Getting started

Create Svelte App

bunx sv create myapp
cd myapp
bun i
bun dev

# official add-ons
bunx sv add drizzle eslint mdsvex prettier storybook sveltekit-adapter tailwindcss

Reactivity

Declare State

App.svelte

<script>
  let name = $state("World");
</script>

<h1>Hello {name}!</h1>

Update State

App.svelte

<script lang='ts'>
  let name: string = $state("World");
  name = "Doe";
</script>

<h1>Hello {name}!</h1>

Computed State

App.svelte

<script>
  let count = $state(10);
  const doubleCount = $derived(count * 2);
</script>

<div>{doubleCount}</div>

Store

App.svelte

<script lang="ts">
	import { userState } from './state.svelte.ts';
</script>

<p>User name: {userState.name}</p>
<button onclick={() => {
	userState.name = 'new name';
}}>
	change name
</button>

state.svelte.ts

export const userState = $state({
	name: 'name',
	/* ... */
});

Templating

Minimal Template

HelloWorld.svelte

<h1>Hello world</h1>

Styling

HelloWorld.svelte

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

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

Loop

HelloWorld.svelte

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

<ul>
  <!-- `index` is optional, and `(color)` is the unique key -->
  {#each colors as color, index (color)}
    <li>{index}: {color}</li>
  {/each}
</ul>

Event Click

HelloWorld.svelte

<script>
  let count = $state(0);

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

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

DOM Ref

HelloWorld.svelte

<script>
  let inputElement;

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

<input bind:this={inputElement} />

Conditional

HelloWorld.svelte

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

  const light = $derived(TRAFFIC_LIGHTS[lightIndex]);

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

<button onclick={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

HelloWorld.svelte

<script>
  let pageTitle = $state("");
  $effect(() => {
    pageTitle = document.title;
  });
</script>

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

On Unmount

HelloWorld.svelte

<script>
  let time = $state(new Date().toLocaleTimeString());

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

    return () => clearInterval(timer);
  });
</script>

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

Component Composition

Props

App.svelte

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

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

UserProfile.svelte

<script>
  const {
    name = "",
    age = null,
    favouriteColors = [],
    isAvailable = false,
  } = $props();
</script>

<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.svelte

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

  let isHappy = $state(true);

  function onAnswerNo() {
    isHappy = false;
  }

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

<p>Are you happy?</p>
<AnswerButton onYes={onAnswerYes} onNo={onAnswerNo} />
<p style="font-size: 50px;">{isHappy ? "Happy" : "Unhappy"}</p>

AnswerButton.svelte

<script>
  let { onYes, onNo } = $props();
</script>

<button onclick={onYes}>YES</button>

<button onclick={onNo}>NO</button>

Slot

App.svelte

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

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

FunnyButton.svelte

<script>
  let { children } = $props();
</script>

<button
  style="background: rgba(0, 0, 0, 0.4); color: #fff; padding: 10px 20px; font-size: 30px; border: 2px solid #fff; margin: 8px; transform: scale(0.9); box-shadow: 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;"
>
  {#if children}
    {@render children()}
  {:else}
    <span>No content found</span>
  {/if}
</button>

Context

App.svelte

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

  const user = createUserState({
    id: 1,
    username: "unicorn42",
    email: "unicorn42@example.com",
  });

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

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

UserProfile.svelte

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

  const user = getContext("user");
</script>

<div>
  <h2>My Profile</h2>
  <p>Username: {user.username}</p>
  <p>Email: {user.email}</p>
  <button onclick={() => (user.username = "Jane")}>
    Update username to Jane
  </button>
</div>

createUserState.svelte.js

export default function createUserState(initial) {
  let username = $state(initial.username);
  return {
    ...initial,
    get username() {
      return username;
    },
    set username(value) {
      username = value;
    },
  };
}

Form Input

Input Text

App.svelte

<script>
  let text = $state("Hello World");
</script>

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

Checkbox

App.svelte

<script>
  let isAvailable = $state(false);
</script>

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

Radio

App.svelte

<script>
  let picked = $state("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

App.svelte

<script>
  let selectedColorId = $state(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

index.html

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

app.js

import { mount } from "svelte";
import App from "./App.svelte";

const app = mount(App, {
  target: document.getElementById("app"),
});

export default app;

App.svelte

<h1>Hello world</h1>

Fetch Data

App.svelte

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

  const response = useFetchUsers();
</script>

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

useFetchUsers.svelte.js

class UsersResponse {
  users = $state();
  error = $state();
  isLoading = $state(false);
}

export default function useFetchUsers() {
  const resp = new UsersResponse();

  async function fetchData() {
    resp.isLoading = true;
    try {
      const response = await fetch("https://randomuser.me/api/?results=3");
      resp.users = (await response.json()).results;
      resp.error = undefined;
    } catch (err) {
      resp.error = err;
      resp.users = undefined;
    }
    resp.isLoading = false;
  }

  fetchData();
  return resp;
}

References

Official resources

Other links