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| Svelte 5 migration guide |
Version 5 comes with an overhauled syntax and reactivity system. While it may look different at first, you'll soon notice many similarities. This guide goes over the changes in detail and shows you how to upgrade. Along with it, we also provide information on why we did these changes.
You don't have to migrate to the new syntax right away - Svelte 5 still supports the old Svelte 4 syntax, and you can mix and match components using the new syntax with components using the old and vice versa. We expect many people to be able to upgrade with only a few lines of code changed initially. There's also a migration script that helps you with many of these steps automatically.
Reactivity syntax changes
At the heart of Svelte 5 is the new runes API. Runes are basically compiler instructions that inform Svelte about reactivity. Syntactically, runes are functions starting with a dollar-sign.
let -> $state
In Svelte 4, a let declaration at the top level of a component was implicitly reactive. In Svelte 5, things are more explicit: a variable is reactive when created using the $state rune. Let's migrate the counter to runes mode by wrapping the counter in $state:
<script>
let count = +++$state(+++0+++)+++;
</script>
Nothing else changes. count is still the number itself, and you read and write directly to it, without a wrapper like .value or getCount().
[!DETAILS] Why we did this
letbeing implicitly reactive at the top level worked great, but it meant that reactivity was constrained - aletdeclaration anywhere else was not reactive. This forced you to resort to using stores when refactoring code out of the top level of components for reuse. This meant you had to learn an entirely separate reactivity model, and the result often wasn't as nice to work with. Because reactivity is more explicit in Svelte 5, you can keep using the same API in an outside the top level of components. Head to TODO LINK TO TUTORIAL to learn more.
$: -> $derived/$effect
In Svelte 4, a $: statement at the top level of a component could be used to declare a derivation, i.e. state that is entirely defined through a computation of other state. In Svelte 5, this is achieved using the $derived rune:
<script>
let count = +++$state(+++0+++)+++;
---$:--- +++const+++ double = +++$derived(+++count * 2+++)+++;
</script>
As with $state, nothing else changes. double is still the number itself, and you read it directly, without a wrapper like .value or getCount().
A $: statement could also be used to create side effects. In Svelte 5, this is achieved using the $effect rune:
<script>
let count = +++$state(+++0+++)+++;
---$:---+++$effect(() =>+++ {
if (count > 5) {
alert('Count is too high!');
}
}+++);+++
</script>
[!DETAILS] Why we did this
$:was a great shorthand and easy to get started with: you could slap a$:in front of most code and it would somehow work. This intuitiveness was also its drawback the more complicated your code became, because it wasn't as easy to reason about. Was the intent of the code to create a derivation, or a side effect? With$derivedand$effect, you have a bit more up-front decision making to do (spoiler alert: 90% of the time you want$derived), but future-you and other developers on your team will have an easier time.There were also gotchas that were hard to spot:
$:only updated directly before rendering, which meant you could read stale values in-between rerenders$:only ran once per tick, which meant that statements may run less often than you think$:dependencies were determined through static analysis of the dependencies. This worked in most cases, but could break in subtle ways during a refactoring where dependencies would be > for example moved into a function and no longer be visible as a result$:statements were also ordered by using static analysis of the dependencies. In some cases there could be ties and the ordering would be wrong as a result, needing manual > interventions. Ordering could also break while refactoring code and some dependencies no longer being visible as a result.Lastly, it wasn't TypeScript-friendly (our editor tooling had to jump through some hoops to make it valid for TypeScript), which was a blocker for making Svelte's reactivity model truly > universal.
