# Svelte Quick Reference

*Components, reactivity, stores, transitions, SvelteKit*

> Source: Svelte Documentation (svelte.dev) · MIT

## Components

### Basic Component

```
<script>
  let name = "world";
</script>
<h1>Hello {name}!</h1>
<style>
  h1 { color: purple; }
</style>
```

### Component Structure

| Command | Description |
|---------|-------------|
| `<script>` | Component logic (JS/TS) |
| `Markup` | HTML template with `{expressions}` |
| `<style>` | Scoped CSS (auto-scoped to component) |
| `<script context="module">` | Runs once per module, not per instance |

## Reactivity

### Reactive Assignments

```
<script>
  let count = 0;
  function increment() { count += 1; } // triggers re-render
</script>
<button on:click={increment}>{count}</button>
```

### Reactive Declarations

```
<script>
  let width = 10;
  let height = 5;
  $: area = width * height;       // recomputes when deps change
  $: console.log("area is", area); // reactive statement
</script>
```

*$: marks reactive declarations and statements*

### Reactive Rules

| Command | Description |
|---------|-------------|
| `Assignment triggers` | `count += 1` triggers update; `obj.x = 1` does too |
| `Array mutation` | Use `arr = [...arr, item]` (reassign to trigger) |
| `$: declaration` | Auto-recomputes when referenced vars change |
| `$: statement` | Runs side effects reactively |

## Props

### Declaring & Passing Props

```
<!-- Child.svelte -->
<script>
  export let name;
  export let greeting = "Hello";  // default value
</script>
<p>{greeting}, {name}!</p>

<!-- Parent.svelte -->
<Child name="Alice" />
```

### Spread Props

```
<script>
  const props = { name: "Alice", greeting: "Hi" };
</script>
<Child {...props} />
```

## Events

### DOM Events

```
<button on:click={handleClick}>Click</button>
<input on:input={(e) => value = e.target.value} />
<form on:submit|preventDefault={handleSubmit}>
```

### Event Modifiers

| Command | Description |
|---------|-------------|
| `preventDefault` | Calls `e.preventDefault()` |
| `stopPropagation` | Stops event bubbling |
| `once` | Handler fires only once |
| `self` | Only if `event.target` is the element |
| `capture` | Use capture phase |

### Component Events

```
<!-- Child.svelte -->
<script>
  import { createEventDispatcher } from "svelte";
  const dispatch = createEventDispatcher();
</script>
<button on:click={() => dispatch("greet", { text: "hi" })}>

<!-- Parent.svelte -->
<Child on:greet={(e) => alert(e.detail.text)} />
```

## Bindings

### Two-Way Binding

```
<input bind:value={name} />
<input type="checkbox" bind:checked={agreed} />
<select bind:value={selected}>
  <option value="a">A</option>
</select>
```

### Element & Component Bindings

```
<div bind:this={element}></div>
<canvas bind:clientWidth={w} bind:clientHeight={h}></canvas>
<Child bind:value={childValue} />
```

### Binding Types

| Command | Description |
|---------|-------------|
| `bind:value` | Input/select/textarea value |
| `bind:checked` | Checkbox state |
| `bind:group` | Radio/checkbox group |
| `bind:this` | DOM element reference |
| `bind:clientWidth/Height` | Read-only element dimensions |

## Stores

### Writable Store

```
// store.js
import { writable } from "svelte/store";
export const count = writable(0);

// Component — auto-subscribe with $
<script>
  import { count } from "./store.js";
</script>
<button on:click={() => $count += 1}>{$count}</button>
```

### Store Methods

```
count.set(10);               // set value
count.update(n => n + 1);    // update from current
const unsub = count.subscribe(v => console.log(v));
```

### Store Types

| Command | Description |
|---------|-------------|
| `writable(val)` | Read-write store |
| `readable(val, fn)` | Read-only, set by start function |
| `derived(stores, fn)` | Computed from other stores |
| `$store` | Auto-subscribe syntax in components |

## Transitions

### Built-in Transitions

```
<script>
  import { fade, slide, fly } from "svelte/transition";
  let visible = true;
</script>
{#if visible}
  <div transition:fade>Fades in/out</div>
  <div in:fly={{ y: 200 }} out:fade>Fly in, fade out</div>
{/if}
```

### Transition Options

| Command | Description |
|---------|-------------|
| `fade` | Opacity 0 to 1 |
| `fly` | Animate x/y offset + opacity |
| `slide` | Slide in/out (height) |
| `scale` | Scale and fade |
| `draw` | SVG path stroke animation |
| `duration` | Transition time in ms |
| `delay` | Delay before start |

## Slots

### Default & Named Slots

```
<!-- Card.svelte -->
<div class="card">
  <slot name="header">Default header</slot>
  <slot>Default content</slot>
</div>

<!-- Usage -->
<Card>
  <h2 slot="header">Title</h2>
  <p>Body content goes here</p>
</Card>
```

### Slot Props

```
<!-- List.svelte -->
{#each items as item}
  <slot {item} index={item.id} />
{/each}

<!-- Usage -->
<List {items} let:item let:index>
  <p>{index}: {item.name}</p>
</List>
```

## Context

### Set & Get Context

```
<!-- Parent.svelte -->
<script>
  import { setContext } from "svelte";
  setContext("theme", { color: "dark" });
</script>

<!-- Descendant.svelte -->
<script>
  import { getContext } from "svelte";
  const theme = getContext("theme"); // { color: "dark" }
</script>
```

### Context vs Stores

| Command | Description |
|---------|-------------|
| `Context` | Component-tree scoped, not reactive by default |
| `Stores` | Global, reactive, importable anywhere |
| `Context + Store` | Pass a store via context for scoped reactivity |

## SvelteKit Basics

### File-Based Routing

```
src/routes/
  +page.svelte          <!-- / -->
  about/+page.svelte    <!-- /about -->
  blog/[slug]/+page.svelte  <!-- /blog/:slug -->
```

### Load Functions

```
// +page.js (runs on client & server)
export async function load({ params, fetch }) {
  const res = await fetch(`/api/posts/${params.slug}`);
  return { post: await res.json() };
}
```

### Key Files

| Command | Description |
|---------|-------------|
| `+page.svelte` | Page component |
| `+page.js / +page.ts` | Client/universal load function |
| `+page.server.js` | Server-only load / form actions |
| `+layout.svelte` | Shared layout wrapper |
| `+error.svelte` | Error page |
| `+server.js` | API endpoint (GET, POST, ...) |