--- NOTE: do not edit this file, it is generated in apps/svelte.dev/scripts/sync-docs/index.ts title: 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](https://github.com/sveltejs/svelte/issues/8566)) - If you are using SvelteKit, upgrade to 1.20.4 or newer ([sveltejs/kit#10172](https://github.com/sveltejs/kit/pull/10172)) - If you are using Vite without SvelteKit, upgrade to `vite-plugin-svelte` 2.4.1 or newer ([#8516](https://github.com/sveltejs/svelte/issues/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](https://github.com/sveltejs/svelte/issues/8515), [198dbcf](https://github.com/sveltejs/svelte/commit/198dbcf)) - If you are using Rollup, upgrade to `rollup-plugin-svelte` 7.1.5 or higher ([198dbcf](https://github.com/sveltejs/svelte/commit/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](https://github.com/sveltejs/svelte/issues/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. If you're using any others, you may observe lifecycle callbacks such as `onMount` not get called and you'll need to update the module resolution configuration. - For Rollup this is done within the `@rollup/plugin-node-resolve` plugin by setting `browser: true` in its options. See the [`rollup-plugin-svelte`](https://github.com/sveltejs/rollup-plugin-svelte/#usage) documentation for more details - For webpack this is done by adding `"browser"` to the `conditionNames` array. You may also have to update your `alias` config, if you have set it. See the [`svelte-loader`](https://github.com/sveltejs/svelte-loader#usage) documentation for more details ([#8516](https://github.com/sveltejs/svelte/issues/8516)) ## Removal of CJS related output 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](https://github.com/sveltejs/svelte/issues/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](https://github.com/sveltejs/svelte/issues/7224)) ```ts // @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](https://github.com/sveltejs/svelte/pull/7442)) ```ts // @noErrors ---const action: Action = (node, params) => { ... } // this is now an error if you use params in any way--- +++const action: Action = (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](https://github.com/sveltejs/svelte/issues/8136)) ```js // @noErrors // 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 ------ ++++++ ``` This change was made to allow [more configurability](custom-elements#Component-options) for advanced use cases. The migration script will adjust your code automatically. The update timing of properties has changed slightly as well. ([#8457](https://github.com/sveltejs/svelte/issues/8457)) ## SvelteComponentTyped is deprecated `SvelteComponentTyped` is deprecated, as `SvelteComponent` now has all its typing capabilities. Replace all instances of `SvelteComponentTyped` with `SvelteComponent`. ```js ---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`. ```svelte ``` The migration script will do both automatically for you. ([#8512](https://github.com/sveltejs/svelte/issues/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`: ```svelte {#if show} ... {#if success}

Success

{/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](https://github.com/sveltejs/svelte/issues/6686)) ## Default slot bindings Default slot bindings are no longer exposed to named slots and vice versa: ```svelte

count in default slot - is available: {count}

count in bar slot - is not available: {count}

``` 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](https://github.com/sveltejs/svelte/issues/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. ```js // @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. ```js // @noErrors preprocess: [ --- vitePreprocess(), mdsvex(mdsvexConfig)--- +++ mdsvex(mdsvexConfig), vitePreprocess()+++ ] ``` Each preprocessor must also have a name. ([#8618](https://github.com/sveltejs/svelte/issues/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](https://github.com/sveltejs/eslint-plugin-svelte). See [this Github post](https://github.com/sveltejs/kit/issues/10242#issuecomment-1610798405) 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](https://github.com/sveltejs/svelte/pull/8628)) - the runtime now uses `classList.toggle(name, boolean)` which may not work in very old browsers. Consider using a [polyfill](https://github.com/eligrey/classList.js) if you need to support these browsers. ([#8629](https://github.com/sveltejs/svelte/issues/8629)) - the runtime now uses the `CustomEvent` constructor which may not work in very old browsers. Consider using a [polyfill](https://github.com/theftprevention/event-constructor-polyfill/tree/master) if you need to support these browsers. ([#8775](https://github.com/sveltejs/svelte/pull/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](https://github.com/sveltejs/svelte/issues/6750)) - `derived` will now throw an error on falsy values instead of stores passed to it. ([#7947](https://github.com/sveltejs/svelte/issues/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](https://github.com/sveltejs/svelte/pull/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](https://github.com/sveltejs/language-tools/blob/master/docs/preprocessors/typescript.md#im-getting-deprecation-warnings-for-sveltejsx--i-want-to-migrate-to-the-new-typings)

v4 migration guide