Intro to React

INFO 253A: Frontend Web Architecture

Kay Ashaolu

Now it is time to learn React

  • You now have a better handle of JavaScript
  • It's time now to learn a new way of developing UI in the browser (and beyond)
  • Note: there are other extensions to React (e.g. React 360)

Review: React is a library

  • React is a JavaScript library for building user interfaces
  • Build declarative components based on the current state of your application
  • Differs from the Event Driven approach we have been doing so far

Example, Event Driven App

  • Lets say you have an array of strings, each representing the subject of a todo
  • And the HTML code was rendered as follows
<ul id="todos">
	<li>Task 1</li>
	<li>Task 2</li>

Example, Event Driven App

  • If you wanted to add a new task on a click of a button, what would you do?
  • If you wanted to delete a task on a click of the task, what would you do?

Example, Event Driven App

  • To add a new task, you would have to select the #todos ul, and then manipulate the html so that there is a new li at the end of the list
  • To delete a new task you would need to find the right li and delete that from the tree

Example, Event Driven App

  • This is doable, but there are some things to consider:
    • What happens if there are multiple todo lists in a single page?
    • What happens if the user tries to add and delete a task at the same time?

Events add up

  • You will need to take several precautions to ensure that each todo list is completely independent and that events do not collide with each other
  • This is not trivial to do for large systems

React: a different approach

  • React uses a declarative programing paradigm
  • Instead of worrying about every action that could happen with your list, you first define what your todo list would look like, given an array of strings.
  • You create a component using your above defintion containing state that contains the titles of all of the tasks
  • On click events, you modify this internal state and the component will update itself

Lets then learn React

  • At this point your browser will not understand your code
  • Reason 1: some browsers do not understand ES6 JavaScript
  • Reason 2: some React syntax is not valid JavaScript

Let's set up your environment

About your dev server

  • The NodeJS server you installed uses NodeJS, Babel, and Webpack, as well as the React codebase to bundle all of your source Javascript in a single file
  • The NodeJS server also builds your single HTML page, as well as keeps a development server running to reload any changes

So without further ado, let's get into React!

Hello World


import React from "react";
import ReactDOM from "react-dom";

const jsx_element = <h1>Hello, world!</h1>;
const dom_element = document.getElementById('root');
ReactDOM.render(jsx_element, dom_element);

Hello World


<!DOCTYPE html>
<html lang="en">
    <div id="root"></div>

Hello World Explained: index.html

  • Your index.html file is an empty file that contains one empty div in the body section
  • Note that the empty div's id is "root"
  • This div is the entry point for our react app: we will tell React in our script to replace this div with our react application
  • This html file will largely remain unchanged

Hello World Explained: index.js

  • We are first importing the React and ReactDOM packages into our JavaScript
  • Next we are telling ReactDOM to render given:
    1. The content of your website
    2. What element in your index.html file that will house your React App

Hello World Explained: JSX

  • JSX stands for JavaScript XML
  • JSX is an extension of JavaScript that enables you to write HTML like syntax directly in your Javascript
  • This enables the ability to write HTML templates directly into your JavaScript code
  • You can also embed expressions, variables, and properties directly into JSX



let formatName = (user) => {
  return user.firstName + ' ' + user.lastName;
const user = {
  firstName: 'Harper',
  lastName: 'Perez'
const element = (
    Hello, {formatName(user)}!

Example explained

  • Note: the index.html has not changed. To use React for your entire app, you can define a single div in the body that is React's entry point
  • We defined a function called formatName that takes a object that has two properties: a firstName and a lastName
  • The formatName function returns a single string with both of those elements
  • We use this function 'formatName' inside of our JSX code (the const element)
  • This shows how you can use these properties within your JSX code

Why JSX?

  • Remember separating content from presentation?
  • Separating HTML (content) from CSS (presentation) is core to the web
  • However once we start using JavaScript, we have the ability to change the HTML rendered on the page
  • That means HTML code can possibly be throughout our JavaScript codebase
  • JSX gives us the ability to write out templated HTML code in a very intuitive fashion

Another Example


import React from "react";
import ReactDOM from "react-dom";

function tick() {
  const element = (
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
  ReactDOM.render(element, document.getElementById('root'));
setInterval(tick, 1000);

Wait, what?

  • Every call to ReactDOM.render tells React to re-render elments given the data that it currently has
  • The code setInterval(tick, 1000) is a special function that tells JavaScript to execute the tick function every 1000 millliseconds.
  • The tick function then defines the element and passes in its properties (namely {new Date().toLocaleTimeString()}) before that components is rendered
  • This is why you see the clock ticking every second


  • This is one of the things I like the most about React
  • The focus on components as independent, resusable pieces that can be placed anywhere
  • This uses the composability relationship: each element can be composed by other elements


  • Using a combination of JSX and JavaScript, you can bundle look and feel and functionality in a single JavaScript class
  • You can consider these React Elements and HTML Elements that you can place wherever you like
  • Let's get into the anatomy of a Component

Component Example

import React from "react";
import ReactDOM from "react-dom";

function FormatName(props) {
	return (
        Hello, {props.firstName} {props.lastName}!

  <FormatName firstName="Kay" lastName="Ashaolu" />,

Component Example Explained

  • We are creating a component by creating a function
  • The name of the function (i.e. FormatName) is the name of the element
  • The React library takes that function and creates a component out of it that can be used¬†


  • A parameter called props is passed into this function
  • props is an object that contains all of the attributes and values that are passed into the element

Component Instance Properties

  • When you add the attribute firstName="Kay", this props object will have a key named "firstName"
  • And a value named "Kay"
  • These properties are immutable
<FormatName firstName="Kay" lastName="Ashaolu" />,


  • What the function returns is the "html" that is generated by the component
  • This function is executed and the "html" is generated in a number of areas in react ¬†(e.g. on a call on ReactDOM.render())
  • This function returns JSX code that the component would render into