Understanding .cursorrules File in Svelte 5 vs Svelte 4

Svelte 5 vs Svelte 4 .cursorrules prompt file

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Adam Shand

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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>
```

stack:

Svelte 5 vs Svelte 4 .cursorrules prompt file

Adam Shand

About .cursorrules prompt file

What you can build

Svelte 5 Interactive Playground: An interactive online platform that allows developers to experiment with Svelte 5's new features such as runes, reactive states, effects, and snippets in a live coding environment. It can provide side-by-side comparisons with Svelte 4 to illustrate differences and improvements.
Svelte 5 Code Migration Tool: A web-based application that assists in migrating projects from Svelte 4 to Svelte 5. It automatically updates deprecated syntax like on: directive, and translates top-level let declarations, reactive variables, and more to the new Svelte 5 format.
Svelte 5 Advanced Tutorial Series: A comprehensive tutorial series that offers detailed lessons, quizzes, and hands-on projects covering all the new Svelte 5 features, including $state, $derived, $effect, and snippets.
Svelte 5 Developer Tools Extension: A Chrome or Firefox browser extension that helps developers debug and visualize Svelte 5 applications, with features to inspect $state changes, track dependencies, and render processes.
Svelte 5 Code Snippet Library: A repository of reusable code snippets specifically for Svelte 5, including new reactive declarations and advanced $derived use cases. These snippets can be integrated into popular editors like VSCode or Sublime Text.
Svelte 5 Project Boilerplate Generator: A CLI-based tool that generates starter project templates built with Svelte 5, providing setup for modern build tools, state management, and component organization.
Svelte 5 Community Forum: An online forum dedicated to discussions, Q&A, and sharing best practices specifically focused on Svelte 5 and its new features. It could serve as a hub for developers to connect and learn from each other.
Svelte 5 Performance Analyzer: A web tool that analyzes the performance of a Svelte 5 application, highlighting areas where $state.raw could improve performance, and suggesting optimizations for effects and derived states.
Svelte 5 Component Marketplace: An online marketplace where developers can buy, sell, and share components built with Svelte 5, emphasizing compatibility with the new reactivity model and snippet handling.
Svelte 5 Workshop Series: Workshops intended for teams or individuals to learn about Svelte 5, focusing on practical applications of the new features in real-world project scenarios, guided by experienced Svelte developers.
Svelte 5 Visualization Library: A library or toolset that leverages Svelte 5’s $effect and $state features to create dynamic, real-time data visualizations, suitable for dashboards and analytics applications.
Svelte 5 Event Modifiers Suggestion Tool: An app that assists developers in replacing deprecated event modifiers with wrapper functions, offering suggestions and best practices for using new event handling paradigms in Svelte 5.

Benefits

Runes Integration: Svelte 5 introduces runes, providing advanced reactivity primitives for finer control over state management and side-effects, departing from the traditional syntax used in Svelte 4.
Event Handling Simplification: Event handlers are now properties, streamlining component interactivity, deprecating on: directives, and replacing createEventDispatcher with callback props for better integration.
Snippets Feature: The addition of snippets and render tags allows for reusable and maintainable markup within components, surpassing the flexibility of deprecated slots.

Synopsis

Developers upgrading from Svelte 4 to Svelte 5 would benefit by constructing applications with enhanced state management and reactivity using runes, $state, $effect, and updated event handler syntax.

Overview of .cursorrules prompt

The .cursorrules file provides a detailed overview of the changes introduced in Svelte 5 compared to Svelte 4. It highlights the introduction of runes, a set of advanced primitives designed to enhance control over reactivity. Key features and their purposes are presented, such as `$state` for declaring reactive state, `$derived` for derived state, and `$effect` for handling side-effects. It includes code examples to demonstrate the usage of each feature. The file also addresses component props with `$props` and bindable props using `$bindable`, and describes the deprecation of certain Svelte 4 constructs like `on:` directives. Furthermore, it covers snippets, a new concept for reusable markup, replacing slots with more flexible usage. The document explains how event handlers are simplified as properties and the deprecated use of event modifiers. Lastly, it provides before-and-after comparisons of common scenarios as examples, aiding in transitioning from Svelte 4 to Svelte 5.

Svelte 5 vs Svelte 4 .cursorrules prompt file

Svelte 5 vs Svelte 4 .cursorrules prompt file

Svelte 5 vs Svelte 4 .cursorrules prompt file

About .cursorrules prompt file

What you can build

Benefits

Synopsis

Overview of .cursorrules prompt

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