Introduction to

web apps & React

Lesson 3: Intro to React

November 2019

Amsul Naeem

twitter.com/amsul_

slides.com/amsul

HTML, Javascript, & React

Topics

React Components

React RenderING

First of all...

HTML, JavaScript, & React

  • HTML & JavaScript are languages
  • React is a UI library

React uses JavaScript & HTML to keep your application UI in sync with your application state.

HTML

HTML, JAVASCRIPT, & REACT

Elements

REACT

Elements

<div>Hi there!</div>
const element = <div>Hi there!</div>

Note the assignment

Attributes

<div id="Title">Hi there!</div>

Props (short for properties)

const element = <div id='Title'>Hi there!</div>

Rendering & Updates

  • Manual
  • Managed with JavaScript
    • DOM querying
    • DOM updating

Rendering & Updates

  • Automatic
  • All in JavaScript
    • No querying
    • Declarative updating

React *Uses* JavaScript

HTML, JAVASCRIPT, & REACT

JSX !== JavaScript

&& JSX !== HTML

JSX is an additional syntax on top of JavaScript to mimic HTML.

JSX compiles to JavaScript in a build step.

var element = React.createElement("div", null, "Hi there!");

Why use React?

HTML, JAVASCRIPT, & REACT

  • DOM updates are slow
    • Updating elements for every change
    • Recalculate UI for every update
    • Repaint UI for every pixel change
  • React components are fast
    • Just functions
    • DOM updates in batches
    • Atomic updates

React Components

HTML, JAVASCRIPT, & REACT

const element = <div>Hi there!</div>
function Component() {
  return <div>Hi there!</div>
}

Components encapsulate logic & markup to be reused and return the composed element to be rendered.

Exercise 1

HTML, JAVASCRIPT, & REACT

Recreate form with pre-filled values:

  • 👉 No styling
  • 👉 No change handlers
  • First Name, Last Name
  • Above 19+
  • Diet Restrictions: Vegetarian, Vegan, Halal/Kosher, None
  • Current Year

Starting point:

  • github.com/amsul/george-brown
  • Under the lesson-03 directory

15 minute break

Handlers & State & Hooks

REACT COMPONENTS

  • Handlers: Manage user actions to trigger a state change.
  • State: Maintains a record of the component’s rendered result.
  • Hooks: Create states and handle component lifecycle.

HANDLERS & STATE & HOOKS

REACT COMPONENTS

function Component() {
  // Create some state..

  const handleButtonClick = () => {
    // Do something to the state..
  }

  // Use the state to show the ### count
  return <button onClick={handleButtonClick}>Clicked ### times!</button>
}
function Component() {
  const [clicked, setClicked] = React.useState(0)

  const handleButtonClick = () => {
    // Do something to the state..
  }

  // Use the state to show the ### count
  return <button onClick={handleButtonClick}>Clicked ### times!</button>
}
function Component() {
  const [clicked, setClicked] = React.useState(0)

  const handleButtonClick = () => {
    setClicked(clicked + 1)
  }

  // Use the state to show the ### count
  return <button onClick={handleButtonClick}>Clicked ### times!</button>
}
function Component() {
  const [clicked, setClicked] = React.useState(0)

  const handleButtonClick = () => {
    setClicked(clicked + 1)
  }


  return <button onClick={handleButtonClick}>Clicked {clicked} times!</button>
}

Props are used to:

  • Bind handlers
  • Share state between components
  • Set attributes of an HTML element

Hooks

REACT COMPONENTS

  • State management useState()
  • Component lifecycle useEffect()
    • More on this in future lessons
  • Other built-in hooks as well

Custom Hooks

  • Wrappers for custom logic around existing hooks
  • Naming convention: use*

React Dev Tools

REACT COMPONENTS

Exercise 2

REACT COMPONENTS

Refactor previous exercise with state & handlers:

  • Define state within correct component(s)
  • Pass props to correct components
  • Add side-effects
    • Hooks
    • Event handlers
      • HINT: Current year requires different handling
  • (Optional) Add gender select menu 

5 minute break

Top-down Approach

React Rendering

  • Initiated from state changes
  • Every subcomponent re-renders
  • Atomic DOM updates
    • React has an internal Virtual DOM
    • Not every re-rendered component results in DOM updates
    • V-DOM diffs the resulting tree to do atomic updates in batches

Rules Of Hooks

REACT RENDERING

  • Only at top-level scope
    • Not inside for-loops
    • Not inside if-statements
    • Not after an early return
  • Only from React Functions
    • Not within React Class Components
    • Can be wrapped easily

Conditional Rendering

React Rendering

Using if statements

Using && conditionals

Using ternary (cond ? a : b) operators

function Component({ someCondition }) {
  if (someCondition) {
    return <div>Condition met</div>
  }
  return <div>Condition not met</div>
}
function Component({ someCondition }) {
  return (
    <div>
      <div>Some content</div>
      {someCondition && <div>Condition met</div>}
    </div>
  )
}
function Component({ someCondition }) {
  return (
    <div>
      {someCondition ? <div>Condition met</div> : <div>Condition not met</div>}
    </div>
  )
}

Looped Rendering

React Rendering

function Component() {
  const list = [
    /*list of items*/
  ]
  return (
    <ul>
      {list.map(item => (
        <li key={item.id}>{item.title}</li>
      ))}
    </ul>
  )
}

Un-Rendering & Un-Mounting

React Rendering

  • Both remove element from DOM
  • Un-rendering: maintains component logic
  • Un-mounting: resets component logic
function ParentComponent() {
  const [someCondition, setSomeCondition] = React.useState(false)
  if (someCondition) {
    return <SubComponentOne />
  }
  return <SubComponentTwo />
}

Exercise 3

React Rendering

Read “Thinking in React”:

End of Lesson

Lesson 3: Intro to React

November 2019

Amsul Naeem

twitter.com/amsul_

slides.com/amsul

Lesson 3: Intro to React

By amsul

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Lesson 3: Intro to React

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