Getting started

Create React App

bun create vite@latest my-app -- --template react
cd my-app
bun i
bun dev

Reactivity

Declare State

Name.jsx

import { useState } from "react";

export default function Name() {
  const [name] = useState("John");

  return <h1>Hello {name}</h1>;
}

Update State

Name.tsx

import { useEffect, useState } from "react";

export default function Name() {
  const [name, setName] = useState("John");

  useEffect(() => {
    setName("Jane");
  }, []);

  return <h1>Hello {name}</h1>;
}

Computed State

DoubleCount.jsx

import { useState } from "react";

export default function DoubleCount() {
  const [count] = useState(10);
  const doubleCount = count * 2;

  return <div>{doubleCount}</div>;
}

Templating

Minimal Template

HelloWorld.jsx

export default function HelloWorld() {
  return <h1>Hello world</h1>;
}

Styling

CssStyle.jsx

import "./style.css";

export default function CssStyle() {
  return (
    <>
      <h1 className="title">I am red</h1>
      <button style={{ fontSize: "10rem" }}>I am a button</button>
    </>
  );
}

style.css

.title {
  color: red;
}

Loop

Colors.tsx

export default function Colors() {
  const colors = ["red", "green", "blue"];
  return (
    <ul>
      {colors.map((color) => (
        <li key={color}>{color}</li>
      ))}
    </ul>
  );
}

Event Click

Counter.jsx

import { useState } from "react";

export default function Counter() {
  const [count, setCount] = useState(0);

  function incrementCount() {
    setCount((count) => count + 1);
  }

  return (
    <>
      <p>Counter: {count}</p>
      <button onClick={incrementCount}>+1</button>
    </>
  );
}

DOM Ref

InputFocused.jsx

import { useEffect, useRef } from "react";

export default function InputFocused() {
  const inputElement = useRef(null);

  useEffect(() => inputElement.current.focus(), []);

  return <input type="text" ref={inputElement} />;
}

Conditional

TrafficLight.jsx

import { useState } from "react";

const TRAFFIC_LIGHTS = ["red", "orange", "green"];

export default function TrafficLight() {
  const [lightIndex, setLightIndex] = useState(0);

  const light = TRAFFIC_LIGHTS[lightIndex];

  function nextLight() {
    setLightIndex((lightIndex + 1) % TRAFFIC_LIGHTS.length);
  }

  return (
    <>
      <button onClick={nextLight}>Next light</button>
      <p>Light is: {light}</p>
      <p>
        You must
        {light === "red" && <span>STOP</span>}
        {light === "orange" && <span>SLOW DOWN</span>}
        {light === "green" && <span>GO</span>}
      </p>
    </>
  );
}

Lifecycle

On Mount

PageTitle.jsx

import { useState, useEffect } from "react";

export default function PageTitle() {
  const [pageTitle, setPageTitle] = useState("");

  useEffect(() => {
    setPageTitle(document.title);
  }, []);

  return <p>Page title: {pageTitle}</p>;
}

On Unmount

Time.jsx

import { useState, useEffect } from "react";

export default function Time() {
  const [time, setTime] = useState(new Date().toLocaleTimeString());

  useEffect(() => {
    const timer = setInterval(() => {
      setTime(new Date().toLocaleTimeString());
    }, 1000);

    return () => clearInterval(timer);
  }, []);

  return <p>Current time: {time}</p>;
}

Component Composition

Props

App.jsx

import UserProfile from "./UserProfile.jsx";

export default function App() {
  return (
    <UserProfile
      name="John"
      age={20}
      favouriteColors={["green", "blue", "red"]}
      isAvailable
    />
  );
}

UserProfile.jsx


export default function UserProfile({
  name = "",
  age = null,
  favouriteColors = [],
  isAvailable = false,
}) {
  return (
    <>
      <p>My name is {name}!</p>
      <p>My age is {age}!</p>
      <p>My favourite colors are {favouriteColors.join(", ")}!</p>
      <p>I am {isAvailable ? "available" : "not available"}</p>
    </>
  );
}

Emit To Parent

App.jsx

import { useState } from "react";
import AnswerButton from "./AnswerButton.jsx";

export default function App() {
  const [isHappy, setIsHappy] = useState(true);

  function onAnswerNo() {
    setIsHappy(false);
  }

  function onAnswerYes() {
    setIsHappy(true);
  }

  return (
    <>
      <p>Are you happy?</p>
      <AnswerButton onYes={onAnswerYes} onNo={onAnswerNo} />
      <p style={{ fontSize: 50 }}>{isHappy ? "Happy" : "Unhappy"}</p>
    </>
  );
}

AnswerButton.jsx

export default function AnswerButton({ onYes, onNo }) {
  return (
    <>
      <button onClick={onYes}>YES</button>

      <button onClick={onNo}>NO</button>
    </>
  );
}

Slot

App.jsx

import FunnyButton from "./FunnyButton.jsx";

export default function App() {
  return (
    <>
      <FunnyButton />
      <FunnyButton>I got content!</FunnyButton>
    </>
  );
}