$derivedand$effectfix all of these by
- always returning the latest value
- running as often as needed to be stable
- determining the dependencies at runtime, and therefore being immune to refactorings
- executing dependencies as needed and therefore being immune to ordering problems
- being TypeScript-friendly
export let -> $props
In Svelte 4, properties of a component were declared using export let. Each property was one declaration. In Svelte 5, all properties are declared through the $props rune, through destructuring:
<script>
---export let optional = 'unset';
export let required;---
+++let { optional = 'unset', required } = $props();+++
</script>
There are multiple cases where declaring properties becomes less straightforward than having a few export let declarations:
- you want to rename the property, for example because the name is a reserved identifier (e.g.
class) - you don't know which other properties to expect in advance
- you want to forward every property to another component
All these cases need special syntax in Svelte 4:
- renaming:
export { klass as class} - other properties:
$$restProps - all properties
$$props
In Svelte 5, the $props rune makes this straightforward without any additional Svelte-specific syntax:
- renaming: use property renaming
let { class: klass } = $props(); - other properties: use spreading
let { foo, bar, ...rest } = $props(); - all properties: don't destructure
let props = $props();
<script>
---let klass = '';
export { klass as class};---
+++let { class: klass, ...rest } = $props();+++
</script>
<button {class} {...---$$restProps---+++rest+++}>click me</button>
[!DETAILS] Why we did this
export letwas one of the more controversial API decisions, and there was a lot of debate about whether you should think about a property beingexported orimported.$propsdoesn't have this trait. It's also in line with the other runes, and the general thinking reduces to "everything special to reactivity in Svelte is a rune".There were also a lot of limitations around
export let, which required additional API, as shown above.$propsunite this in one syntactical concept that leans heavily on regular JavaScript destructuring syntax.
Event changes
Event handlers have been given a facelift in Svelte 5. Whereas in Svelte 4 we use the on: directive to attach an event listener to an element, in Svelte 5 they are properties like any other (in other words - remove the colon):
<script>
let count = $state(0);
</script>
<button on---:---click={() => count++}>
clicks: {count}
</button>
Since they're just properties, you can use the normal shorthand syntax...
<script>
let count = $state(0);
function onclick() {
count++;
}
</script>
<button {onclick}>
clicks: {count}
</button>
...though when using a named event handler function it's usually better to use a more descriptive name.
Component events
In Svelte 4, components could emit events by creating a dispatcher with createEventDispatcher.
This function is deprecated in Svelte 5. Instead, components should accept callback props - which means you then pass functions as properties to these components:
<!--- file: App.svelte --->
<script>
import Pump from './Pump.svelte';
let size = $state(15);
let burst = $state(false);
function reset() {
size = 15;
burst = false;
}
</script>
<Pump
---on:---inflate={(power) => {
size += power---.details---;
if (size > 75) burst = true;
}}
---on:---deflate={(power) => {
if (size > 0) size -= power---.details---;
}}
/>
{#if burst}
<button onclick={reset}>new balloon</button>
<span class="boom">💥</span>
{:else}
<span class="balloon" style="scale: {0.01 * size}">
🎈
</span>
{/if}
<!--- file: Pump.svelte --->
<script>
---import { createEventDispatcher } from 'svelte';
const dispatch = createEventDispatcher();
---
+++let { inflate, deflate } = $props();+++
let power = $state(5);
</script>
<button onclick={() => ---dispatch('inflate', power)---+++inflate(power)+++}>
inflate
</button>
<button onclick={() => ---dispatch('deflate', power)---+++deflate(power)+++}>
deflate
</button>
<button onclick={() => power--}>-</button>
Pump power: {power}
<button onclick={() => power++}>+</button>
Bubbling events
Instead of doing <button on:click> to 'forward' the event from the element to the component, the component should accept an onclick callback prop:
<script>
+++let { onclick } = $props();+++
</script>
<button ---on:click--- +++{onclick}+++>
click me
</button>
Note that this also means you can 'spread' event handlers onto the element along with other props instead of tediously forwarding each event separately:
<script>
let props = $props();
</script>
<button ---{...$$props} on:click on:keydown on:all_the_other_stuff--- +++{...props}+++>
click me
</button>
Event modifiers
In Svelte 4, you can add event modifiers to handlers:
<button on:click|once|preventDefault={handler}>...</button>
Modifiers are specific to on: and as such do not work with modern event handlers. Adding things like event.preventDefault() inside the handler itself is preferable, since all the logic lives in one place rather than being split between handler and modifiers.
