
Why don’t we set the event listener attribute value to a function call?
Answer
If we set the event listener attribute value to a function call, the function will get called automatically on the page load (when our JSX element renders to the browser) instead of listening for the event and then calling the function.
For example:
import React from 'react';
import ReactDOM from 'react-dom';
function switchHeading() {
console.log('switchHeading was called!');
}
const heading = (
<h1 onClick={switchHeading}>Click Me!</h1> // onClick will listen for the click event, then will call switchHeading. When that occurrs we will see 'switchHeading was called!' logged to the console
// if <h1 onClick={switchHeading()}>Click Me!</h1> is used instead, and we set onClick to a function call, we will see 'switchHeading was called!' logged to the console immediately when our JSX renders and not on the click event
)
ReactDOM.render(heading, document.getElementById('app'));import React from 'react';
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container);
function makeDoggy(e) {
// Call this extremely useful function on an <img>.
// The <img> will become a picture of a doggy.
e.target.setAttribute('src', 'https://content.codecademy.com/courses/React/react_photo-puppy.jpeg');
e.target.setAttribute('alt', 'doggy');
}
const kitty = (
<img
src="https://content.codecademy.com/courses/React/react_photo-kitty.jpg"
alt="kitty" onClick={makeDoggy} />
);
root.render(kitty);JSX Conditionals: If Statements That Don't Work
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:
(
<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?import React from 'react';
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container);
function coinToss() {
// This function will randomly return either 'heads' or 'tails'.
return Math.random() < 0.5 ? 'heads' : 'tails';
}
const pics = {
kitty: 'https://content.codecademy.com/courses/React/react_photo-kitty.jpg',
doggy: 'https://content.codecademy.com/courses/React/react_photo-puppy.jpeg'
};
let img;
// if/else statement begins here:
if (coinToss() == 'heads') {
img = <img src={pics.kitty} />
} else {
img = <img src={pics.doggy} />
}
root.render(img);
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>
);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: &&
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:
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>
);&& 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 .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:
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.
// 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
<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:
- 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 is true, then you don’t have to worry about keys. If you aren’t sure, then it never hurts to use them!
React.createElement
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:
const h1 = <h1>Hello world</h1>;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() to learn more.
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Thank you :)