Week 8 Lecture Introduction to React

React Introduction¶

Building UIs with Components & TypeScript¶

What We’ll Cover Today¶

Why React exists and what problem it solves
Components: the building block of React
JSX/TSX: writing HTML inside TypeScript
Props: passing data into components
State: making components dynamic
Handling events
Putting it all together

50 min lecture → 60 min lab: you’ll build a working UI

The Problem React Solves¶

Without React: Manual DOM updates

<!-- index.html -->
<div id="count">0</div>
<button onclick="increment()">+</button>

// app.js
let count = 0;
function increment() {
  count++;
  // We must manually find & update
  document.getElementById('count').textContent = count;
}

Every change = find the element, update it, remember to do it everywhere. It scales badly.

The Problem React Solves¶

With React: Describe what it should look like

// Counter.tsx
function Counter() {
  // React tracks this value
  const [count, setCount] = useState(0);

  // Just describe the UI — React handles the DOM
  return (
    <div>
      <p>{count}</p>
      <button onClick={() => setCount(count + 1)}>
        +
      </button>
    </div>
  );
}

¶

Declarative: you say what to show; React figures out how to update the page.

What Is React?¶

A JavaScript/TypeScript library for building user interfaces
Created by Facebook/Meta in 2013
The most widely-used front-end library today
Core idea: UI = f(state)

The UI is a function of your data. Change the data → React updates the screen.

What React is NOT:¶

Not a full framework (no router, no HTTP client built in)
Not a templating engine
Not a replacement for HTML/CSS — it uses them

Setting Up a React + TypeScript Project¶

# Vite is the recommended modern
# way to scaffold a React project
npm create vite@latest my-app \
  -- --template react-ts

cd my-app
npm install
npm run dev

¶

What you get:¶

my-app/
├── src/
│   ├── App.tsx        ← main component
│   ├── main.tsx       ← entry point
│   └── index.css
├── index.html
└── package.json

Entry Point: `main.tsx`¶

// main.tsx
import { StrictMode } from 'react'
import { createRoot } from 'react-dom/client'
import App from './App'

// Find the <div id="root"> in index.html
// and hand it to React
createRoot(document.getElementById('root')!).render(
  <StrictMode>
    <App />
  </StrictMode>
)

React takes over one <div> in your HTML. Everything inside is React’s world.

Components: The Core Idea¶

A component is a reusable piece of UI — like a custom HTML tag you design yourself.

Think of UI as a tree of components:¶

<App>
  ├── <Header />
  ├── <ProductList>
  │     ├── <ProductCard />
  │     ├── <ProductCard />
  │     └── <ProductCard />
  └── <Footer />

Each box = one .tsx file, one function, one job.

Your First Component¶

// Greeting.tsx

// A component is just a TypeScript
// function that returns JSX
function Greeting() {
  return (
    <div>
      <h1>Hello, World!</h1>
      <p>Welcome to React.</p>
    </div>
  );
}

// Always export your component
export default Greeting;

¶

Rules:¶

Function name must start with a capital letter
Must return one root element (wrap in <div> or <>)
File extension is .tsx (TypeScript + JSX)

What Is JSX / TSX?¶

JSX = JavaScript XML. A syntax extension that lets you write HTML-like markup inside TypeScript.

It looks like HTML — but it’s not:¶

HTML	TSX
`class="btn"`	`className="btn"`
`for="name"`	`htmlFor="name"`
`<br>`	`<br />` (self-closing)
`onclick="fn()"`	`onClick={fn}`
`<!-- comment -->`	`{/* comment */}`

JSX compiles down to React.createElement() calls. It’s just a nicer syntax.

Embedding Expressions in TSX¶

Use { } (curly braces) to drop any TypeScript expression into your markup:

function UserCard() {
  const name = "Alex";
  const score = 42;
  const isAdmin = true;

  return (
    <div>
      {/* Variables */}
      <h2>{name}</h2>
      
      {/* Math & expressions */}
      <p>Score: {score * 2}</p>
      
      {/* Ternary (if/else inline) */}
      <p>{isAdmin ? "Admin" : "User"}</p>
    </div>
  );
}

Anything valid as a TypeScript expression can go inside { }.

Rendering Lists¶

function FruitList() {
  const fruits = [
    "Apple",
    "Banana",
    "Cherry"
  ];

  return (
    <ul>
      {/* .map() turns array → JSX */}
      {fruits.map((fruit) => (
        // key helps React track items
        <li key={fruit}>
          {fruit}
        </li>
      ))}
    </ul>
  );
}

⚠️ Always add a key prop when rendering lists. Use a unique ID, not the array index if you can avoid it.

Props: Passing Data In¶

Props (properties) let you pass data into a component — like attributes on an HTML tag.