Since event handlers are just functions, you can create your own wrappers as necessary:
<script>
function once(fn) {
return function (event) {
if (fn) fn.call(this, event);
fn = null;
};
}
function preventDefault(fn) {
return function (event) {
event.preventDefault();
fn.call(this, event);
};
}
</script>
<button onclick={once(preventDefault(handler))}>...</button>
There are three modifiers — capture, passive and nonpassive — that can't be expressed as wrapper functions, since they need to be applied when the event handler is bound rather than when it runs.
For capture, we add the modifier to the event name:
<button onclickcapture={...}>...</button>
Changing the passive option of an event handler, meanwhile, is not something to be done lightly. If you have a use case for it — and you probably don't! — then you will need to use an action to apply the event handler yourself.
Multiple event handlers
In Svelte 4, this is possible:
<button on:click={one} on:click={two}>...</button>
Duplicate attributes/properties on elements — which now includes event handlers — are not allowed. Instead, do this:
<button
onclick={(e) => {
one(e);
two(e);
}}
>
...
</button>
When spreading props, local event handlers must go after the spread, or they risk being overwritten:
<button
{...props}
onclick={(e) => {
doStuff(e);
props.onclick?.(e);
}}
>
...
</button>
[!DETAILS] Why we did this
createEventDispatcherwas always a bit boilerplate-y:
- import the function
- call the function to get a dispatch function
- call said dispatch function with a string and possibly a payload
- retrieve said payload on the other end through a
.detailsproperty, because the event itself was always aCustomEventIt was always possible to use component callback props, but because you had to listen to dom events using
on:, it made sense to usecreateEventDispatcherfor component events due to syntactical consistency. Now that we have event attributes (onclick), it's the other way around: Callback props are now the more sensible thing to do.The removal of event modifiers is arguably one of the changes that seems like a step back for those who've liked the shorthand syntax of event modifiers. Given that they are not used that frequently, we traded a smaller surface area for more explicitness. Modifiers also were inconsistent, because most of them were only useable on Dom elements.
Multiple listeners for the same event are also no longer possible, but it was something of an anti-pattern anyway, since it impedes readability: if there are many attributes, it becomes harder to spot that there are two handlers unless they are right next to each other. It also implies that the two handlers are independent, when in fact something like
event.stopImmediatePropagation()insideonewould preventtwofrom being called.By deprecating
createEventDispatcherand theon:directive in favour of callback props and normal element properties, we:
- reduce Svelte's learning curve
- remove boilerplate, particularly around
createEventDispatcher- remove the overhead of creating
CustomEventobjects for events that may not even have listeners- add the ability to spread event handlers
- add the ability to know which event handlers were provided to a component
- add the ability to express whether a given event handler is required or optional
- increase type safety (previously, it was effectively impossible for Svelte to guarantee that a component didn't emit a particular event)
Snippets instead of slots
In Svelte 4, content can be passed to components using slots. Svelte 5 replaces them with snippets which are more powerful and flexible, and as such slots are deprecated in Svelte 5.
They continue to work, however, and you can mix and match snippets and slots in your components.
When using custom elements, you should still use <slot /> like before. In a future version, when Svelte removes its internal version of slots, it will leave those slots as-is, i.e. output a regular DOM tag instead of transforming it.
