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Intro to React

Joel Ross
Autumn 2024

View of the Day

  • Hello React! (lecture)

  • JSX (code demo)

  • Components (code demo)

  • Props (code demo) [if time, or check video!]

Any lingering JavaScript Questions?

Updating Lecture Code

# switch to starter branch to get new starter code
git checkout starter

# download new starter code
git pull

# switch back to main branch for coding
git checkout main

# merge in new starter code (use default msg)
git merge starter --no-edit

# code and enjoy!

Get the starter code from the starter branch, but do all of your work on main.

https://reactjs.org/

  • Dynamically generate and interact with the DOM
     

  • Organize the DOM into User Interface "Components" for easy page design
     

  • Efficiently make changes to the rendered DOM (see here)

Ethical Consumption of Libraries?

source, source, source, source, source, source

Creating DOM Elements

//DOM - element to show
const msgElem = document.createElement('h1');
msgElem.id = 'hello';
msgElem.classList.add('myClass');
msgElem.textContent = 'Hello World!';





//show the content in the web page 
//(inside #root)
document.getElementById('root').appendChild(msgElem);

Hello React

//React - element to show
const msgElem = React.createElement(
  //html tag
  'h1',
  //object of attributes
  { id: 'hello', className: 'myClass' },
  //content
  'Hello World!'
); 

//Create a "React root" out of the `#root` elemment
//then render the React element at that root
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(msgElem)

can't use "class" since a keyword

React can be used to create and render DOM elements.

React v18 (March 2022)

JSX

An XML syntax extension for the JavaScript language. You define React elements in a way that looks like HTML!

//JSX - element to show
const msgElem = <h1 id="hello" className="myclass">Hello World</h1>;








//Create a "React root" out of the `#root` elemment
//then render the React element at that root
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(msgElem)

shortcut syntax for React.createElement()

Transpiling with Vite

Since JSX is not actually valid JavaScript, we need to "translate" it into real JavaScript, called transpiling.

 

There are a number of tools that can do this work; we'll be using a build tool called Vite.js

https://vite.dev/

Vite Dev Server

Vite provides a development server which will:

  • Automatically transpile React code into pure JavaScript
  • Manage module dependencies, including external libraries
  • Show build and syntax errors in the console
  • Automatically reload the page (replaces live-server)!
# Make sure you are in the project directory
cd path/to/project

# Install dependencies for existing project (installs vite)
npm install

# Run the development server script
npm run dev

JSX

Elements defined using JSX can include children.

const headerElem = (
  <header>
    <h1>Hello world!</h1>
    <p>This is a fine demo</p>
  </header>
);

//values need to have a "single" parent
const invalidCode = (
  <p>First sentence</p>
  <p>Second sentence</p>
)

const validCode = (
  <> {/* a fragment: a parent that won't be rendered */}
    <p>First sentence</p>
    <p>Second sentence</p>
  </>
);

use parentheses for JSX on multiple lines

a comment in JSX!

Inline Expressions

Use {} to include JavaScript expressions in the JSX. These expressions will be evaluated and inserted into the element's "HTML".

//Can include JavaScript expressions in React elements
const message = "Hello world!";
const element = <h1>{message}</h1>;


//Can include arbitrary expressions
const element = (
  <p>
    A leap year has {(365 + 1) * 24 * 60} minutes!
  </p>
);

//Can use inline expressions in attributes
const imgUrl = 'path/to/my_picture.png';
const pic = <img src={imgUrl} alt="A picture" />;

replace with expression (value)

React elements
must be closed

React Components

React lets us describe the page in terms of UI components, instead of HTML elements.

 

React Components

React lets us describe the page in terms of UI components, instead of HTML elements.

In effect, we will create our own XML Elements! 

