From 6e9b2a6fd85e01a12aaca4b883eea4d15ae14d36 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 27 Feb 2026 15:35:19 -0500
Subject: [PATCH] docs: best practices (#17804)

Based on #17727 but with a few changes:

- smaller (fewer tokens)
- removed some bits that are really just summarising existing docs
- added an explicit 'don't use these legacy features' section
- shuffled some things around a bit

---------

Co-authored-by: paoloricciuti <ricciutipaolo@gmail.com>
Co-authored-by: Federico Varano <50919335+fvarano@users.noreply.github.com>
Co-authored-by: ComputerGuy <63362464+Ocean-OS@users.noreply.github.com>
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+---
+title: Best practices
+name: svelte-core-bestpractices
+description: Guidance on writing fast, robust, modern Svelte code. Load this skill whenever in a Svelte project and asked to write/edit or analyze a Svelte component or module. Covers reactivity, event handling, styling, integration with libraries and more.
+---
+
+<!-- llm-ignore-start -->
+This document outlines some best practices that will help you write fast, robust Svelte apps. It is also available as a `svelte-core-bestpractices` skill for your agents.
+<!-- llm-ignore-end -->
+
+## `$state`
+
+Only use the `$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`:
+
+```js
+// @errors: 2451
+let num = 0;
+// ---cut---
+// 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 ...}`](@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](bind#Function-bindings) as appropriate
+- If you need to log values for debugging purposes, use [`$inspect`]($inspect)
+- If you need to observe something external to Svelte, use [`createSubscriber`](svelte-reactivity#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`:
+
+```js
+// @errors: 2451
+let { type } = $props();
+
+// do this
+let color = $derived(type === 'danger' ? 'red' : 'green');
+
+// don't do this — `color` will not update if `type` changes
+let color = type === 'danger' ? 'red' : 'green';
+```
+
+## `$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:
+
+```svelte
+<button onclick={() => {...}}>click me</button>
+
+<!-- attribute shorthand also works -->
+<button {onclick}>...</button>
+
+<!-- so do spread attributes -->
+<button {...props}>...</button>
+```
+
+If you need to attach listeners to `window` or `document` you can use `<svelte:window>` and `<svelte:document>`:
+
+```svelte
+<svelte:window onkeydown={...} />
+<svelte:document onvisibilitychange={...} />
+```
+
+Avoid using `onMount` or `$effect` for this.
+
+## Snippets
+
+[Snippets](snippet) are a way to define reusable chunks of markup that can be instantiated with the [`{@render ...}`](@render) tag, or passed to components as props. They must be declared within the template.
+
+```svelte
+{#snippet greeting(name)}
+  <p>hello {name}!</p>
+{/snippet}
+
+{@render greeting('world')}
+```
+
+> [!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](each#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.
+
+```svelte
+<div style:--columns={columns}>...</div>
+```
+
+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:
+
+```svelte
+<!-- Parent.svelte -->
+<Child --color="red" />
+
+<!-- Child.svelte -->
+<h1>Hello</h1>
+
+<style>
+	h1 {
+		color: var(--color);
+	}
+</style>
+```
+
+If this impossible (for example, the child component comes from a library) you can use `:global` to override styles:
+
+```svelte
+<div>
+	<Child />
+</div>
+
+<style>
+	div :global {
+		h1 {
+			color: red;
+		}
+	}
+</style>
+```
+
+## 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](await-expressions) and [hydratable](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