Default content
In Svelte 4, the easiest way to pass a piece of UI to the child was using a <slot />. In Svelte 5, this is done using the children prop instead, which is then shown with {@render children()}:
<script>
+++let { children } = $props();+++
</script>
---<slot />---
+++{@render children?.()}+++
Multiple content placeholders
If you wanted multiple UI placeholders, you had to use named slots. In Svelte 5, use props instead, name them however you like and {@render ...} them:
<script>
+++let { header, main, footer } = $props();+++
</script>
<header>
---<slot name="header" />---
+++{@render header()}+++
</header>
<main>
---<slot name="main" />---
+++{@render main()}+++
</main>
<footer>
---<slot name="header" />---
+++{@render footer()}+++
</footer>
Passing data back up
In Svelte 4, you would pass data to a <slot /> and then retrieve it with let: in the parent component. In Svelte 5, snippets take on that responsibility:
<!--- file: App.svelte --->
<script>
import List from './List.svelte';
</script>
<List items={['one', 'two', 'three']} ---let:item--->
+++{#snippet item(text)}+++
<span>{text}</span>
+++{/snippet}+++
---<span slot="empty">No items yet</span>---
+++{#snippet empty()}
<span>No items yet</span>
{/snippet}+++
</List>
<!--- file: List.svelte --->
<script>
let { items, +++item, empty+++ } = $props();
</script>
{#if items.length}
<ul>
{#each items as entry}
<li>
---<slot item={entry} />---
+++{@render item(entry)}+++
</li>
{/each}
</ul>
{:else}
---<slot name="empty" />---
+++{@render empty?.()}+++
{/if}
[!DETAILS] Why we did this Slots were easy to get started with, but the more advanced the use case became, the more involved and confusing the syntax became:
- the
let:syntax was confusing to many people as it creates a variable whereas all other:directives receive a variable- the scope of a variable declared with
let:wasn't clear. In the example above, it may look like you can use theitemslot prop in theemptyslot, but that's not true- named slots had to be applied to an element using the
slotattribute. Sometimes you didn't want to create an element, so we had to add the<svelte:fragment>API- named slots could also be applied to a component, which changed the semantics of where
let:directives are available (even today us maintainers often don't know which way around it > works)Snippets solve all of these problems by being much more readable and clear. At the same time they're more powerful as they allow you to define sections of UI that you can render anywhere, not just passing them as props to a component.
Migration script
By now you should have a pretty good understanding of the before/after and how the old syntax relates to the new syntax. It probably also became clear that a lot of these migrations are rather technical and repetitive - something you don't want to do by hand.
We thought the same, which is why we provide a migration script to do most of the migration automatically. You can upgrade your project by using npx sv migrate svelte-5. This will do the following things:
- bump core dependencies in your
package.json - migrate to runes (
let->$stateetc) - migrate to event attributes for Dom elements (
on:click->onclick) - migrate slot creations to render tags (
<slot />->{@render children()}) - migrate slot usages to snippets (
<div slot="x">...</div>->{#snippet x()}<div>...</div>{/snippet}) - migrate obvious component creations (
new Component(...)->mount(Component, ...))
You can also migrate a single component in VS Code through the Migrate Component to Svelte 5 Syntax command, or in our Playground through the Migrate button.
Not everything can be migrated automatically, and some migrations need manual cleanup afterwards. The following sections describe these in more detail.
run
You may see that the migration script converts some of your $: statements to a run function which is imported from svelte/legacy. This happens if the migration script couldn't reliably migrate the statement to a $derived and concluded this is a side effect instead. In some cases this may be wrong and it's best to change this to use a $derived instead. In other cases it may be right, but since $: statements also ran on the server but $effect does not, it isn't safe to transform it as such. Instead, run is used as a stopgap solution. run mimics most of the characteristics of $:, in that it runs on the server once, and runs as $effect.pre on the client ($effect.pre runs before changes are applied to the DOM; most likely you want to use $effect instead).
<script>
---import { run } from 'svelte/legacy';---
---run(() => {---
+++$effect(() => {+++
// some side effect code
})
</script>
Event modifiers
Event modifiers are not applicable to event attributes (e.g. you can't do onclick|preventDefault={...}). Therefore, when migrating event directives to event attributes, we need a function-replacement for these modifiers. These are imported from svelte/legacy, and should be migrated away from in favor of e.g. just using event.preventDefault().
<script>
---import { preventDefault } from 'svelte/legacy';---
</script>
<button
onclick={---preventDefault---((event) => {
+++event.preventDefault();+++
// ...