FunnyButton.jsx

export default function FunnyButton({ children }) {
  return (
    <button
      style={{
        background: "rgba(0, 0, 0, 0.4)",
        color: "#fff",
        padding: "10px 20px",
        fontSize: "30px",
        border: "2px solid #fff",
        margin: "8px",
        transform: "scale(0.9)",
        boxShadow: "4px 4px rgba(0, 0, 0, 0.4)",
        transition: "transform 0.2s cubic-bezier(0.34, 1.65, 0.88, 0.925) 0s",
        outline: "0",
      }}
    >
      {children || <span>No content found</span>}
    </button>
  );
}

Context

App.jsx

import { useState } from "react";
import UserProfile from "./UserProfile";
import { UserContext } from "./UserContext";

export default function App() {
  const [user, setUser] = useState({
    id: 1,
    username: "unicorn42",
    email: "unicorn42@example.com",
  });

  function updateUsername(newUsername) {
    setUser((userData) => ({ ...userData, username: newUsername }));
  }

  return (
    <>
      <h1>Welcome back, {user.username}</h1>
      <UserContext.Provider value={{ ...user, updateUsername }}>
        <UserProfile />
      </UserContext.Provider>
    </>
  );
}

UserProfile.jsx

import { createContext } from "react";

export const UserContext = createContext();

UserProfile.jsx

import { useContext } from "react";
import { UserContext } from "./UserContext";

export default function UserProfile() {
  const { username, email, updateUsername } = useContext(UserContext);

  return (
    <div>
      <h2>My Profile</h2>
      <p>Username: {username}</p>
      <p>Email: {email}</p>
      <button onClick={() => updateUsername("Jane")}>
        Update username to Jane
      </button>
    </div>
  );
}

Form Input

Input Text

InputHello.jsx

import { useState } from "react";

export default function InputHello() {
  const [text, setText] = useState("Hello world");

  function handleChange(event) {
    setText(event.target.value);
  }

  return (
    <>
      <p>{text}</p>
      <input value={text} onChange={handleChange} />
    </>
  );
}

Checkbox

IsAvailable.jsx

import { useState } from "react";

export default function IsAvailable() {
  const [isAvailable, setIsAvailable] = useState(false);

  function handleChange() {
    setIsAvailable(!isAvailable);
  }

  return (
    <>
      <input
        id="is-available"
        type="checkbox"
        checked={isAvailable}
        onChange={handleChange}
      />
      <label htmlFor="is-available">Is available</label>
    </>
  );
}

Radio

PickPill.jsx

import { useState } from "react";

export default function PickPill() {
  const [picked, setPicked] = useState("red");

  function handleChange(event) {
    setPicked(event.target.value);
  }

  return (
    <>
      <div>Picked: {picked}</div>

      <input
        id="blue-pill"
        checked={picked === "blue"}
        type="radio"
        value="blue"
        onChange={handleChange}
      />
      <label htmlFor="blue-pill">Blue pill</label>

      <input
        id="red-pill"
        checked={picked === "red"}
        type="radio"
        value="red"
        onChange={handleChange}
      />
      <label htmlFor="red-pill">Red pill</label>
    </>
  );
}

Select

ColorSelect.jsx

import { useState } from "react";

const colors = [
  { id: 1, text: "red" },
  { id: 2, text: "blue" },
  { id: 3, text: "green" },
  { id: 4, text: "gray", isDisabled: true },
];

export default function ColorSelect() {
  const [selectedColorId, setSelectedColorId] = useState(2);

  function handleChange(event) {
    setSelectedColorId(event.target.value);
  }

  return (
    <select value={selectedColorId} onChange={handleChange}>
      {colors.map((color) => (
        <option key={color.id} value={color.id} disabled={color.isDisabled}>
          {color.text}
        </option>
      ))}
    </select>
  );
}

Webapp Features

Render App

index.html

<!doctype html>
<html>
  <body>
    <div id="app"></div>
    <script type="module" src="./main.jsx"></script>
  </body>
</html>

main.jsx

import React from "react";
import ReactDOM from "react-dom/client";
import App from "./App";

ReactDOM.createRoot(document.getElementById("app")).render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

App.jsx

export default function App() {
  return <h1>Hello world</h1>;
}

Fetch Data

App.jsx

import useFetchUsers from "./useFetchUsers";

export default function App() {
  const { isLoading, error, data: users } = useFetchUsers();

  return (
    <>
      {isLoading ? (
        <p>Fetching users...</p>
      ) : error ? (
        <p>An error occurred while fetching users</p>
      ) : (
        users && (
          <ul>
            {users.map((user) => (
              <li key={user.login.uuid}>
                <img src={user.picture.thumbnail} alt="user" />
                <p>
                  {user.name.first} {user.name.last}
                </p>
              </li>
            ))}
          </ul>
        )
      )}
    </>
  );
}

useFetchUsers.js

import { useEffect, useState } from "react";

export default function useFetchUsers() {
  const [data, setData] = useState();
  const [error, setError] = useState();
  const [isLoading, setIsLoading] = useState(false);

  useEffect(() => {
    async function fetchData() {
      setIsLoading(true);
      try {
        const response = await fetch("https://randomuser.me/api/?results=3");
        const { results: users } = await response.json();
        setData(users);
        setError();
      } catch (err) {
        setData();
        setError(err);
      }
      setIsLoading(false);
    }
    fetchData();
  }, []);

  return { isLoading, error, data };
}

References

Official resources

Other links