<App>
  <HomePage>
    <Header />
    <SearchBar />
    <EmployeeList>
      <EmployeeListItem person="James King" />
      <EmployeeListItem person="Julie Taylor" />
      <EmployeeListItem person="Eugene Lee" />
    </EmployList>
  </HomePage>
  <EmployeePage>
    <Header />
    ...
  </EmployeePage>
</App>

React Components

We define components as functions that return the DOM elements to be rendered

//declare a function to define a component -- this is like a class
function HelloMessage(props) {

  
  //this function returns the elements (JSX)
  //that make up the component
  return (
     <h1>Hello World!</h1>
  );
}

//"call" function to create a new element value!
const msgElem = <HelloMessage />;

//show the content in the web page (inside #root)
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(msgElem)

what is rendered when
component is shown

our own HTML tags!

Capitalize!

NEVER CALL A COMPONENT FUNCTION WITH ()

 

ALWAYS RENDER AS A COMPONENT WITH <>

Composing Components

function HelloMessage(props) {
   return <p>Hello World!</p>;
}

function GoodbyeMessage(props) {
   return <p>See ya later!</p>;
}

function MessageList(props) {
   return (
      <div>
         <HelloMessage /> {/* A HelloMessage component */}
       	 <GoodbyeMessage /> {/* A GoodbyeMessage component */}
      </div>
   );
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(msgElem)

Components can render other components ("call" functions to create new elements), and mix those with regular DOM elements

comments in JSX

Component Modules

Components are usually defined in separate modules (files), and then imported by modules that need to use them.

/*** in App.js ***/

//import from other components; HelloMessage.js, Messages.js, etc
import { HelloMessage } from './Messages.js'

//declare a function component
export default function App(props) {
  return (
    <HelloMessage /> {/* render imported Component */}
  )
}
/*** in index.js ***/
import App from './App.js' //default import

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />)

Properties (props) 

//Passing a prop called `message` with value "Hello property"
const messageA = <MessageItem message="Hello property!" />;

//A component can accept multiple props
//This component takes in a `name` property as well as 
//a `descriptor` property
const userInfo = <UserInfo name="Ethel" descriptor="Aardvark" />;

//Passing a value as a prop using an inline expression
const secret = "Shave and a haircut";
const messageB = <MessageItem message={secret} />;

We specify attributes for a component (called "props") when we instantiate a component by specifying the XML attributes (key-value).

Props are the "input parameters" into a component!

Properties (props) 

function MessageItem(props) {
   const message = props.message; //access the prop



    //can use prop for logic or processing
   const messageUpper = message.toUpperCase(); 

   return <li>{messageUpper}</li>; //render based on prop
}

ReactDOM.createRoot(document.getElementById('root'))
   .render(<MessageItem message="Be quiet" />)

Inside the Component function definition, all the passed in props are passed in as a single argument object (conventionally called props). This object is the collection of all of the attributes/arguments.

ALL props stored in this object

Properties (props) 

//Pass an array as a prop!
const array = [1,2,3,4,5];
const suitcaseElem = <Suitcase luggageCombo={array} />;


//Pass a function as a prop (like a callback)!
function sayHello() { 
  console.log('Hello world!');
}
const greetingElem = <Greeting callback={sayHello} />;

Importantly, props can be any kind of variable! This includes arrays, objects and functions

Props and Composition

function MessageList(props) {
  //msgComponents will be an array of components!
  const msgComponents = props.messages.map((msgStr) => {
    const elem = <MessageItem message={msgStr} key={msgStr} />; //pass prop down!
    return elem
  }

  return (
    <ul>
      {/* An array of components renders as siblings */}
      {msgComponents} 
    </ul>
  );
}

const messagesArray = ["Hello world", "No borders", "Go huskies!"];

ReactDOM.createRoot(document.getElementById('root'))
   .render(<MessageList messages={messagesArray} />)

Props will often need to be "passed down" to child components. A common pattern is to map an array of prop values to an array of children components to render!

unique "id" for the element

Action Items!

Action Items!

  • Complete task list for Week 6 (all items!)

  • Problem Set 06 due Friday

    • ​Get it done early so you can focus on React!​

  • Problem Set 07 due in 2 weeks

    • This is the biggest one!

    • Get started early; project draft 2 is due right afterwards

 

Next time: More React -- working with Components