})}
>
click me
</button>
Things that are not automigrated
The migration script does not convert createEventDispatcher. You need to adjust those parts manually. It doesn't do it because it's too risky because it could result in breakage for users of the component, which the migration script cannot find out.
The migration script does not convert beforeUpdate/afterUpdate. It doesn't do it because it's impossible to determine the actual intent of the code. As a rule of thumb you can often go with a combination of $effect.pre (runs at the same time as beforeUpdate did) and tick (imported from svelte, allows you to wait until changes are applied to the DOM and then do some work).
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 accessors: 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] Note that using
eventsis 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({ foo: 'baz' });---
+++const props = $state({ foo: 'bar' });
const app = mount(App, { target: document.getElementById("app"), props });
props.foo = 'baz';+++
For $destroy, use unmount instead.
+++import { mount, unmount } from 'svelte';+++
import App from './App.svelte'
---const app = new App({ target: document.getElementById("app"), props: { foo: 'bar' } });
app.$destroy();---
+++const app = mount(App, { target: document.getElementById("app") });
unmount(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 compatibility.componentApi compiler option for auto-applied backwards compatibility, which means code using new Component(...) keeps working without adjustments (note that this adds a bit of overhead to each component). This will also add $set and $on methods for all component instances you get through bind:this.
/// svelte.config.js
export default {
compilerOptions: {
compatibility: {
componentApi: 4
}
}
};
Note that mount and hydrate are not synchronous, so things like onMount won't have been called by the time the function returns and the pending block of promises will not have been rendered yet (because #await waits a microtask to wait for a potentially immediately-resolved promise). If you need that guarantee, call flushSync (import from 'svelte') after calling mount/hydrate.
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({ props: { message: 'hello' }});---
+++const { html, head } = render(App, { props: { message: 'hello' }});+++
In Svelte 4, rendering a component to a string also returned the CSS of all components. In Svelte 5, this is no longer the case by default because most of the time you're using a tooling chain that takes care of it in other ways (like SvelteKit). If you need CSS to be returned from render, you can set the css compiler option to 'injected' and it will add <style> elements to the head.
Component typing changes
The change from classes towards functions is also reflected in the typings: SvelteComponent, the base class from Svelte 4, is deprecated in favour of the new Component type which defines the function shape of a Svelte component. To manually define a component shape in a d.ts file:
import type { Component } from 'svelte';
export declare const MyComponent: Component<{
foo: string;
}>;
To declare that a component of a certain type is required:
<script lang="ts">
import type { Component } from 'svelte';
import {
ComponentA,
ComponentB
} from 'component-library';
let component: Component<{ foo: string }> = $state(
Math.random() ? ComponentA : ComponentB
);
</script>
<svelte:component this={component} foo="bar" />
The two utility types ComponentEvents and ComponentType are also deprecated. ComponentEvents is obsolete because events are defined as callback props now, and ComponentType is obsolete because the new Component type is the component type already (e.g. ComponentType<SvelteComponent<{ prop: string }>> == Component<{ prop: string }>).
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
pretags
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>.
Modern browser required
Svelte 5 requires a modern browser (in other words, not Internet Explorer) for various reasons:
- it uses
Proxies - elements with
clientWidth/clientHeight/offsetWidth/offsetHeightbindings use aResizeObserverrather than a convoluted<iframe>hack <input type="range" bind:value={...} />only uses aninputevent listener, rather than also listening forchangeevents as a fallback
The legacy compiler option, which generated bulkier but IE-friendly code, no longer exists.
Changes to compiler options
- The
false/true(already deprecated previously) and the"none"values were removed as valid values from thecssoption - The
legacyoption was repurposed - The
hydratableoption has been removed. Svelte components are always hydratable now - The
enableSourcemapoption has been removed. Source maps are always generated now, tooling can choose to ignore it - The
tagoption was removed. Use<svelte:options customElement="tag-name" />inside the component instead - The
loopGuardTimeout,format,sveltePath,errorModeandvarsReportoptions 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.
