$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);

		// ceci va être rejoué lorsque `color` ou `size` change
		context.fillStyle = color;
		context.fillRect(0, 0, size, size);
	});
</script>

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

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

	$effect(() => {
		// ceci sera recréé lorsque `milliseconds` change
		const interval = setInterval(() => {
			count += 1;
		}, milliseconds);

		return () => {
			// si une fonction de nettoyage est fourni, il sera rejoué
			// a) immédiatement avant que l'effet ne se rejoue
			// b) lorsque que le composant est démonté
			clearInterval(interval);
		};
	});
</script>

<h1>{count}</h1>

<button onclick={() => (milliseconds *= 2)}>ralentir</button>
<button onclick={() => (milliseconds /= 2)}>accélérer</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);

	// cet effet sera rejoué lorsque `color` change...
	const context: CanvasRenderingContext2Dcontext.CanvasFillStrokeStyles.fillStyle: string | CanvasGradient | CanvasPattern
MDN Reference
fillStyle = let color: stringcolor;

	function setTimeout<[]>(callback: () => void, ms?: 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, 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.
@paramargs Optional arguments to pass when the callback is called.
@returnfor use with {@link clearTimeout}setTimeout(() => {
		// ... mais pas lorsque `size` change
		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 });

	// ceci sera joué une seule fois, car `state` n'est jamais réassigné (seulement muté)
	$effect(() => {
		state;
	});

	// ceci sera rejoué à chaque fois que `state.value` change...
	$effect(() => {
		state.value;
	});

	// ... et ceci également, car `derived` est un nouvel objet à chaque fois
	$effect(() => {
		derived;
	});
</script>

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

<p>Le double de {state.value} vaut {derived.value}</p>

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(() => {
	var console: Console
The console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without calling require('console').

Warning: The global console object’s methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@seesourceconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)
Prints to stdout with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
See util.format() for more information.
@sincev0.1.100log('effet');

	if (let a: falsea) {
		var console: Console
The console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without calling require('console').

Warning: The global console object’s methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@seesourceconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)
Prints to stdout with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
See util.format() for more information.
@sincev0.1.100log('b:', let b: falseb);
	}
});

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

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

	// ...

	$effect.pre(() => {
		if (!div) return; // pas encore monté

		// référence à la longueur du tableau `messages` afin que ce code soit rejoué lorsque celle-ci
		// change
		messages.length;

		// scroll automatique lorsque de nouveaux messages sont ajoutés
		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('lors de la mise en place du composant :', $effect.tracking()); // false

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

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

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

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

		return () => {
			console.log('nettoyage de l\'effet racine');
		};
	});
</script>

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

	// ne faites pas ça !
	$effect(() => {
		doubled = count * 2;
	});
</script>

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

<script>
	let 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} dépensé
</label>

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

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

	function updateSpent(value) {
		spent = value;
		left = total - spent;
	}

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

<label>
	<input type="range" bind:value={() => spent, updateSpent} max={total} />
	{spent}/{total} dépensé
</label>

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