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Svelte 5 vs Svelte 4 .cursorrules prompt file

I'm using svelte 5 instead of svelte 4 here is an overview of the changes.

Overview

Svelte 5 introduces runes, a set of advanced primitives for controlling reactivity. The runes replace certain non-runes features and provide more explicit control over state and effects.

$state

Purpose: Declare reactive state.
Usage:

`<script>`let count = $state(0);`</script>`

Replaces: Top-level let declarations in non-runes mode.
Class Fields:

class Todo {	done = $state(false);	text = $state();	constructor(text) {		this.text = text;	}}

Deep Reactivity: Only plain objects and arrays become deeply reactive.

$state.raw

Purpose: Declare state that cannot be mutated, only reassigned.
Usage:

`<script>`let numbers = $state.raw([1, 2, 3]);`</script>`

Performance: Improves with large arrays and objects.

$state.snapshot

Purpose: Take a static snapshot of $state.
Usage:

`<script>`	let counter = $state({ count: 0 });	function onClick() {		console.log($state.snapshot(counter));	}`</script>`

$derived

Purpose: Declare derived state.
Usage:

`<script>`let count = $state(0); let doubled = $derived(count * 2);`</script>`

Replaces: Reactive variables computed using $: in non-runes mode.

$derived.by

Purpose: Create complex derivations with a function.
Usage:

`<script>`	let numbers = $state([1, 2, 3]); let total = $derived.by(() => numbers.reduce((a, b) => a + b,	0));`</script>`

$effect

Purpose: Run side-effects when values change.
Usage:

`<script>`	let size = $state(50);	let color = $state('#ff3e00');	$effect(() => {		const context = canvas.getContext('2d');		context.clearRect(0, 0, canvas.width, canvas.height);		context.fillStyle = color;		context.fillRect(0, 0, size, size);	});`</script>`

Replacements: $effect replaces a substantial part of $: {} blocks triggering side-effects.

$effect.pre

Purpose: Run code before the DOM updates.
Usage:

`<script>`	$effect.pre(() =>{' '}	{		// logic here	}	);`</script>`

Replaces: beforeUpdate.

$effect.tracking

Purpose: Check if code is running inside a tracking context.
Usage:

`<script>`console.log('tracking:', $effect.tracking());`</script>`

$props

Purpose: Declare component props.
Usage:

`<script>`let {(prop1, prop2)} = $props();`</script>`

Replaces: export let syntax for declaring props.

$bindable

Purpose: Declare bindable props.
Usage:

`<script>`let {(bindableProp = $bindable('fallback'))} = $props();`</script>`

$inspect

Purpose: Equivalent to console.log but re-runs when its argument changes.
Usage:

`<script>`let count = $state(0); $inspect(count);`</script>`

$host

Purpose: Retrieve the this reference of the custom element.
Usage:

`<script>`	function greet(greeting) {		$host().dispatchEvent(new CustomEvent('greeting', { detail: greeting }));	}`</script>`

Note: Only available inside custom element components on the client-side.

Overview of snippets in svelte 5

Snippets, along with render tags, help create reusable chunks of markup inside your components, reducing duplication and enhancing maintainability.

Snippets Usage

Definition: Use the #snippet syntax to define reusable markup sections.
Basic Example:

{#snippet figure(image)}	`<figure>`		`<img src={image.src} alt={image.caption} width={image.width} height={image.height} />`		`<figcaption>`{image.caption}`</figcaption>`	`</figure>`{/snippet}

Invocation: Render predefined snippets with @render:

{@render figure(image)}

Destructuring Parameters: Parameters can be destructured for concise usage:

{#snippet figure({ src, caption, width, height })}	`<figure>`		`<img alt={caption} {src} {width} {height} />`		`<figcaption>`{caption}`</figcaption>`	`</figure>`{/snippet}

Snippet Scope

Scope Rules: Snippets have lexical scoping rules; they are visible to everything in the same lexical scope:

`<div>`	{#snippet x()}		{#snippet y()}...{/snippet}		`<!-- valid usage -->`		{@render y()}	{/snippet}	`<!-- invalid usage -->`	{@render y()}`</div>``<!-- invalid usage -->`{@render x()}

Recursive References: Snippets can self-reference or reference other snippets:

{#snippet blastoff()}	`<span>`🚀`</span>`{/snippet}{#snippet countdown(n)}	{#if n > 0}		`<span>`{n}...`</span>`		{@render countdown(n - 1)}	{:else}		{@render blastoff()}	{/if}{/snippet}{@render countdown(10)}

Passing Snippets to Components

Direct Passing as Props:

`<script>`	import Table from './Table.svelte';	const fruits = [{ name: 'apples', qty: 5, price: 2 }, ...];`</script>`{#snippet header()}	`<th>`fruit`</th>`	`<th>`qty`</th>`	`<th>`price`</th>`	`<th>`total`</th>`{/snippet}{#snippet row(fruit)}	`<td>`{fruit.name}`</td>`	`<td>`{fruit.qty}`</td>`	`<td>`{fruit.price}`</td>`	`<td>`{fruit.qty * fruit.price}`</td>`{/snippet}`<Table data={fruits} {header} {row} />`

Implicit Binding:

`<table data="{fruits}">`	{#snippet header()}	`<th>`fruit`</th>`	`<th>`qty`</th>`	`<th>`price`</th>`	`<th>`total`</th>`	{/snippet} {#snippet row(fruit)}	`<td>`{fruit.name}`</td>`	`<td>`{fruit.qty}`</td>`	`<td>`{fruit.price}`</td>`	`<td>`{fruit.qty * fruit.price}`</td>`	{/snippet}`</table>`

Children Snippet: Non-snippet content defaults to the children snippet:

`<table data="{fruits}">`	`<th>`fruit`</th>`	`<th>`qty`</th>`	`<th>`price`</th>`	`<th>`total`</th>`	`<!-- additional content -->``</table>``<script>`	let { data, children, row } = $props();`</script>``<table>`	`<thead>`		`<tr>`			{@render children()}		`</tr>`	`</thead>`	`<!-- table body -->``</table>`

Avoid Conflicts: Do not use a prop named children if also providing content inside the component.

Typing Snippets

TypeScript Integration:

`<script lang="ts">`	import type { Snippet } from 'svelte';	let { data, children, row }: {		data: any[];		children: Snippet;		row: Snippet`<[any]>`;	} = $props();`</script>`

Generics for Improved Typing:

`<script lang="ts" generics="T">`	import type { Snippet } from 'svelte';	let { data, children, row }: {		data: T[];		children: Snippet;		row: Snippet`<[T]>`;	} = $props();`</script>`

Creating Snippets Programmatically

Advanced Use: Create snippets programmatically using createRawSnippet where necessary.

Snippets and Slots

Mixing with Slots: Slots are deprecated but still work. Snippets provide more flexibility and power.
Custom Elements: Continue using <slot /> for custom elements as usual.

Sure! Here are the succinct instructions for handling Event Handlers in Svelte 5, tailored for the AI-integrated code editor to help it understand and utilize these features effectively.

Custom Instructions for Svelte 5 Event Handlers in Cursor AI

Overview

In Svelte 5, event handlers are treated as properties, simplifying their use and integrating them more closely with the rest of the properties in the component.

Basic Event Handlers

Declaration: Use properties to attach event handlers.

`<script>`	let count = $state(0);`</script>``<button onclick={() =>` count++}>	clicks: {count}`</button>`

Shorthand Syntax:

`<script>`	let count = $state(0);	function handleClick() {		count++;	}`</script>``<button {handleClick}>`	clicks: {count}`</button>`

Deprecation: The traditional on: directive is deprecated.

Component Events

Replacing createEventDispatcher: Components should accept callback props instead of using createEventDispatcher.

`<script>`	import Pump from './Pump.svelte';	let size = $state(15);	let burst = $state(false);	function reset() {		size = 15;		burst = false;	}`</script>``<Pump	inflate={(power) =>` { size += power; if (size > 75) burst = true; }}	deflate={(power) => { if (size > 0) size -= power; }}/>{#if burst}	`<button onclick={reset}>`new balloon`</button>`	`<span class="boom">`💥`</span>`{:else}	`<span class="balloon" style="scale: {0.01 * size}">` 🎈 `</span>`{/if}

Bubbling Events

Accept Callback Props:

`<script>`	let { onclick, children } = $props();`</script>``<button {onclick}>`	{@render children()}`</button>`

Spreading Props:

`<script>`	let { children, ...props } = $props();`</script>``<button {...props}>`	{@render children()}`</button>`

Event Modifiers

Avoiding Modifiers: Modifiers like |once, |preventDefault, etc., are not supported. Use wrapper functions instead.
Example Wrapper Functions:

`<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>`

Special Modifiers: For capture:

`<button onclickcapture={...}>`...`</button>`

Multiple Event Handlers

Combining Handlers: Instead of using multiple handlers, combine them into one.