Dot notation indicates a component
In Svelte 4, <foo.bar> would create an element with a tag name of "foo.bar". In Svelte 5, foo.bar is treated as a component instead. This is particularly useful inside each blocks:
{#each items as item}
<item.component {...item.props} />
{/each}
Breaking changes in runes mode
Some breaking changes only apply once your component is in runes mode.
Bindings to component exports are not allowed
Exports from runes mode components cannot be bound to directly. For example, having export const foo = ... in component A and then doing <A bind:foo /> causes an error. Use bind:this instead — <A bind:this={a} /> — and access the export as a.foo. This change makes things easier to reason about, as it enforces a clear separation between props and exports.
Bindings need to be explicitly defined using $bindable()
In Svelte 4 syntax, every property (declared via export let) is bindable, meaning you can bind: to it. In runes mode, properties are not bindable by default: you need to denote bindable props with the $bindable rune.
If a bindable property has a default value (e.g. let { foo = $bindable('bar') } = $props();), you need to pass a non-undefined value to that property if you're binding to it. This prevents ambiguous behavior — the parent and child must have the same value — and results in better performance (in Svelte 4, the default value was reflected back to the parent, resulting in wasteful additional render cycles).
accessors option is ignored
Setting the accessors option to true makes properties of a component directly accessible on the component instance. In runes mode, properties are never accessible on the component instance. You can use component exports instead if you need to expose them.
immutable option is ignored
Setting the immutable option has no effect in runes mode. This concept is replaced by how $state and its variations work.
Classes are no longer "auto-reactive"
In Svelte 4, doing the following triggered reactivity:
<script>
let foo = new Foo();
</script>
<button on:click={() => (foo.value = 1)}>{foo.value}</button
>
This is because the Svelte compiler treated the assignment to foo.value as an instruction to update anything that referenced foo. In Svelte 5, reactivity is determined at runtime rather than compile time, so you should define value as a reactive $state field on the Foo class. Wrapping new Foo() with $state(...) will have no effect — only vanilla objects and arrays are made deeply reactive.
<svelte:component> is no longer necessary
In Svelte 4, components are static — if you render <Thing>, and the value of Thing changes, nothing happens. To make it dynamic you must use <svelte:component>.
This is no longer true in Svelte 5:
<script>
import A from './A.svelte';
import B from './B.svelte';
let Thing = $state();
</script>
<select bind:value={Thing}>
<option value={A}>A</option>
<option value={B}>B</option>
</select>
<!-- these are equivalent -->
<Thing />
<svelte:component this={Thing} />
Touch and wheel events are passive
When using onwheel, onmousewheel, ontouchstart and ontouchmove event attributes, the handlers are passive to align with browser defaults. This greatly improves responsiveness by allowing the browser to scroll the document immediately, rather than waiting to see if the event handler calls event.preventDefault().
In the very rare cases that you need to prevent these event defaults, you should use on instead (for example inside an action).
Attribute/prop syntax is stricter
In Svelte 4, complex attribute values needn't be quoted:
<Component prop=this{is}valid />
This is a footgun. In runes mode, if you want to concatenate stuff you must wrap the value in quotes:
<Component prop="this{is}valid" />
Note that Svelte 5 will also warn if you have a single expression wrapped in quotes, like answer="{42}" — in Svelte 6, that will cause the value to be converted to a string, rather than passed as a number.
HTML structure is stricter
In Svelte 4, you were allowed to write HTML code that would be repaired by the browser when server side rendering it. For example you could write this...
<table>
<tr>
<td>hi</td>
</tr>
</table>
... and the browser would auto-insert a <tbody> element:
<table>
<tbody>
<tr>
<td>hi</td>
</tr>
</tbody>
</table>
Svelte 5 is more strict about the HTML structure and will throw a compiler error in cases where the browser would repair the DOM.
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.