// 1. Define the shape with a type
type GreetingProps = {
  name: string;
  age: number;
};

// 2. Receive props as a parameter
function Greeting(props: GreetingProps) {
  return (
    <p>
      Hi {props.name},
      you are {props.age} years old.
    </p>
  );
}

// 3. Use it like an HTML tag
// <Greeting name="Sam" age={21} />

Props: Destructuring (Cleaner Syntax)¶

type CardProps = {
  title: string;
  description: string;
  imageUrl: string;
};

// Destructure props directly
function Card({
  title,
  description,
  imageUrl,
}: CardProps) {
  return (
    <div className="card">
      <img src={imageUrl} alt={title} />
      <h3>{title}</h3>
      <p>{description}</p>
    </div>
  );
}

TypeScript will catch mismatched prop types before you run the code. This is why we use .tsx.

Props: Using a Component¶

// App.tsx
import Card from './Card';

function App() {
  return (
    <div>
      <Card
        title="React Basics"
        description="Learn React step by step"
        imageUrl="/react.png"
      />
      <Card
        title="TypeScript"
        description="Types save you from bugs"
        imageUrl="/ts.png"
      />
    </div>
  );
}

One component definition → used many times with different data. This is the power of components.

Props Are Read-Only¶

type LabelProps = {
  text: string;
};

function Label({ text }: LabelProps) {
  // ✅ Reading props is fine
  return <span>{text}</span>;

  // ❌ NEVER mutate props
  // text = "different"; // Error!
}

Props flow down from parent → child.
A child component never changes its own props.

If data needs to change, that’s a job for State.

State: Making Components Dynamic¶

State is data that belongs to a component and can change over time. When state changes → React re-renders the component.

import { useState } from 'react';

function Counter() {
  // useState returns [value, setter]
  // TypeScript infers type as number
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={
        // Call the setter to update
        () => setCount(count + 1)
      }>
        Increment
      </button>
    </div>
  );
}

`useState` Anatomy¶

const [count, setCount] = useState(0);
//     ^       ^                   ^
//     |       |                   |
//     |       |          initial value
//     |       |
//     |    function to UPDATE the value
//     |    (triggers a re-render)
//     |
//   current value (read-only here)

TypeScript explicit type (when needed):¶

// Inferred (TypeScript figures it out)
const [name, setName] = useState('');

// Explicit (for complex types)
const [user, setUser] =
  useState<User | null>(null);

State with Strings & Booleans¶

import { useState } from 'react';

function ToggleMessage() {
  const [visible, setVisible] =
    useState(false);

  return (
    <div>
      <button onClick={
        () => setVisible(!visible)
      }>
        {visible ? 'Hide' : 'Show'}
      </button>

      {/* Conditional rendering */}
      {visible && (
        <p>Now you see me!</p>
      )}
    </div>
  );
}

Props vs State — The Key Distinction¶

	Props	State
Who owns it?	Parent	Component itself
Can it change?	No (read-only)	Yes (via setter)
Where does it come from?	Passed in	`useState()`
What triggers re-render?	Parent re-renders	Calling the setter

Mental model:¶

Props = settings passed to you by someone else
State = your own internal memory

Handling Events¶

function EventDemo() {
  // Button click
  function handleClick() {
    console.log('Button clicked!');
  }

  // Input change
  function handleChange(
    // TypeScript needs the event type
    e: React.ChangeEvent<HTMLInputElement>
  ) {
    console.log(e.target.value);
  }

  return (
    <div>
      <button onClick={handleClick}>
        Click me
      </button>
      <input onChange={handleChange} />
    </div>
  );
}

Controlled Inputs¶

A controlled input ties an <input> to a state variable. React is the single source of truth.

import { useState } from 'react';

function NameInput() {
  const [name, setName] = useState('');

  return (
    <div>
      <input
        // value comes FROM state
        value={name}
        // changes go BACK to state
        onChange={(e) => setName(e.target.value)}
        placeholder="Enter your name"
      />
      {/* Live preview */}
      <p>Hello, {name || 'stranger'}!</p>
    </div>
  );
}

Putting It All Together¶

// A small, complete React component
import { useState } from 'react';

type Item = { id: number; text: string };

function TodoList() {
  const [items, setItems] =
    useState<Item[]>([]);
  const [input, setInput] = useState('');

  function addItem() {
    if (!input.trim()) return;
    setItems([
      ...items,
      { id: Date.now(), text: input }
    ]);
    setInput('');
  }

  return ( /* next slide */ );
}

Putting It All Together (cont.)¶

  // ...continued from previous slide
  return (
    <div>
      <input
        value={input}
        onChange={(e) =>
          setInput(e.target.value)
        }
        placeholder="Add a task..."
      />
      <button onClick={addItem}>
        Add
      </button>
      <ul>
        {items.map((item) => (
          <li key={item.id}>
            {item.text}
          </li>
        ))}
      </ul>
    </div>
  );