`<button	onclick={(e) =>` {		handlerOne(e);		handlerTwo(e);	}}>	...`</button>`

examples old vs new

Counter Example

Svelte 4 vs. Svelte 5:
Before:
```html
<script>
let count = 0;
$: double = count * 2;
$: {
if (count > 10) alert('Too high!');
}
</script>
<button on:click="{()" ="">count++}> {count} / {double}</button>
```
After:
```html
<script>
let count = $state(0);
let double = $derived(count * 2);
$effect(() => {
if (count > 10) alert('Too high!');
});
</script>
<button onclick="{()" ="">count++}> {count} / {double}</button>
```

Tracking Dependencies

Svelte 4 vs. Svelte 5:
Before:
```html
<script>
let a = 0;
let b = 0;
$: sum = add(a, b);
function add(x, y) {
return x + y;
}
</script>
<button on:click="{()" ="">a++}>a++</button>
<button on:click="{()" ="">b++}>b++</button>
<p>{a} + {b} = {sum}</p>
```
After:
```html
<script>
let a = $state(0);
let b = $state(0);
let sum = $derived(add());
function add() {
return a + b;
}
</script>
<button onclick="{()" ="">a++}>a++</button>
<button onclick="{()" ="">b++}>b++</button>
<p>{a} + {b} = {sum}</p>
```

Untracking Dependencies

Svelte 4 vs. Svelte 5:
Before:
```html
<script>
let a = 0;
let b = 0;
$: sum = a + noTrack(b);
function noTrack(value) {
return value;
}
</script>
<button on:click="{()" ="">a++}>a++</button>
<button on:click="{()" ="">b++}>b++</button>
<p>{a} + {b} = {sum}</p>
```
After:
```html
<script>
import { untrack } from 'svelte';
let a = $state(0);
let b = $state(0);
let sum = $derived(add());
function add() {
return a + untrack(() => b);
}
</script>
<button onclick="{()" ="">a++}>a++</button>
<button onclick="{()" ="">b++}>b++</button>
<p>{a} + {b} = {sum}</p>
```

Simple Component Props

Svelte 5:
```html
<script>
let { count = 0 } = $props();
</script>
{count}
```

Advanced Component Props

Svelte 5:
```html
<script>
let { class: classname, ...others } = $props();
</script>
<pre class="{classname}">
{JSON.stringify(others)}
</pre>
```

Autoscroll Example

Svelte 4 vs. Svelte 5:
Before:
```html
<script>
import { tick, beforeUpdate } from 'svelte';
let theme = 'dark';
let messages = [];
let viewport;
let updatingMessages = false;
beforeUpdate(() => {
if (updatingMessages) {
const autoscroll =
viewport && viewport.offsetHeight + viewport.scrollTop > viewport.scrollHeight - 50;
if (autoscroll) {
tick().then(() => viewport.scrollTo(0, viewport.scrollHeight));
}
}
});
function handleKeydown(event) {
if (event.key === 'Enter') {
const text = event.target.value;
if (text) {
messages = [...messages, text];
updatingMessages = true;
event.target.value = '';
}
}
}
function toggle() {
theme = theme === 'dark' ? 'light' : 'dark';
}
</script>
<div class:dark="{theme" ="" ="" ="dark" }>
<div bind:this="{viewport}">
{#each messages as message}
<p>{message}</p>
{/each}
</div>
<input on:keydown="{handleKeydown}" />
<button on:click="{toggle}">Toggle dark mode</button>
</div>
```
After:
```html
<script>
import { tick } from 'svelte';
let theme = $state('dark');
let messages = $state([]);
let viewport;
$effect.pre(() => {
messages;
const autoscroll =
viewport && viewport.offsetHeight + viewport.scrollTop > viewport.scrollHeight - 50;
if (autoscroll) {
tick().then(() => viewport.scrollTo(0, viewport.scrollHeight));
}
});
function handleKeydown(event) {
if (event.key === 'Enter') {
const text = event.target.value;
if (text) {
messages = [...messages, text];
event.target.value = '';
}
}
}
function toggle() {
theme = theme === 'dark' ? 'light' : 'dark';
}
</script>
<div class:dark="{theme" ="" ="" ="dark" }>
<div bind:this="{viewport}">
{#each messages as message}
<p>{message}</p>
{/each}
</div>
<input onkeydown="{handleKeydown}" />
<button onclick="{toggle}">Toggle dark mode</button>
</div>
```

Forwarding Events

Svelte 5:
```html
<script>
let { ...props } = $props();
</script>
<button {...props}>a button</button>
```

Passing UI Content to a Component

Passing content using snippets:
```html

<script>
import Button from './Button.svelte';
</script>
<button>{#snippet children(prop)} click {prop} {/snippet}</button>

<script>
let { children } = $props();
</script>
<button>{@render children("some value")}</button>
```