@unlazy/svelte

The Svelte integration provides an UnLazyImage component for Svelte 5+ applications using runes.

Installation

Install the @unlazy/svelte package using your favorite package manager:

pnpm

pnpm add -D @unlazy/svelte

yarn

yarn add -D @unlazy/svelte

npm

npm install -D @unlazy/svelte

Import the UnLazyImage component in your component file:

<script lang='ts'>
  import { UnLazyImage } from '@unlazy/svelte'
</script>

<UnLazyImage
  blurhash='LKO2:N%2Tw=w]~RBVZRi};RPxuwH'
  srcSet='image-320w.jpg 320w, image-640w.jpg 640w'
  autoSizes
/>

UnLazyImage Component

The UnLazyImage component allows you to easily implement unlazy in your Svelte application, providing a smoother image loading experience.

The component supports automatic calculation of the sizes attribute with the autoSizes prop. It also enables you to specify a blurhash or thumbhash for the blurry placeholder image.

Props

The UnLazyImage component accepts the following props:

Prop	Type	Default	Description
src	String	-	Image source URL to be lazy-loaded.
srcSet	String	-	Image source set to be lazy-loaded.
sources	Array	-	Array of UnLazySource objects to render a <picture> element.
autoSizes	Boolean	false	Whether the sizes attribute should be automatically calculated.
blurhash	String	-	A BlurHash string representing the blurry placeholder image.
thumbhash	String	-	A ThumbHash string representing the blurry placeholder image. If both are provided, thumbhash takes precedence.
placeholderSrc	String	-	Optional image source URL for a custom placeholder image. Ignored if a hash is provided.
placeholderSize	Number	32	The size of the longer edge for BlurHash decoding. Ignored for ThumbHash.
preload	Boolean	false	Whether the image should be preloaded immediately, bypassing lazy loading.
loading	String	'lazy'	Loading strategy for the image ('lazy' or 'eager').
onImageLoad	Function	-	Callback function invoked when the image has been successfully loaded. Receives the HTMLImageElement as an argument.
onImageError	Function	-	Callback function invoked when an error occurs during image loading. Receives the HTMLImageElement and Event as arguments.

The component also accepts all standard <img> HTML attributes.

Svelte 5 Runes

This component is built using Svelte 5's modern runes syntax:

• $props(): Declares component props with TypeScript typing and default values
• $state(): Creates a reactive state variable for the img element reference
• $effect(): Runs reactive effects when dependencies change, with automatic cleanup on unmount

Examples

BlurHash

<UnLazyImage
  width={800}
  height={600}
  blurhash='LKO2:N%2Tw=w]~RBVZRi};RPxuwH'
  srcSet='image-320w.jpg 320w, image-640w.jpg 640w'
  autoSizes
/>

ThumbHash

<UnLazyImage
  width={800}
  height={600}
  thumbhash='1QcSHQRnh493V4dIh4eXh1h4kJUI'
  srcSet='image-320w.jpg 320w, image-640w.jpg 640w'
  autoSizes
/>

Inlined placeholder image

<UnLazyImage
  width={800}
  height={600}
  placeholderSrc='data:image/svg+xml, ...'
  srcSet='image-320w.jpg 320w, image-640w.jpg 640w'
  autoSizes
/>

INFO

When using BlurHash, set explicit width and height props. See Hash-Based Placeholders for why.

TIP

In each example, the sizes attribute is automatically calculated given the auto-sizes prop.

Multiple Image Sources

Pass sources to render a <picture> element with format negotiation and art direction. Each entry becomes a <source> child:

<script lang='ts'>
  import type { UnLazySource } from 'unlazy'
  import { UnLazyImage } from '@unlazy/svelte'

  const sources: UnLazySource[] = [
    { type: 'image/avif', srcSet: 'hero.avif 1x, hero@2x.avif 2x' },
    {
      media: '(max-width: 600px)',
      srcSet: 'hero-mobile.jpg',
      width: 480,
      height: 640,
    },
  ]
</script>

<UnLazyImage
  src='/hero.jpg'
  {sources}
  width={640}
  height={427}
/>

TIP

Set width and height per source to prevent layout shift when the breakpoint changes. Modern browsers use the matched <source>'s dimensions for the rendered <img>'s aspect ratio.

Preload Image

Useful if the UnLazyImage is part of e.g. a slider, and you want to preload the next image.

<UnLazyImage
  blurhash='LKO2:N%2Tw=w]~RBVZRi};RPxuwH'
  srcSet='image-320w.jpg 320w, image-640w.jpg 640w'
  autoSizes
  preload
/>