logo Component party

Reactivity

Declare state

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import { useState } from "react";

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

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

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<script>
  let name = "John";
</script>

<h1>Hello {name}</h1>

Update state

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import { useEffect, useState } from "react";

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

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

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

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<script>
  let name = "John";
  name = "Jane";
</script>

<h1>Hello {name}</h1>

Computed state

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import { useState } from "react";

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

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

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<script>
  let count = 10;
  $: doubleCount = count * 2;
</script>

<div>{doubleCount}</div>

Templating

Minimal template

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

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<h1>Hello world</h1>

Styling

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import "./style.css";

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

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<h1 class="title">I am red</h1>
<button style="font-size: 10rem;">I am a button</button>

<style>
  .title {
    color: red;
  }
</style>

Loop

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

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<script>
  const colors = ["red", "green", "blue"];
</script>

<ul>
  {#each colors as color (color)}
    <li>{color}</li>
  {/each}
</ul>

Event click

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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>
    </>
  );
}

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<script>
  let count = 0;

  function incrementCount() {
    count++;
  }
</script>

<p>Counter: {count}</p>
<button on:click={incrementCount}>+1</button>

Dom ref

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import { useEffect, useRef } from "react";

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

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

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

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<script>
  import { onMount } from "svelte";

  let inputElement;

  onMount(() => {
    inputElement.focus();
  });
</script>

<input bind:this={inputElement} />

Conditional

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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>
    </>
  );
}

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<script>
  const TRAFFIC_LIGHTS = ["red", "orange", "green"];
  let lightIndex = 0;

  $: light = TRAFFIC_LIGHTS[lightIndex];

  function nextLight() {
    lightIndex = (lightIndex + 1) % TRAFFIC_LIGHTS.length;
  }
</script>

<button on:click={nextLight}>Next light</button>
<p>Light is: {light}</p>
<p>
  You must
  {#if light === "red"}
    <span>STOP</span>
  {:else if light === "orange"}
    <span>SLOW DOWN</span>
  {:else if light === "green"}
    <span>GO</span>
  {/if}
</p>

Lifecycle

On mount

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import { useState, useEffect } from "react";

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

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

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

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<script>
  import { onMount } from "svelte";
  let pageTitle = "";
  onMount(() => {
    pageTitle = document.title;
  });
</script>

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

On unmount

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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>;
}

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<script>
  import { onDestroy } from "svelte";

  let time = new Date().toLocaleTimeString();

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

  onDestroy(() => clearInterval(timer));
</script>

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

Component composition

Props

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import UserProfile from "./UserProfile.jsx";

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

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<script>
  import UserProfile from "./UserProfile.svelte";
</script>

<UserProfile
  name="John"
  age={20}
  favouriteColors={["green", "blue", "red"]}
  isAvailable
/>

Emit to parent

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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 ? "😀" : "😥"}</p>
    </>
  );
}

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<script>
  import AnswerButton from "./AnswerButton.svelte";

  let isHappy = true;

  function onAnswerNo() {
    isHappy = false;
  }

  function onAnswerYes() {
    isHappy = true;
  }
</script>

<p>Are you happy?</p>
<AnswerButton on:yes={onAnswerYes} on:no={onAnswerNo} />
<p style="font-size: 50px;">{isHappy ? "😀" : "😥"}</p>

Slot

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import FunnyButton from "./FunnyButton.jsx";

export default function App() {
  return <FunnyButton>Click me!</FunnyButton>;
}

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<script>
  import FunnyButton from "./FunnyButton.svelte";
</script>

<FunnyButton>Click me!</FunnyButton>

Slot fallback

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import FunnyButton from "./FunnyButton.jsx";

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

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<script>
  import FunnyButton from "./FunnyButton.svelte";
</script>

<FunnyButton />
<FunnyButton>I got content!</FunnyButton>

Context

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

export default function App() {
  // In a real app, you would fetch the user data from an API
  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>
    </>
  );
}

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<script>
  import { setContext } from "svelte";
  import UserProfile from "./UserProfile.svelte";
  import createUserStore from "./createUserStore.js";

  // In a real app, you would fetch the user data from an API
  const userStore = createUserStore({
    id: 1,
    username: "unicorn42",
    email: "unicorn42@example.com",
  });

  setContext("user", userStore);
</script>

<h1>Welcome back, {$userStore.username}</h1>
<UserProfile />

Form input

Input text

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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} />
    </>
  );
}

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<script>
  let text = "Hello World";
</script>

<p>{text}</p>
<input bind:value={text} />

Checkbox

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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>
    </>
  );
}

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<script>
  let isAvailable = false;
</script>

<input id="is-available" type="checkbox" bind:checked={isAvailable} />
<label for="is-available">Is available</label>

Radio

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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>
    </>
  );
}

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<script>
  let picked = "red";
</script>

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

<input id="blue-pill" bind:group={picked} type="radio" value="blue" />
<label for="blue-pill">Blue pill</label>

<input id="red-pill" bind:group={picked} type="radio" value="red" />
<label for="red-pill">Red pill</label>

Select

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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>
  );
}

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<script>
  let selectedColorId = 2;

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

<select bind:value={selectedColorId}>
  {#each colors as color}
    <option value={color.id} disabled={color.isDisabled}>
      {color.text}
    </option>
  {/each}
</select>

Webapp features

Render app

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

logo of Svelte 4 Svelte 4

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

Fetch data

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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>
        )
      )}
    </>
  );
}

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<script>
  import useFetchUsers from "./useFetchUsers";

  const { isLoading, error, data: users } = useFetchUsers();
</script>

{#if $isLoading}
  <p>Fetching users...</p>
{:else if $error}
  <p>An error occurred while fetching users</p>
{:else if $users}
  <ul>
    {#each $users as user}
      <li>
        <img src={user.picture.thumbnail} alt="user" />
        <p>
          {user.name.first}
          {user.name.last}
        </p>
      </li>
    {/each}
  </ul>
{/if}