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Breaking changes

While Svelte 5 is a complete rewrite, we have done our best to ensure that most codebases can upgrade with a minimum of hassle. That said, there are a few small breaking changes which may require action on your part. They are listed here.

Components are no longer classes

In Svelte 3 and 4, components are classes. In Svelte 5 they are functions and should be instantiated differently. If you need to manually instantiate components, you should use mount or hydrate (imported from svelte) instead. If you see this error using SvelteKit, try updating to the latest version of SvelteKit first, which adds support for Svelte 5. If you're using Svelte without SvelteKit, you'll likely have a main.js file (or similar) which you need to adjust:

 import { mount } from 'svelte';
import App from './App.svelte'

 const app = new App({ target: document.getElementById("app") });
 const app = mount(App, { target: document.getElementById("app") });

export default app;

mount and hydrate have the exact same API. The difference is that hydrate will pick up the Svelte's server-rendered HTML inside its target and hydrate it. Both return an object with the exports of the component and potentially property accessors (if compiled with accesors: true). They do not come with the $on, $set and $destroy methods you may know from the class component API. These are its replacements:

For $on, instead of listening to events, pass them via the events property on the options argument.

 import { mount } from 'svelte';
import App from './App.svelte'

 const app = new App({ target: document.getElementById("app") });
 app.$on('event', callback);
 const app = mount(App, { target: document.getElementById("app"), events: { event: callback } });

Note that using events is discouraged — instead, use callbacks

For $set, use $state instead to create a reactive property object and manipulate it. If you're doing this inside a .js or .ts file, adjust the ending to include .svelte, i.e. .svelte.js or .svelte.ts.

 import { mount } from 'svelte';
import App from './App.svelte'

 const app = new App({ target: document.getElementById("app"), props: { foo: 'bar' } });
 app.$set('event', { foo: 'baz' });
 const props = $state({ foo: 'bar' });
 const app = mount(App, { target: document.getElementById("app"), props }); = 'baz';

For $destroy, use unmount instead.

 import { mount, unmount } from 'svelte';
import App from './App.svelte'

 const app = new App({ target: document.getElementById("app"), props: { foo: 'bar' } });
 const app = mount(App, { target: document.getElementById("app") });

As a stop-gap-solution, you can also use createClassComponent or asClassComponent (imported from svelte/legacy) instead to keep the same API known from Svelte 4 after instantiating.

 import { createClassComponent } from 'svelte/legacy';
import App from './App.svelte'

 const app = new App({ target: document.getElementById("app") });
 const app = createClassComponent({ component: App, target: document.getElementById("app") });

export default app;

If this component is not under your control, you can use the legacy.componentApi compiler option for auto-applied backwards compatibility (note that this adds a bit of overhead to each component).

Server API changes

Similarly, components no longer have a render method when compiled for server side rendering. Instead, pass the function to render from svelte/server:

 import { render } from 'svelte/server';
import App from './App.svelte';

 const { html, head } = App.render({ message: 'hello' });
 const { html, head } = render(App, { props: { message: 'hello' } });

render also no longer returns CSS; it should be served separately from a CSS file.

bind:this changes

Because components are no longer classes, using bind:this no longer returns a class instance with $set, $on and $destroy methods on it. It only returns the instance exports (export function/const) and, if you're using the accessors option, a getter/setter-pair for each property.

Whitespace handling changed

Previously, Svelte employed a very complicated algorithm to determine if whitespace should be kept or not. Svelte 5 simplifies this which makes it easier to reason about as a developer. The rules are:

  • Whitespace between nodes is collapsed to one whitespace
  • Whitespace at the start and end of a tag is removed completely
  • Certain exceptions apply such as keeping whitespace inside pre tags

As before, you can disable whitespace trimming by setting the preserveWhitespace option in your compiler settings or on a per-component basis in <svelte:options>.

More recent browser required

Svelte now use Mutation Observers instead of IFrames to measure dimensions for bind:clientWidth/clientHeight/offsetWidth/offsetHeight. It also no longer listens to the change event on range inputs. Lastly, the legacy option was removed (or rather, replaced with a different set of settings).

Changes to compiler options

  • The false/true (already deprecated previously) and the "none" values were removed as valid values from the css option
  • The legacy option was repurposed
  • The hydratable option has been removed. Svelte components are always hydratable now
  • The enableSourcemap option has been removed. Source maps are always generated now, tooling can choose to ignore it
  • The tag option was removed. Use <svelte:options customElement="tag-name" /> inside the component instead
  • The loopGuardTimeout, format, sveltePath, errorMode and varsReport options were removed

The children prop is reserved

Content inside component tags becomes a snippet prop called children. You cannot have a separate prop by that name.

Other breaking changes

Stricter @const assignment validation

Assignments to destructured parts of a @const declaration are no longer allowed. It was an oversight that this was ever allowed.

Stricter CSS :global selector validation

Previously, a compound selector starting with a global modifier which has universal or type selectors (like :global(span).foo) was valid. In Svelte 5, this is a validation error instead. The reason is that in this selector the resulting CSS would be equivalent to one without :global - in other words, :global is ignored in this case.

CSS hash position no longer deterministic

Previously Svelte would always insert the CSS hash last. This is no longer guaranteed in Svelte 5. This is only breaking if you have very weird css selectors.

Scoped CSS uses :where(...)

To avoid issues caused by unpredictable specificity changes, scoped CSS selectors now use :where(.svelte-xyz123) selector modifiers alongside .svelte-xyz123 (where xyz123 is, as previously, a hash of the <style> contents). You can read more detail here.

In the event that you need to support ancient browsers that don't implement :where, you can manually alter the emitted CSS, at the cost of unpredictable specificity changes:

css = css.replace(/:where\((.+?)\)/, '$1');

Renames of various error/warning codes

Various error and warning codes have been renamed slightly.

Reduced number of namespaces

The number of valid namespaces you can pass to the compiler option namespace has been reduced to html (the default), svg and foreign.

beforeUpdate/afterUpdate changes

beforeUpdate no longer runs twice on initial render if it modifies a variable referenced in the template.

afterUpdate callbacks in a parent component will now run after afterUpdate callbacks in any child components.

Both functions are disallowed in runes mode — use $effect.pre(...) and $effect(...) instead.

contenteditable behavior change

If you have a contenteditable node with a corresponding binding and a reactive value inside it (example: <div contenteditable=true bind:textContent>count is {count}</div>), then the value inside the contenteditable will not be updated by updates to count because the binding takes full control over the content immediately and it should only be updated through it.

oneventname attributes no longer accept string values

In Svelte 4, it was possible to specify event attributes on HTML elements as a string:

<button onclick="alert('hello')">...</button>

This is not recommended, and is no longer possible in Svelte 5, where properties like onclick replace on:click as the mechanism for adding event handlers.

null and undefined become the empty string

In Svelte 4, null and undefined were printed as the corresponding string. In 99 out of 100 cases you want this to become the empty string instead, which is also what most other frameworks out there do. Therefore, in Svelte 5, null and undefined become the empty string.

bind:files values can only be null, undefined or FileList

bind:files is now a two-way binding. As such, when setting a value, it needs to be either falsy (null or undefined) or of type FileList.