Guide de migration vers Svelte 4

import { function createEventDispatcher<EventMap extends Record<string, any> = any>(): EventDispatcher<EventMap>
Creates an event dispatcher that can be used to dispatch component events.
Event dispatchers are functions that can take two arguments: name and detail.
Component events created with createEventDispatcher create a
CustomEvent.
These events do not bubble.
The detail argument corresponds to the CustomEvent.detail
property and can contain any type of data.
The event dispatcher can be typed to narrow the allowed event names and the type of the detail argument:
const dispatch = createEventDispatcher&#x3C;{
 loaded: never; // does not take a detail argument
 change: string; // takes a detail argument of type string, which is required
 optional: number | null; // takes an optional detail argument of type number
}>();
@deprecatedUse callback props and/or the $host() rune instead — see migration guidecreateEventDispatcher } from 'svelte';

const 
const dispatch: EventDispatcher<{
    optional: number | null;
    required: string;
    noArgument: null;
}>dispatch = 
createEventDispatcher<{
    optional: number | null;
    required: string;
    noArgument: null;
}>(): EventDispatcher<{
    optional: number | null;
    required: string;
    noArgument: null;
}>
Creates an event dispatcher that can be used to dispatch component events.
Event dispatchers are functions that can take two arguments: name and detail.
Component events created with createEventDispatcher create a
CustomEvent.
These events do not bubble.
The detail argument corresponds to the CustomEvent.detail
property and can contain any type of data.
The event dispatcher can be typed to narrow the allowed event names and the type of the detail argument:
const dispatch = createEventDispatcher&#x3C;{
 loaded: never; // does not take a detail argument
 change: string; // takes a detail argument of type string, which is required
 optional: number | null; // takes an optional detail argument of type number
}>();
@deprecatedUse callback props and/or the $host() rune instead — see migration guidecreateEventDispatcher<{
	optional: number | nulloptional: number | null;
	required: stringrequired: string;
	noArgument: nullnoArgument: null;
}>();

// Svelte version 3:
const dispatch: EventDispatcher
<"optional">(type: "optional", parameter?: number | null | undefined, options?: DispatchOptions | undefined) => booleandispatch('optional');
const dispatch: EventDispatcher
<"required">(type: "required", parameter: string, options?: DispatchOptions | undefined) => booleandispatch('required'); // je peux toujours omettre l'argument de détail
const dispatch: EventDispatcher
<"noArgument">(type: "noArgument", parameter?: null | undefined, options?: DispatchOptions | undefined) => booleandispatch('noArgument', 'surprise'); // je peux toujours ajouter un argument de détail

// Svelte version 4 avec TypeScript mode strict:
const dispatch: EventDispatcher
<"optional">(type: "optional", parameter?: number | null | undefined, options?: DispatchOptions | undefined) => booleandispatch('optional');
const dispatch: EventDispatcher
<"required">(type: "required", parameter: string, options?: DispatchOptions | undefined) => booleandispatch('required'); // erreur, argument manquant
const dispatch: EventDispatcher
<"noArgument">(type: "noArgument", parameter?: null | undefined, options?: DispatchOptions | undefined) => booleandispatch('noArgument', 'surprise'); // erreur, vous ne pouvez pas fournir d'argument

const action: Action = (node, params) => { ... } // ceci est maintenant un erreur si vous fournissez n'importe quel argument
const const action: Action<HTMLElement, string>action: type Action = /*unresolved*/ anyAction<HTMLElement, string> = (node: anynode, params: anyparams) => { ... } // l'argument est de type string

// exemple où ce changement révèle un bug
onMount(
	// nettoyage() n'est pas appelé car la fonction fournie à onMount est asynchrone
	async () => {
		const qqch = await foo();
           	// nettoyage est appelé car la fonction fournie à onMount est synchrone
	() => {
		foo().then(qqch: anyqqch => {...});
		// ...
		return () => nettoyage();
	}
);

<svelte:options tag="mon-composant" />
<svelte:options customElement="mon-composant" />

import { SvelteComponentTyped } from 'svelte';
import { class SvelteComponent<Props extends Record<string, any> = Record<string, any>, Events extends Record<string, any> = any, Slots extends Record<string, any> = any>
This was the base class for Svelte components in Svelte 4. Svelte 5+ components
are completely different under the hood. For typing, use Component instead.
To instantiate components, use mount instead.
See migration guide for more info.
SvelteComponent } from 'svelte';

export class Foo extends SvelteComponentTyped<{ aProp: string }> {}
export class class FooFoo extends class SvelteComponent<Props extends Record<string, any> = Record<string, any>, Events extends Record<string, any> = any, Slots extends Record<string, any> = any>
This was the base class for Svelte components in Svelte 4. Svelte 5+ components
are completely different under the hood. For typing, use Component instead.
To instantiate components, use mount instead.
See migration guide for more info.
SvelteComponent<{ aProp: stringaProp: string }> {}

<script>
	import ComponentA from './ComponentA.svelte';
	import ComponentB from './ComponentB.svelte';
	import { SvelteComponent } from 'svelte';

	let component: typeof SvelteComponent<any>;

	function choseRandomly() {
		component = Math.random() > 0.5 ? ComponentA : ComponentB;
	}
</script>

<button on:click={choseRandomly}>random</button>
<svelte:element this={component} />

{#if show}
	...
	{#if success}
		<p in:slide>Succès</p>
	{/each}
{/if}

<script>
	import Nested from './Nested.svelte';
</script>

<Nested let:count>
	<p>
		count dans le slot par défaut — disponible : {count}
	</p>
	<p slot="bar">
		count dans le slot "bar" — non disponible : {count}
	</p>
</Nested>

import { 
function preprocess(source: string, preprocessor: PreprocessorGroup | PreprocessorGroup[], options?: {
    filename?: string;
} | undefined): Promise<Processed>
The preprocess function provides convenient hooks for arbitrarily transforming component source code.
For example, it can be used to convert a &#x3C;style lang="sass"> block into vanilla CSS.
preprocess } from 'svelte/compiler';

const { const code: string
The new code
code } = await 
function preprocess(source: string, preprocessor: PreprocessorGroup | PreprocessorGroup[], options?: {
    filename?: string;
} | undefined): Promise<Processed>
The preprocess function provides convenient hooks for arbitrarily transforming component source code.
For example, it can be used to convert a &#x3C;style lang="sass"> block into vanilla CSS.
preprocess(
	source,
	[
		{
			PreprocessorGroup.markup?: MarkupPreprocessor | undefinedmarkup: () => {
				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('markup-1');
			},
			PreprocessorGroup.script?: Preprocessor | undefinedscript: () => {
				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('script-1');
			},
			PreprocessorGroup.style?: Preprocessor | undefinedstyle: () => {
				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('style-1');
			}
		},
		{
			PreprocessorGroup.markup?: MarkupPreprocessor | undefinedmarkup: () => {
				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('markup-2');
			},
			PreprocessorGroup.script?: Preprocessor | undefinedscript: () => {
				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('script-2');
			},
			PreprocessorGroup.style?: Preprocessor | undefinedstyle: () => {
				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('style-2');
			}
		}
	],
	{
		filename?: string | undefinedfilename: 'App.svelte'
	}
);

// Svelte 3 affiche :
// markup-1
// markup-2
// script-1
// script-2
// style-1
// style-2

// Svelte 4 affiche :
// markup-1
// script-1
// style-1
// markup-2
// script-2
// style-2

preprocess: [
	vitePreprocess(),
	mdsvex(mdsvexConfig)
	mdsvex(mdsvexConfig),
	vitePreprocess()
]