@unlazy/react

The React integration provides an UnLazyImage component for your React 19+ application.

Installation

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

pnpm

pnpm add -D @unlazy/react

yarn

yarn add -D @unlazy/react

npm

npm install -D @unlazy/react

Import the UnLazyImage component in your component file:

import { UnLazyImage } from '@unlazy/react'

export default function MyComponent() {
  return (
    <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 React 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 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 (e.g., alt, className, style).

Lifecycle

The component uses React's useEffect hook to initialize lazy loading when mounted:

1. Renders with placeholder image or hash-generated placeholder.
2. Sets up lazy loading via lazyLoad() from the core library.
3. Decodes BlurHash/ThumbHash if provided (thumbhash takes precedence).
4. Swaps data-src to src when image enters viewport.
5. Cleans up event listeners on unmount or when props change.

Examples

BlurHash

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

ThumbHash

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

Inlined placeholder image

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

TIP

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

INFO

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

Multiple Image Sources

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

import type { UnLazySource } from 'unlazy'

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,
  },
]

return (
  <UnLazyImage
    src="/hero.jpg"
    sources={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.

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