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March 03, 2025 00:44
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Svelte Cheatsheet

A concise reference for Svelte syntax, directives, lifecycle methods, and best practices. This cheat sheet provides a quick guide to building reactive web applications with Svelte.

Svelte Basics

Component Syntax

Svelte components are defined in .svelte files, containing HTML, CSS, and JavaScript.

<!-- MyComponent.svelte -->
<script>
  let name = 'world';
</script>

<h1>Hello {name}!</h1>

<style>
  h1 {
    color: blue;
  }
</style>

Key Features:

  • Declarative components
  • Reactive statements ($:)
  • Built-in transitions and animations
  • Scoped CSS

Reactivity

let keyword

Declares a reactive variable. Svelte tracks assignments to these variables and updates the DOM accordingly.

let count = 0;

function increment() {
  count += 1;
}

$: reactive statements

Re-executes the code block whenever any of the referenced variables change.

let count = 0;
$: doubled = count * 2;

Updating Arrays and Objects

Arrays and objects require special handling to trigger reactivity. Use methods like push, pop, shift, unshift, splice for arrays, or reassign the entire object/array.

// Array update
myArray = [...myArray, newValue];

// Object update
myObject = { ...myObject, newProperty: value };

Basic Directives

  • bind:: Binds an element property to a component variable.
  • on:: Attaches an event listener to an element.
  • class:: Conditionally adds or removes a CSS class.
  • style:: Conditionally applies a CSS style.

Example:

<input bind:value={name}>
<button on:click={handleClick}>Click me</button>
<div class:highlighted={isHighlighted}></div>
<p style:color={textColor}></p>

Control Flow & Components

Conditional Rendering

Svelte uses {#if}, {:else if}, {:else}, and {/if} blocks for conditional rendering.

{#if user.loggedIn}
  <p>Welcome, {user.name}!</p>
{:else}
  <p>Please log in.</p>
{/if}

List Rendering

Use {#each} blocks to iterate over arrays and render lists. Include a key attribute for efficient updates.

<ul>
  {#each items as item (item.id)}
    <li>{item.name}</li>
  {/each}
</ul>

Component Communication

Props

Pass data from parent to child components using props.

<!-- Parent.svelte -->
<Child name="{parentName}" />

<!-- Child.svelte -->
<script>
  export let name;
</script>
<h1>Hello {name}!</h1>

Events

Dispatch custom events from child to parent components.

<!-- Child.svelte -->
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();

  function handleClick() {
    dispatch('message', { text: 'Hello from Child!' });
  }
</script>
<button on:click={handleClick}>Send Message</button>

<!-- Parent.svelte -->
<Child on:message={handleMessage} />

Context API

Share data between components without explicitly passing props through every level. Use setContext and getContext.

// Parent.svelte
import { setContext } from 'svelte';
setContext('theme', { color: 'dark' });

// Child.svelte
import { getContext } from 'svelte';
const theme = getContext('theme');

Lifecycle and Advanced Features

Lifecycle Methods

  • onMount(callback): Runs after the component is first rendered to the DOM.
  • onDestroy(callback): Runs when the component is unmounted.
  • beforeUpdate(callback): Runs immediately before the component updates.
  • afterUpdate(callback): Runs immediately after the component updates.
  • tick(): Returns a promise that resolves once any pending state changes have been applied to the DOM.

Example:

import { onMount } from 'svelte';

onMount(() => {
  console.log('Component mounted');
  return () => {
    console.log('Component unmounted');
  };
});

Transitions

transition:

Apply transitions when elements enter or leave the DOM.

<div transition:fade>...

in: and out:

Apply different transitions for entering and leaving.

<div in:fly={{ y: 200, duration: 2000 }} out:fade>...

Built-in transitions

fade, fly, slide, scale, draw

Animations

animate:

Animate changes to numeric values.

<div animate:flip>...

Custom animations

Use the animate: directive with a custom animation function for more control.

Stores and Actions

Stores

Stores are a way to manage state outside of components.

  • Writable: Allows setting and updating values.
  • Readable: Allows reading values but not directly setting them.
  • Derived: A store that derives its value from other stores.
// store.js
import { writable } from 'svelte/store';

export const count = writable(0);

<!-- Component.svelte -->
<script>
  import { count } from './store.js';
  import { increment } from './actions.js';
</script>

<h1>{$count}</h1>
<button on:click={() => $count++}>Increment</button>
<button on:click={increment}>Increment with Action</button>

Actions

Actions

Actions are functions that run when an element is created and can return an object with a destroy method that runs when the element is unmounted.

// actions.js
export function increment() {
  count.update(n => n + 1);
}

Using Actions

 
<button use:increment>Increment with Action</button>

Advanced Directives

  • use:: Applies an action to an element.
  • transition:: Applies a transition when an element enters or leaves the DOM.
  • animate:: Animates changes to numeric values.