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Svelte 4 migration guide

This migration guide provides an overview of how to migrate from Svelte version 3 to 4. See the linked PRs for more details about each change. Use the migration script to migrate some of these automatically: npx svelte-migrate@latest svelte-4

If you're a library author, consider whether to only support Svelte 4 or if it's possible to support Svelte 3 too. Since most of the breaking changes don't affect many people, this may be easily possible. Also remember to update the version range in your peerDependencies.

Minimum version requirements

Upgrade to Node 16 or higher. Earlier versions are no longer supported. (#8566)
If you are using SvelteKit, upgrade to 1.20.4 or newer (sveltejs/kit#10172)
If you are using Vite without SvelteKit, upgrade to vite-plugin-svelte 2.4.1 or newer (#8516)
If you are using webpack, upgrade to webpack 5 or higher and svelte-loader 3.1.8 or higher. Earlier versions are no longer supported. (#8515, 198dbcf)
If you are using Rollup, upgrade to rollup-plugin-svelte 7.1.5 or higher (198dbcf)
If you are using TypeScript, upgrade to TypeScript 5 or higher. Lower versions might still work, but no guarantees are made about that. (#8488)

Browser conditions for bundlers

Bundlers must now specify the browser condition when building a frontend bundle for the browser. SvelteKit and Vite will handle this automatically for you. For Rollup or webpack you may need to adjust your config to ensure it matches what is shown in the rollup-plugin-svelte and svelte-loader documentation. (#8516)

Svelte no longer supports the CommonJS (CJS) format for compiler output and has also removed the svelte/register hook and the CJS runtime version. If you need to stay on the CJS output format, consider using a bundler to convert Svelte's ESM output to CJS in a post-build step. (#8613)

Stricter types for Svelte functions

There are now stricter types for createEventDispatcher, Action, ActionReturn, and onMount:

createEventDispatcher now supports specifying that a payload is optional, required, or non-existent, and the call sites are checked accordingly (#7224)

// @errors: 2554 2345
import { createEventDispatcher } from 'svelte';

const dispatch = createEventDispatcher<{
	optional: number | null;
	required: string;
	noArgument: null;
}>();

// Svelte version 3:
dispatch('optional');
dispatch('required'); // I can still omit the detail argument
dispatch('noArgument', 'surprise'); // I can still add a detail argument

// Svelte version 4 using TypeScript strict mode:
dispatch('optional');
dispatch('required'); // error, missing argument
dispatch('noArgument', 'surprise'); // error, cannot pass an argument

Action and ActionReturn have a default parameter type of undefined now, which means you need to type the generic if you want to specify that this action receives a parameter. The migration script will migrate this automatically (#7442)

-const action: Action = (node, params) => { .. } // this is now an error if you use params in any way
+const action: Action<HTMLElement, string> = (node, params) => { .. } // params is of type string

onMount now shows a type error if you return a function asynchronously from it, because this is likely a bug in your code where you expect the callback to be called on destroy, which it will only do for synchronously returned functions (#8136)

// Example where this change reveals an actual bug
onMount(
- // someCleanup() not called because function handed to onMount is async
- async () => {
-   const something = await foo();
+ // someCleanup() is called because function handed to onMount is sync
+ () => {
+  foo().then(something =>  ..
   // ..
   return () => someCleanup();
}
);

Custom Elements with Svelte

The creation of custom elements with Svelte has been overhauled and significantly improved. The tag option is deprecated in favor of the new customElement option:

-<svelte:options tag="my-component" />
+<svelte:options customElement="my-component" />

This change was made to allow more configurability for advanced use cases. The migration script will adjust your code automatically. The update timing of properties has changed slightly as well. (#8457)

SvelteComponentTyped is deprecated

SvelteComponentTyped is deprecated, as SvelteComponent now has all its typing capabilities. Replace all instances of SvelteComponentTyped with SvelteComponent.

- import { SvelteComponentTyped } from 'svelte';
+ import { SvelteComponent } from 'svelte';

- export class Foo extends SvelteComponentTyped<{ aProp: string }> {}
+ export class Foo extends SvelteComponent<{ aProp: string }> {}

If you have used SvelteComponent as the component instance type previously, you may see a somewhat opaque type error now, which is solved by changing : typeof SvelteComponent to : typeof SvelteComponent<any>.

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

-  let component: typeof SvelteComponent;
+  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} />

The migration script will do both automatically for you. (#8512)

Transitions are local by default

Transitions are now local by default to prevent confusion around page navigations. "local" means that a transition will not play if it's within a nested control flow block (each/if/await/key) and not the direct parent block but a block above it is created/destroyed. In the following example, the slide intro animation will only play when success goes from false to true, but it will not play when show goes from false to true:

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

To make transitions global, add the |global modifier - then they will play when any control flow block above is created/destroyed. The migration script will do this automatically for you. (#6686)

Default slot bindings

Default slot bindings are no longer exposed to named slots and vice versa:

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

<Nested let:count>
	<p>
		count in default slot - is available: {count}
	</p>
	<p slot="bar">
		count in bar slot - is not available: {count}
	</p>
</Nested>

This makes slot bindings more consistent as the behavior is undefined when for example the default slot is from a list and the named slot is not. (#6049)

Preprocessors

The order in which preprocessors are applied has changed. Now, preprocessors are executed in order, and within one group, the order is markup, script, style.

// @errors: 2304
import { preprocess } from 'svelte/compiler';

const { code } = await preprocess(
	source,
	[
		{
			markup: () => {
				console.log('markup-1');
			},
			script: () => {
				console.log('script-1');
			},
			style: () => {
				console.log('style-1');
			}
		},
		{
			markup: () => {
				console.log('markup-2');
			},
			script: () => {
				console.log('script-2');
			},
			style: () => {
				console.log('style-2');
			}
		}
	],
	{
		filename: 'App.svelte'
	}
);

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

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

This could affect you for example if you are using MDsveX - in which case you should make sure it comes before any script or style preprocessor.

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

Each preprocessor must also have a name. (#8618)

New eslint package

eslint-plugin-svelte3 is deprecated. It may still work with Svelte 4 but we make no guarantees about that. We recommend switching to our new package eslint-plugin-svelte. See this Github post for an instruction how to migrate. Alternatively, you can create a new project using npm create svelte@latest, select the eslint (and possibly TypeScript) option and then copy over the related files into your existing project.

Other breaking changes

the inert attribute is now applied to outroing elements to make them invisible to assistive technology and prevent interaction. (#8628)
the runtime now uses classList.toggle(name, boolean) which may not work in very old browsers. Consider using a polyfill if you need to support these browsers. (#8629)
the runtime now uses the CustomEvent constructor which may not work in very old browsers. Consider using a polyfill if you need to support these browsers. (#8775)
people implementing their own stores from scratch using the StartStopNotifier interface (which is passed to the create function of writable etc) from svelte/store now need to pass an update function in addition to the set function. This has no effect on people using stores or creating stores using the existing Svelte stores. (#6750)
derived will now throw an error on falsy values instead of stores passed to it. (#7947)
type definitions for svelte/internal were removed to further discourage usage of those internal methods which are not public API. Most of these will likely change for Svelte 5
Removal of DOM nodes is now batched which slightly changes its order, which might affect the order of events fired if you're using a MutationObserver on these elements (#8763)
if you enhanced the global typings through the svelte.JSX namespace before, you need to migrate this to use the svelteHTML namespace. Similarly if you used the svelte.JSX namespace to use type definitions from it, you need to migrate those to use the types from svelte/elements instead. You can find more information about what to do here

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