More course study notes from moving through React - The Complete Guide section 3.

Why have I seen both filenames .js and .jsx in React?
Both work fine, but .js is convention these days.

Not technically HTML, it’s JSX

return (
  <div className="App">
    <h1>I'm a React app</h1>

That ☝️ get’s compiled to something like this 👇

return React.createElement('div', null,
       React.createElement('h1', {className: 'App'}, 'I\'m a React app')

Cool exercise in the course, rewriting the JSX-ish "HTML" from the first example to the second.

This gave me a lot of context for the concept of syntactic sugar:

syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an alternative style that some may prefer.

  • JSX is syntactic sugar for JavaScript
  • It allows me to write HTML-ish code
  • …without having to grapple with nested React.createElement(...) calls

🤯 Soooo… this is in fact why a front-of-the-frontend developer like me has been able to be productive in React applications without actually knowing a lot of neither js or react. I think I’m writing HTML, but React is converting it behind the scenes.

Restrictions of JSX

Here is some of the stuff that has annoyed me about React in the past, and understanding properly why helps to accept different syntax. It’s not just because library authors “opted to be weird”, there are technical restrictions with reasons that I can now learn.

  • className because class is a reserved name in JS
  • one root element, because the JSX expression requires it

Capitalize name of custom components

  • Uncapitalized components are reserved for native HTML elements
  • Uppercase first letter in my own components, like <Person>
  • React will then identify it as a custom component

Two ways of creating components

Functional components are also know as "presentational", "dumb" or "stateless" components. They are created for example by using using ES6 arrow functions.

Class-based components are called "containers", "smart" or "stateful" components. I can create them with using a ES6 class to extend React’s component.

(Currently confused why the instructor calls the first as best practice. If I only need presentational components, why do I need React?! But I’m sure this will become clear as I proceed.)

Use props

  1. Receive props in the component: const person = (props) => {...}
  2. Output props dynamically: <p>I’m {} and {props.age} years old</p>
  3. Pass props as arguments: <Person name="Alice" age="37" />

Parentheses for multiple code lines

  return <p>whatever on a single line of code</p>
  return (
      <p>whatever across
          multiple lines as needed

Related: Adjacent JSX elements need to be wrapped in an enclosing tag. This I am used to from before, and I’ve even bumped into the syntax for fragments to group a list of children inside <React.Fragment> and new short syntax with empty tags <></>.

Give props children

Here is how I can pass content as props directly where my component is being used. Simple text here, but this feature becomes useful when it’s complex html.

  return (
      <p>I’m {} and {props.age} years old
        <span> {props.children}</span>
  <Person name="Carmen" age="66">
    and I love cars