<info 340/>

Intro to React

Joel Ross
Spring 2023

View of the Day

ES6 Modules (code demo)
Hello React! (lecture)
JSX (code demo)
Components (code demo) [if time]
Props (code demo) [if time]

ES 6 Modules

Modules are self-contained, isolated scripts that are able to run in their own namespace (without polluting the global environment). Modules can export values that may be imported and used by other modules.

Web pages load modules by including a script tag with the type="module" attribute.

<!-- load a script as a module -->
<script type="module" src="path/to/module.js"></script>

Live-Server

cd path/to/project
npx live-server .

where index.html is!

dot to serve entire folder

When an HTML page is opened with the file:// protocol, JavaScript cannot access system files. You need to run a local web server instead to use the http:// protocol.
You can do this with the live-server package

Access the webpage at http://localhost:8080/index.html

Stop the server by using ctrl+c in the command shell.

Importing

It is "possible" to import external libraries and make them available in your script from JavaScript (not HTML)

CommonJS (used by Node.js)

ES 6 Modules (used by browsers)

const util = require('java-util');

node version of "import"

global to refer
to the library

module library name

import { ArrayList } from 'java-util'; //named import

module library name

Java

import java.util.ArrayList;

module name

variable to import

Python

from java-util import ArrayList

Exporting Values

export variables, functions, and classes to be available for other modules to import.

/*** my-module.js ***/
export function foo() { return 'foo'; } //named export

export const bar = "bar"; //export a variable

//will not be available (a "private" function)
function baz() { return 'baz'; }

/*** index.js ***/
//example of named imports:
import {foo} from './my-module.js'; //single named import
import {foo, bar} from './my-module.js'; //multiple named import
foo() //=> 'foo'
console.log(bar) //=> 'bar'

relative path to file (start with ./)
.js extension is implied by default in Node

Importing Options

Use the as keyword to "alias" a value.

Use import * syntax to import all values from a module.

/*** my-module.js ***/
export function foo() { return 'foo'; } //"named" export

//provide an "alias" (consumer name) for value
export { bar as yourBar };

/*** index.js ***/
import { yourBar } from './my-module.js'; //import value by name
yourBar() //=> 'bar'

import { bar } from './my-module.js'; //error, no value `bar` exported

//provide "alias" for value when importing!
import {foo as myFoo} from './my-module.js';
myFoo(); //=> 'foo'

//import everything that was exported
//loads as a single object with values as properties
import * as theModule from './my-module.js'; 
theModule.foo(); //=> 'foo'
theModule.yourBar(); //=> 'bar'

Default Exports

Each module can have a single default export, which provides a shortcut when importing.

/*** my-module.js ***/
export default function sayHello() {
    return 'Hello world!';
}

/*** index.js ***/

//default import can assign own alias without needing name!
//think: "import {default as greet} from './mymodule.js'"
import greet from './my-module.js';

greet(); //=> "Hello world!"

Be careful about whether an export is named or default!

External libraries especially are not consistent.

When to export?

Only export a value if it must be used by another module. When in doubt, keep values private.
In general, prefer named exports. This makes it easier to export additional values later if needed.
If a module is designed around a single value or function (e.g., a single React Component), make it a default export. A single exported value isn't necessarily a default export!

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

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

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.

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
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 & 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)

use parentheses for JSX on multiple lines

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
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'

//declare a function component
export default function App(props) {
  return (
    <HelloMessage /> {/* render imported Component */}
  )
}

/*** in index.js ***/
import App from './App' //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!

Read/Review Ch 15: React
Problem Set 06 due Friday
Problem Set 07 due 10 days after that
- This is a big one!
- Get started early; the project draft is due at the same time

<info 340/>

Intro to React

Joel Ross Spring 2023

View of the Day

ES6 Modules (code demo)

Hello React! (lecture)

JSX (code demo)

Components (code demo) [if time]

Props (code demo) [if time]

ES 6 Modules

Live-Server

Importing

CommonJS (used by Node.js)

ES 6 Modules (used by browsers)

Java

Python

Exporting Values

Importing Options

Default Exports

When to export?

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?

Ethical Consumption of Libraries?

Creating DOM Elements

Hello React

JSX

shortcut syntax for React.createElement()

Transpiling

Create React App

JSX & Inline Expressions

React Components

React Components

React Components

NEVER CALL A COMPONENT FUNCTION WITH ()

ALWAYS RENDER AS A COMPONENT WITH <>

Composing Components

Component Modules

Properties (props)

Properties (props)

Properties (props)

Props and Composition

Action Items!

Action Items!

info340sp23-react-intro

More from Joel Ross

Joel Ross
Spring 2023

Properties (`props`)

Properties (`props`)

Properties (`props`)