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 classespermalink

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 createRoot (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;

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

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.

Whitespace handling changedpermalink

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 requiredpermalink

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 optionspermalink

• 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 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 reservedpermalink

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

Other breaking changespermalink

Stricter @const assignment validationpermalink

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

CSS hash position no longer deterministicpermalink

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.

beforeUpdate changepermalink

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

contenteditable behavior changepermalink

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 valuespermalink

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.