$effect

<script>
	let size = $state(50);
	let color = $state('#ff3e00');

	let canvas;

	$effect(() => {
		const context = canvas.getContext('2d');
		context.clearRect(0, 0, canvas.width, canvas.height);

		// これは `color` や `size` が変更されるたびに再実行されます
		context.fillStyle = color;
		context.fillRect(0, 0, size, size);
	});
</script>

<canvas bind:this={canvas} width="100" height="100"></canvas>

<script>
	let count = $state(0);
	let milliseconds = $state(1000);

	$effect(() => {
		// これは `milliseconds` が変更されるたびに再作成されます
		const interval = setInterval(() => {
			count += 1;
		}, milliseconds);

		return () => {
			// teardown 関数が提供される場合、以下のときに実行されます
			// a) effect が再実行される直前
			// b) コンポーネントが破棄される前
			clearInterval(interval);
		};
	});
</script>

<h1>{count}</h1>

<button onclick={() => (milliseconds *= 2)}>slower</button>
<button onclick={() => (milliseconds /= 2)}>faster</button>

function $effect(fn: () => void | (() => void)): void
namespace $effect
Runs code when a component is mounted to the DOM, and then whenever its dependencies change, i.e. $state or $derived values.
The timing of the execution is after the DOM has been updated.
Example:
$effect(() => console.log('The count is now ' + count));
If you return a function from the effect, it will be called right before the effect is run again, or when the component is unmounted.
Does not run during server side rendering.
https://svelte.dev/docs/svelte/$effect
@paramfn The function to execute$effect(() => {
	const const context: CanvasRenderingContext2Dcontext = 
let canvas: {
    width: number;
    height: number;
    getContext(type: "2d", options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2D;
}canvas.function getContext(type: "2d", options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2DgetContext('2d');
	const context: CanvasRenderingContext2Dcontext.CanvasRect.clearRect(x: number, y: number, w: number, h: number): void
MDN Reference
clearRect(0, 0, 
let canvas: {
    width: number;
    height: number;
    getContext(type: "2d", options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2D;
}canvas.width: numberwidth, 
let canvas: {
    width: number;
    height: number;
    getContext(type: "2d", options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2D;
}canvas.height: numberheight);

	// これは `color` が変更されるたびに再実行されます...
	const context: CanvasRenderingContext2Dcontext.CanvasFillStrokeStyles.fillStyle: string | CanvasGradient | CanvasPattern
MDN Reference
fillStyle = let color: stringcolor;

	function setTimeout<[]>(callback: () => void, delay?: number): NodeJS.Timeout (+2 overloads)
Schedules execution of a one-time callback after delay milliseconds.
The callback will likely not be invoked in precisely delay milliseconds.
Node.js makes no guarantees about the exact timing of when callbacks will fire,
nor of their ordering. The callback will be called as close as possible to the
time specified.
When delay is larger than 2147483647 or less than 1 or NaN, the delay
will be set to 1. Non-integer delays are truncated to an integer.
If callback is not a function, a TypeError will be thrown.
This method has a custom variant for promises that is available using
timersPromises.setTimeout().
@sincev0.0.1
@paramcallback The function to call when the timer elapses.
@paramdelay The number of milliseconds to wait before calling the
callback. Default: 1.
@paramargs Optional arguments to pass when the callback is called.
@returnsfor use with clearTimeout()setTimeout(() => {
		// ...しかしこれは `size` が変更されても再実行されません
		const context: CanvasRenderingContext2Dcontext.CanvasRect.fillRect(x: number, y: number, w: number, h: number): void
MDN Reference
fillRect(0, 0, let size: numbersize, let size: numbersize);
	}, 0);
});

<script>
	let state = $state({ value: 0 });
	let derived = $derived({ value: state.value * 2 });

	// これは一度だけ実行されます。なぜなら `state` は再代入されていないからです (変異(mutated)のみです)
	$effect(() => {
		state;
	});

	// これは `state.value` が変更されるたびに実行されます...
	$effect(() => {
		state.value;
	});

	// ...これも `state.value` が変更されるたびに実行されます。なぜなら `derived` は毎回新しいオブジェクトになるからです
	$effect(() => {
		derived;
	});
</script>