The React Mental Model — Summary¶

      DATA FLOWS DOWN (props)
            │
     ┌──────▼───────┐
     │    App.tsx   │  ← owns state
     └──────┬───────┘
            │ props
     ┌──────▼───────┐
     │  Component   │  ← displays data
     └──────┬───────┘
            │ events bubble UP
     ┌──────▼───────┐
     │  User Action │  (click, type…)
     └─────────────-┘
            │ calls setter
            └──── state updates
                  → re-render

Data down, events up. This is the React data flow.

Key Concepts Recap¶

Concept	What it is	Syntax
Component	Reusable UI function	`function Foo() { return <div/> }`
JSX/TSX	HTML-in-TypeScript	`<h1>{value}</h1>`
Props	Data passed in	`function Foo({ name }: Props)`
State	Component’s own data	`const [x, setX] = useState(0)`
Event	User interaction	`onClick={handler}`
Conditional	Show/hide UI	`{flag && <div/>}`
Lists	Render arrays	`arr.map(i => <li key={i.id}>)`

Common Mistakes to Avoid¶

❌ Mutating state directly

// WRONG — React won't see the change
items.push(newItem);

// RIGHT — create a new array
setItems([...items, newItem]);

Common Mistakes to Avoid¶

❌ Missing key on list items

// WRONG
{names.map(n => <li>{n}</li>)}

// RIGHT
{names.map(n => <li key={n}>{n}</li>)}

Common Mistakes to Avoid¶

❌ Component name starts lowercase

// WRONG — React treats this as HTML
function card() { ... }

// RIGHT
function Card() { ... }

What’s Coming Next¶

You’ll build a multi-component app with:

A form component with controlled inputs
A live-preview ProfileCard component
State lifted between components

Last update : July 6, 2026
Created : July 1, 2026

Week 8 Lecture Introduction to React

React Introduction¶

Building UIs with Components & TypeScript¶

What We’ll Cover Today¶

The Problem React Solves¶

The Problem React Solves¶

// Counter.tsx function Counter() { // React tracks this value const [count, setCount] = useState(0); // Just describe the UI — React handles the DOM return ( <div> <p>{count}</p> <button onClick={() => setCount(count + 1)}> + </button> </div> ); } ¶

What Is React?¶

What React is NOT:¶

Setting Up a React + TypeScript Project¶

# Vite is the recommended modern # way to scaffold a React project npm create vite@latest my-app \ -- --template react-ts cd my-app npm install npm run dev ¶

What you get:¶

Entry Point: main.tsx¶

Components: The Core Idea¶

Think of UI as a tree of components:¶

Your First Component¶

// Greeting.tsx // A component is just a TypeScript // function that returns JSX function Greeting() { return ( <div> <h1>Hello, World!</h1> <p>Welcome to React.</p> </div> ); } // Always export your component export default Greeting; ¶

Rules:¶

What Is JSX / TSX?¶

It looks like HTML — but it’s not:¶

Embedding Expressions in TSX¶

Rendering Lists¶

Props: Passing Data In¶

Props: Destructuring (Cleaner Syntax)¶

Props: Using a Component¶

Props Are Read-Only¶

State: Making Components Dynamic¶

useState Anatomy¶

TypeScript explicit type (when needed):¶

State with Strings & Booleans¶

Props vs State — The Key Distinction¶

Mental model:¶

Handling Events¶

Controlled Inputs¶

Putting It All Together¶

Putting It All Together (cont.)¶

The React Mental Model — Summary¶

Key Concepts Recap¶

Common Mistakes to Avoid¶

Common Mistakes to Avoid¶

Common Mistakes to Avoid¶

What’s Coming Next¶

Demo (60 min): Profile Card Builder¶

`// Counter.tsx function Counter() { // React tracks this value const [count, setCount] = useState(0); // Just describe the UI — React handles the DOM return ( <div> <p>{count}</p> <button onClick={() => setCount(count + 1)}> + </button> </div> ); }`
¶

`# Vite is the recommended modern # way to scaffold a React project npm create vite@latest my-app \ -- --template react-ts cd my-app npm install npm run dev`
¶

Entry Point: `main.tsx`¶

`// Greeting.tsx // A component is just a TypeScript // function that returns JSX function Greeting() { return ( <div> <h1>Hello, World!</h1> <p>Welcome to React.</p> </div> ); } // Always export your component export default Greeting;`
¶

`useState` Anatomy¶