:is(...) and :where(...) are scoped
Previously, Svelte did not analyse selectors inside :is(...) and :where(...), effectively treating them as global. Svelte 5 analyses them in the context of the current component. As such, some selectors may now be treated as unused if they were relying on this treatment. To fix this, use :global(...) inside the :is(...)/:where(...) selectors.
When using Tailwind's @apply directive, add a :global selector to preserve rules that use Tailwind-generated :is(...) selectors:
main +++:global+++ {
@apply bg-blue-100 dark:bg-blue-900;
}
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:
// @errors: 2552
css = css.replace(/:where\((.+?)\)/, '$1');
Error/warning codes have been renamed
Error and warning codes have been renamed. Previously they used dashes to separate the words, they now use underscores (e.g. foo-bar becomes foo_bar). Additionally, a handful of codes have been reworded 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), mathml and svg.
The foreign namespace was only useful for Svelte Native, which we're planning to support differently in a 5.x minor.
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.
Bindings now react to form resets
Previously, bindings did not take into account reset event of forms, and therefore values could get out of sync with the DOM. Svelte 5 fixes this by placing a reset listener on the document and invoking bindings where necessary.
walk no longer exported
svelte/compiler reexported walk from estree-walker for convenience. This is no longer true in Svelte 5, import it directly from that package instead in case you need it.
Content inside svelte:options is forbidden
In Svelte 4 you could have content inside a <svelte:options /> tag. It was ignored, but you could write something in there. In Svelte 5, content inside that tag is a compiler error.
<slot> elements in declarative shadow roots are preserved
Svelte 4 replaced the <slot /> tag in all places with its own version of slots. Svelte 5 preserves them in the case they are a child of a <template shadowrootmode="..."> element.
<svelte:element> tag must be an expression
In Svelte 4, <svelte:element this="div"> is valid code. This makes little sense — you should just do <div>. In the vanishingly rare case that you do need to use a literal value for some reason, you can do this:
<svelte:element this=+++{+++"div"+++}+++>
Note that whereas Svelte 4 would treat <svelte:element this="input"> (for example) identically to <input> for the purposes of determining which bind: directives could be applied, Svelte 5 does not.
mount plays transitions by default
The mount function used to render a component tree plays transitions by default unless the intro option is set to false. This is different from legacy class components which, when manually instantiated, didn't play transitions by default.
<img src={...}> and {@html ...} hydration mismatches are not repaired
In Svelte 4, if the value of a src attribute or {@html ...} tag differ between server and client (a.k.a. a hydration mismatch), the mismatch is repaired. This is very costly: setting a src attribute (even if it evaluates to the same thing) causes images and iframes to be reloaded, and reinserting a large blob of HTML is slow.
Since these mismatches are extremely rare, Svelte 5 assumes that the values are unchanged, but in development will warn you if they are not. To force an update you can do something like this:
<script>
let { markup, src } = $props();
if (typeof window !== 'undefined') {
// stash the values...
const initial = { markup, src };
// unset them...
markup = src = undefined;
$effect(() => {
// ...and reset after we've mounted
markup = initial.markup;
src = initial.src;
});
}
</script>
{@html markup}
<img {src} />
Hydration works differently
Svelte 5 makes use of comments during server side rendering which are used for more robust and efficient hydration on the client. As such, you shouldn't remove comments from your HTML output if you intend to hydrate it, and if you manually authored HTML to be hydrated by a Svelte component, you need to adjust that HTML to include said comments at the correct positions.
onevent attributes are delegated
Event attributes replace event directives: Instead of on:click={handler} you write onclick={handler}. For backwards compatibility the on:event syntax is still supported and behaves the same as in Svelte 4. Some of the onevent attributes however are delegated, which means you need to take care to not stop event propagation on those manually, as they then might never reach the listener for this event type at the root.
--style-props uses a different element
Svelte 5 uses an extra <svelte-css-wrapper> element instead of a <div> to wrap the component when using CSS custom properties.