svelte-core-bestpractices

$state

Only use the $state rune for variables that should be reactive — in other words, variables that cause an $effect, $derived or template expression to update. Everything else can be a normal variable.

Objects and arrays ($state({...}) or $state([...])) are made deeply reactive, meaning mutation will trigger updates. This has a trade-off: in exchange for fine-grained reactivity, the objects must be proxied, which has performance overhead. In cases where you're dealing with large objects that are only ever reassigned (rather than mutated), use $state.raw instead. This is often the case with API responses, for example.

$derived

To compute something from state, use $derived rather than $effect:

// do this
let square = $derived(num * num);
 
// don't do this
let square;
 
$effect(() => {
	square = num * num;
});

[!NOTE] $derived is given an expression, not a function. If you need to use a function (because the expression is complex, for example) use $derived.by.

Deriveds are writable — you can assign to them, just like $state, except that they will re-evaluate when their expression changes.

If the derived expression is an object or array, it will be returned as-is — it is not made deeply reactive. You can, however, use $state inside $derived.by in the rare cases that you need this.

$effect

Effects are an escape hatch and should mostly be avoided. In particular, avoid updating state inside effects.

• If you need to sync state to an external library such as D3, it is often neater to use {@attach ...}
• If you need to run some code in response to user interaction, put the code directly in an event handler or use a function binding as appropriate
• If you need to log values for debugging purposes, use $inspect
• If you need to observe something external to Svelte, use createSubscriber

Never wrap the contents of an effect in if (browser) {...} or similar — effects do not run on the server.

$props

Treat props as though they will change. For example, values that depend on props should usually use $derived:

$inspect.trace

$inspect.trace is a debugging tool for reactivity. If something is not updating properly or running more than it should you can add $inspect.trace(label) as the first line of an $effect or $derived.by (or any function they call) to trace their dependencies and discover which one triggered an update.

Events

Any element attribute starting with on is treated as an event listener:

If you need to attach listeners to window or document you can use <svelte:window> and <svelte:document>:

Avoid using onMount or $effect for this.

Snippets

Snippets are a way to define reusable chunks of markup that can be instantiated with the {@render ...} tag, or passed to components as props. They must be declared within the template.

[!NOTE] Snippets declared at the top level of a component (i.e. not inside elements or blocks) can be referenced inside <script>. A snippet that doesn't reference component state is also available in a <script module>, in which case it can be exported for use by other components.

Each blocks

Prefer to use keyed each blocks — this improves performance by allowing Svelte to surgically insert or remove items rather than updating the DOM belonging to existing items.

[!NOTE] The key must uniquely identify the object. Do not use the index as a key.

Avoid destructuring if you need to mutate the item (with something like bind:value={item.count}, for example).

Using JavaScript variables in CSS

If you have a JS variable that you want to use inside CSS you can set a CSS custom property with the style: directive.

You can then reference var(--columns) inside the component's <style>.

Styling child components

The CSS in a component's <style> is scoped to that component. If a parent component needs to control the child's styles, the preferred way is to use CSS custom properties:

If this is impossible (for example, the child component comes from a library) you can use :global to override styles:

Context

Consider using context instead of declaring state in a shared module. This will scope the state to the part of the app that needs it, and eliminate the possibility of it leaking between users when server-side rendering.

Use createContext rather than setContext and getContext, as it provides type safety.

Async Svelte

If using version 5.36 or higher, you can use await expressions and hydratable to use promises directly inside components. Note that these require the experimental.async option to be enabled in svelte.config.js as they are not yet considered fully stable.

Avoid legacy features

Always use runes mode for new code, and avoid features that have more modern replacements:

• use $state instead of implicit reactivity (e.g. let count = 0; count += 1)
• use $derived and $effect instead of $: assignments and statements (but only use effects when there is no better solution)
• use $props instead of export let, $$props and $$restProps
• use onclick={...} instead of on:click={...}
• use {#snippet ...} and {@render ...} instead of <slot> and $$slots and <svelte:fragment>
• use <DynamicComponent> instead of <svelte:component this={DynamicComponent}>
• use import Self from './ThisComponent.svelte' and <Self> instead of <svelte:self>
• use classes with $state fields to share reactivity between components, instead of using stores
• use {@attach ...} instead of use:action
• use clsx-style arrays and objects in class attributes, instead of the class: directive