JSX Basics

Expressions & Attributes

const name = "Alice"; const el =

Hello, {name}!

; const img =

;

JSX Rules

{expression}	Embed any JS expression
className	Use instead of `class`
htmlFor	Use instead of `for`
style={{color: 'red'}}	Inline styles as object
<Component />	Self-closing tags required
<> ... </>	Fragment (no extra DOM node)

Components

Function Components

function Greeting({ name }) {
  return Hello, {name}!;
}

// Arrow function variant
const Greeting = ({ name }) => (
  Hello, {name}!
);

Props

function Card({ title, children }) { return (

{title}

{children}

); }

Content here

Default Props

function Button({ label = "Click me", onClick }) { return ; }

State (useState)

Basic State

import { useState } from "react"; function Counter() { const [count, setCount] = useState(0); return ( ); }

Functional Updates

setCount(prev => prev + 1); // use prev state
setItems(prev => [...prev, newItem]); // arrays
setUser(prev => ({...prev, name: "Bob"})); // objects

State Rules

Immutable updates	Never mutate state directly
Async batching	Updates may be batched
Functional form	Use `prev =>` when depending on prior state

Effects (useEffect)

Effect Patterns

import { useEffect } from "react";

// Run on every render
useEffect(() => { /* ... */ });

// Run once on mount
useEffect(() => { /* ... */ }, []);

// Run when deps change
useEffect(() => { /* ... */ }, [count]);

// Cleanup on unmount
useEffect(() => {
  const id = setInterval(tick, 1000);
  return () => clearInterval(id);
}, []);

Lists & Keys

function TodoList({ items }) {
  return (
    
      {items.map(item => (
        {item.text}

      ))}
    
  );
}

Keys must be stable, unique IDs -- avoid array index as key

Event Handling

Events

setVal(e.target.value)} />

Common Events

onClick	Mouse click
onChange	Input value change
onSubmit	Form submission
onKeyDown	Key press
onFocus / onBlur	Focus gained / lost
onMouseEnter	Mouse enters element

Conditional Rendering

// Ternary
{isLoggedIn ?  : }

// Logical AND (short-circuit)
{hasError && }

// Early return
function Page({ user }) {
  if (!user) return ;
  return ;
}

Hooks

useRef

const inputRef = useRef(null); // Access: inputRef.current.focus();

useRef persists values across renders without triggering re-render

useMemo & useCallback

// Memoize expensive computation
const sorted = useMemo(
  () => items.sort(compareFn),
  [items]
);

// Memoize callback
const handleClick = useCallback(
  () => setCount(c => c + 1),
  []
);

useContext

import { useContext } from "react";
const value = useContext(MyContext);

Custom Hooks

function useLocalStorage(key, initial) {
  const [value, setValue] = useState(() => {
    const saved = localStorage.getItem(key);
    return saved ? JSON.parse(saved) : initial;
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(value));
  }, [key, value]);

  return [value, setValue];
}

// Usage
const [name, setName] = useLocalStorage("name", "");

Custom hooks must start with 'use'

Context API

Create & Provide

import { createContext, useContext } from "react";

const ThemeCtx = createContext("light");

function App() {
  return (
    
  );
}

Consume

function Page() {
  const theme = useContext(ThemeCtx); // "dark"
  return ...
;
}

REACT QUICK REFERENCE

Hello, {name}!

Hello, {name}!

Hello, {name}!

{title}

REACT QUICK REFERENCE (continued)