Reactivity
Declare state
Update state
Templating
Styling
Loop
Event click
<script>
let count = $state(0);
function incrementCount() {
count++;
}
</script>
<p>Counter: {count}</p>
<button onclick={incrementCount}>+1</button>
Dom ref
Conditional
<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>
<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
On unmount
<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
<script>
import UserProfile from "./UserProfile.svelte";
</script>
<UserProfile
name="John"
age={20}
favouriteColors={["green", "blue", "red"]}
isAvailable
/>
Emit to parent
<script>
import AnswerButton from "./AnswerButton.svelte";
let isHappy = $state(true);
function onAnswerNo() {
isHappy = false;
}
function onAnswerYes() {
isHappy = true;
}
</script>
<p>Are you happy?</p>
<AnswerButton onYes={onAnswerYes} onNo={onAnswerNo} />
<p style="font-size: 50px;">{isHappy ? "😀" : "😥"}</p>
<script>
import AnswerButton from "./AnswerButton.svelte";
let isHappy = true;
function onAnswerNo() {
isHappy = false;
}
function onAnswerYes() {
isHappy = true;
}
</script>
<p>Are you happy?</p>
<AnswerButton on:yes={onAnswerYes} on:no={onAnswerNo} />
<p style="font-size: 50px;">{isHappy ? "😀" : "😥"}</p>
Slot
Slot fallback
Context
<script>
import { setContext } from "svelte";
import UserProfile from "./UserProfile.svelte";
import createUserState from "./createUserState.svelte.js";
const user = createUserState({
id: 1,
username: "unicorn42",
email: "[email protected]",
});
setContext("user", user);
</script>
<h1>Welcome back, {user.username}</h1>
<UserProfile />
<script>
import { setContext } from "svelte";
import UserProfile from "./UserProfile.svelte";
import createUserStore from "./createUserStore.js";
const userStore = createUserStore({
id: 1,
username: "unicorn42",
email: "[email protected]",
});
setContext("user", userStore);
</script>
<h1>Welcome back, {$userStore.username}</h1>
<UserProfile />
Form input
Input text
Checkbox
<script>
let isAvailable = $state(false);
</script>
<input id="is-available" type="checkbox" bind:checked={isAvailable} />
<label for="is-available">Is available</label>
Radio
<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
<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>
<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
Svelte 5
<!doctype html>
<html>
<body>
<div id="app"></div>
<script type="module" src="./app.js"></script>
</body>
</html>
Svelte 4
<!doctype html>
<html>
<body>
<div id="app"></div>
<script type="module" src="./app.js"></script>
</body>
</html>
Fetch data
<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}
<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}