<button onclick={() => (state.value += 1)}>
	{state.value}
</button>

<p>{state.value} doubled is {derived.value}</p>

import function confetti(opts?: ConfettiOptions): voidconfetti from 'canvas-confetti';

let let condition: booleancondition = 
function $state<true>(initial: true): true (+1 overload)
namespace $state
Declares reactive state.
Example:
let count = $state(0);
https://svelte.dev/docs/svelte/$state
@paraminitial The initial value$state(true);
let let color: stringcolor = 
function $state<"#ff3e00">(initial: "#ff3e00"): "#ff3e00" (+1 overload)
namespace $state
Declares reactive state.
Example:
let count = $state(0);
https://svelte.dev/docs/svelte/$state
@paraminitial The initial value$state('#ff3e00');

function $effect(fn: () => void | (() => void)): void
namespace $effect
Runs code when a component is mounted to the DOM, and then whenever its dependencies change, i.e. $state or $derived values.
The timing of the execution is after the DOM has been updated.
Example:
$effect(() => console.log('The count is now ' + count));
If you return a function from the effect, it will be called right before the effect is run again, or when the component is unmounted.
Does not run during server side rendering.
https://svelte.dev/docs/svelte/$effect
@paramfn The function to execute$effect(() => {
	if (let condition: truecondition) {
		function confetti(opts?: ConfettiOptions): voidconfetti({ ConfettiOptions.colors: string[]colors: [let color: stringcolor] });
	} else {
		function confetti(opts?: ConfettiOptions): voidconfetti();
	}
});

<script>
	import { tick } from 'svelte';

	let div = $state();
	let messages = $state([]);

	// ...

	$effect.pre(() => {
		if (!div) return; // まだマウントされていない場合

		// `messages` 配列の length が変わるたびにこのコードを再実行するため、ここで参照する
		messages.length;

		// 新しい message が追加されるたびに自動でスクロールする
		if (div.offsetHeight + div.scrollTop > div.scrollHeight - 20) {
			tick().then(() => {
				div.scrollTo(0, div.scrollHeight);
			});
		}
	});
</script>

<div bind:this={div}>
	{#each messages as message}
		<p>{message}</p>
	{/each}
</div>

<script>
	console.log('in component setup:', $effect.tracking()); // false

	$effect(() => {
		console.log('in effect:', $effect.tracking()); // true
	});
</script>

<p>in template: {$effect.tracking()}</p> <!-- true -->

<button onclick={() => a++}>a++</button>
<button onclick={() => b++}>b++</button>

<p>{a} + {b} = {await add(a, b)}</p>

{#if $effect.pending()}
	<p>pending promises: {$effect.pending()}</p>
{/if}

const const destroy: () => voiddestroy = 
namespace $effect
function $effect(fn: () => void | (() => void)): void
Runs code when a component is mounted to the DOM, and then whenever its dependencies change, i.e. $state or $derived values.
The timing of the execution is after the DOM has been updated.
Example:
$effect(() => console.log('The count is now ' + count));
If you return a function from the effect, it will be called right before the effect is run again, or when the component is unmounted.
Does not run during server side rendering.
https://svelte.dev/docs/svelte/$effect
@paramfn The function to execute$effect.function $effect.root(fn: () => void | (() => void)): () => void
The $effect.root rune is an advanced feature that creates a non-tracked scope that doesn’t auto-cleanup. This is useful for
nested effects that you want to manually control. This rune also allows for creation of effects outside of the component
initialisation phase.
Example:
<script>
  let count = $state(0);

  const cleanup = $effect.root(() => {
	$effect(() => {
			console.log(count);
		})

	 return () => {
	   console.log('effect root cleanup');
			}
  });
</script>

<button onclick={() => cleanup()}>cleanup</button>
https://svelte.dev/docs/svelte/$effect#$effect.root
root(() => {
	
function $effect(fn: () => void | (() => void)): void
namespace $effect
Runs code when a component is mounted to the DOM, and then whenever its dependencies change, i.e. $state or $derived values.
The timing of the execution is after the DOM has been updated.
Example:
$effect(() => console.log('The count is now ' + count));
If you return a function from the effect, it will be called right before the effect is run again, or when the component is unmounted.
Does not run during server side rendering.
https://svelte.dev/docs/svelte/$effect
@paramfn The function to execute$effect(() => {
		// setup
	});

	return () => {
		// cleanup
	};
});

// later...
const destroy: () => voiddestroy();

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

	// don't do this!
	$effect(() => {
		doubled = count * 2;
	});
</script>

<script>
	let count = $state(0);
	let doubled = $derived(count * 2);
</script>

<script>
	const total = 100;
	let spent = $state(0);
	let left = $state(total);

	$effect(() => {
		left = total - spent;
	});

	$effect(() => {
		spent = total - left;
	});
</script>

<label>
	<input type="range" bind:value={spent} max={total} />
	{spent}/{total} spent
</label>

<label>
	<input type="range" bind:value={left} max={total} />
	{left}/{total} left
</label>

<script>
	const total = 100;
	let spent = $state(0);
	let left = $derived(total - spent);

	function updateLeft(left) {
		spent = total - left;
	}
</script>

<label>
	<input type="range" bind:value={spent} max={total} />
	{spent}/{total} spent
</label>

<label>
	<input type="range" bind:value={() => left, updateLeft} max={total} />
	{left}/{total} left
</label>