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Intro to JSX
Lesson 1 of 2
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  1. 1
    React.js is a JavaScript library. It was developed by engineers at Facebook. Here are just a few of the reasons why people choose to program with React: - React is fast. Apps made in React …
  2. 2
    Take a look at the following line of code: const h1 = Hello world ; What kind of weird hybrid code is that? Is it JavaScript? HTML? Or something else? It seems like it must be JavaScript,…
  3. 3
    Good! Take another look at the line of code that you wrote. Does this code belong in a JavaScript file, an HTML file, or somewhere else? The answer is…a JavaScript file! Despite what it looks…
  4. 4
    JSX is a syntax extension for JavaScript. It was written to be used with React. JSX code looks a lot like HTML. What does “syntax extension” mean? In this case, it means that JSX is not valid…
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    A basic unit of JSX is called a JSX element. Here’s an example of a JSX element: Hello world This JSX element looks exactly like HTML! The only noticeable difference is that you would find i…
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    JSX elements are treated as JavaScript expressions. They can go anywhere that JavaScript expressions can go. That means that a JSX element can be saved in a variable, passed to a function, sto…
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    JSX elements can have attributes, just like HTML elements can. A JSX attribute is written using HTML-like syntax: a name, followed by an equals sign, followed by a value. The value shoul…
  8. 8
    You can nest JSX elements inside of other JSX elements, just like in HTML. Here’s an example of a JSX element, nested inside of a JSX element: Click me! To make this more readable, …
  9. 9
    There’s a rule that we haven’t mentioned: a JSX expression must have exactly one outermost element. In other words, this code will work: const paragraphs = ( I am a paragraph. I,…
  10. 10
    You’ve learned how to write JSX elements! Now it’s time to learn how to render them. To render a JSX expression means to make it appear onscreen.
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    Let’s examine the code that you just wrote. Start in previous.js, on line 5, all the way to the left. You can see something called ReactDOM. What’s that? ReactDOM is the name of a JavaScript…
  12. 12
    Move to the right a little more, and you will see this expression: document.getElementById(‘app’) You just learned that ReactDOM.render() makes its first argument appear onscreen. But where…
  13. 13
    ReactDOM.render()’s first argument should evaluate to a JSX expression, it doesn’t have to literally be a JSX expression. The first argument could also be a variable, so long as that variable…
  14. 14
    One special thing about ReactDOM.render() is that it only updates DOM elements that have changed. That means that if you render the exact same thing twice in a row, the second render will do no…
  15. 15
    Congratulations! You’ve learned to create and render JSX elements! This is the first step towards becoming fluent in React. In the next lesson, you’ll learn some powerful things that you can do w…
  1. 1
    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 wat…
  2. 2
    Another JSX ‘gotcha’ involves 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…
  3. 3
    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, wri…
  4. 4
    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 a…
  5. 5
    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 th…
  6. 6
    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 expr…
  7. 7
    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”…
  8. 8
    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 sp…
  9. 9
    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. Th…
  10. 10
    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. Look at if.js. Follow the if stat…
  11. 11
    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 s…
  12. 12
    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 surprisingly often. In the la…
  13. 13
    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. It can …
  14. 14
    When you make a list in JSX, sometimes your list will need to include something called keys: Example1 Example2 Example3 A key is a JSX attribute. The attribute’s name is key. …
  15. 15
    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 possib…
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    Congratulations! You have completed the unit on JSX. 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 …

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