Intro to React

Joel Ross

Getting Started with React

Joel Ross

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

Ethical Consumption of Libraries?

source, source, source, source, source, source

Create React App

create-react-app is a command line application that generates scaffolding ("starter code") for a React website.

# EITHER create a new react app (only once!)
npx create-react-app app-name --use-npm

# OR install dependencies for existing project
cd path/to/project
npm install

# run the server
npm start

Runs a script that starts a development server which will:

Automatically transpile React code into pure JavaScript
Combine (bundle) different modules into a single file
Show build and syntax errors in the console, including style warnings
Automatically reload the page (replaces live-server)!

JSX

Joel Ross

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

Since JSX is not actually valid JavaScript, we need to "translate" it into real JavaScript, using the Babel compiler. Transpiling and other build steps are handled by the webpack bundler.

JSX

CSS

JPG

CSS

JPG

combined

minimized

compressed

etc.

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

Joel Ross

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>

Components

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 />)

React: Props

Joel Ross

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` prop as well as 
//a `descriptor` prop
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 collected 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!

Intro to React

Joel Ross

Getting Started with React

Joel Ross

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

Ethical Consumption of Libraries?

Create React App

JSX

Joel Ross

Creating DOM Elements

Hello React

JSX

shortcut syntax for React.createElement()

Transpiling

JSX

Inline Expressions

React: Components

Joel Ross

Components

Components

React Components

NEVER CALL A COMPONENT FUNCTION WITH ()

ALWAYS RENDER AS A COMPONENT WITH <>

Composing Components

Component Modules

React: Props

Joel Ross

Properties (`props`)

Properties (`props`)

Properties (`props`)

Props and Composition

info340-react-intro

info340-react-intro

Joel Ross PRO

Intro to React

Joel Ross

Getting Started with React

Joel Ross

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

Ethical Consumption of Libraries?

Create React App

JSX

Joel Ross

Creating DOM Elements

Hello React

JSX

shortcut syntax for React.createElement()

Transpiling

JSX

Inline Expressions

React: Components

Joel Ross

Components

Components

React Components

NEVER CALL A COMPONENT FUNCTION WITH ()

ALWAYS RENDER AS A COMPONENT WITH <>

Composing Components

Component Modules

React: Props

Joel Ross

Properties (props)

Properties (props)

Properties (props)

Props and Composition

info340-react-intro

More from Joel Ross

Properties (`props`)

Properties (`props`)

Properties (`props`)