What is React.js?

React.js is a JavaScript library for building user interfaces. It is a front-end web framework. React.js breaks down the user interface into customizable components. It combines pieces of HTML, CSS, and JavaScript in each component. It allows the browser to refresh a single component without having to refresh the entire page. It is one of the most popular and in-demand web frameworks

Most people put all of their body content within a div with the id “root.” This is React convention.

<div id="root">

Before we can start coding, we must install our dependencies

npm i react react-dom
ReactDOM.render(WHAT TO SHOW, WHERE TO SHOW it, optional callback after render function completed)
var React = require("react");
var ReactDOM = require("react-dom");

ReactDOM.render(<h1>Hello world!</h1>, document.getElementById("root"));

React works by creating these JSX files. HTML is inputted into compiler which converts it into Vanilla JavaScript. The compiler comes from including the React module.

Inside the React Module, there is something called Babel. Babel is a JavaScript compiler. It’s able to take next-gen JavaScript (es6, es7, es8) and compile it down to a version that any browser can understand. This includes compiling JSX into plain JS.

var h1 = document.createElement("h1");
h1.innerHTML = "Hello World!";
import React from "react";
import ReactDOM from "react-dom";

Render method can only take a single HTML element. If you put two back-to-back, it will crash. The way around this is wrapping n elements into a div, so that it’s “1 element.”

   <h1>Hello world!</h1>
   <p>This is the first paragraph.</p>
const name = "Joshua";

ReactDOM.render(<h1>Hello {name}!</h1>, document.getElementById("root"));
const num = 4;

ReactDOM.render(<p>Your lucky number is {num}!</p>, document.getElementById("root"));
ReactDOM.render(<p>Your lucky number is {Math.floor(Math.random() * 10)}!</p>, document.getElementById("root"));

You can add any JavaScript value or expression inside the curly braces, but you can’t write JavaScript statements(if statements, etc.)


External Stylesheets

<h1 class="heading">Title</h1> // no 
<h1 className="heading">Title</h1> // yes

Although it looks like HTML, it’s JSX. It’s being converted to Vanilla JavaScript. There is no “class” property in JS; the correct JS equivalent is the className property.

HTML uses all lowercase. When we write JSX we use camel case because JS uses camel case.

contenteditable vs contentEditable

<h1 contentEditable="true" spellCheck="false">My Favourite Foods</h1>

It is recommended to do all of your styling in an external CSS file then apply those styles to your JSX content with class and id tags.

.heading {
   color: 'red';
<h1 className="heading">My Favorite Foods</h1>

Inline Styling

The style property requires a JavaScript object

JavaScript object

  key: value,
<h1 style="color: 'red'">My Title</h1> // no
<h1 style={{color: 'red'}}>My Title</h1> // yes

Any JavaScript within HTML must be wrapped in curly braces. JavaScript objects must be wrapped in curly braces. Thus, 2 sets of curly braces

const customStyle = {
  color: "red",         // use commas instead of semicolons
  fontSize: "20px",     // font-size becomes fontSize
  border: "1px solid #000" // every value goes in quotes

customStyle.color = "blue";  // change styles after definition

    <h1 style={customStyle}>My Favourite Foods</h1>

React Components

We always capitalize the names of our components because that’s how React differentiates that they are custom components

function Heading() {
   return <h1>My Heading!</h1>

      <Heading />

In the same folder, create a new file called “Heading.jsx”

All of our components separated into individual files with the .jsx extension

import React from "react";

function Heading() {
   return <h1>My Heading!</h1>;

export default Heading;

Don’t use parentheses. That will make it run immediately; we want to use it as a component

import React from "react";
import ReactDOM from "react-dom";
import Heading from "./Heading";

Most times, you won’t see any div or child elements in the index.js file. Instead, it will just be a single App custom component.

import React from "react";
import Heading from "./Heading";   // child component
import List from "./List";         // child component

function App(){
   return <div>
      <Heading />
      <List />

export default App;
   <App />,

Create a components folder and put all of your components in it

  • src
    • components
      • App.jsx
      • Heading.jsx
      • List.jsx
  • public
    • index.html
    • styles.css
import App from "./components/App";

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