React Component to lazy load images and other elements

React Lazy Load Image Component

React Component to lazy load images and other components/elements. Includes a HOC to track window scroll position to improve performance.

React Component to lazy load images and components using a HOC to track window scroll position.

Features

• Includes two components (LazyLoadImage and LazyLoadComponent) and a HOC (trackWindowScroll) which adds scroll position tracking to any component you wish.
• Handles scroll events, resize events and re-renders that might change the position of the components. And, of course, above-the-fold on initial render.
• Placeholder by default with the same size of the image/component.
• A custom placeholder component or image can be specified.
• Built-in on-visible effects (blur, black and white and opacity transitions).
• threshold is set to 100px by default and can be modified.
• beforeLoad and afterLoad events.
• debounce and throttle included by default and configurable.

Installation

# Yarn
$ yarn add react-lazy-load-image-component

# NPM
$ npm i --save react-lazy-load-image-component

LazyLoadImage usage

import React from 'react';
import { LazyLoadImage } from 'react-lazy-load-image-component';

const MyImage = ({ image }) => (
  <div>
    <LazyLoadImage
      alt={image.alt}
      height={image.height}
      src={image.src} // use normal <img> attributes as props
      width={image.width} />
    <span>{image.caption}</span>
  </div>
);

export default MyImage;

Props

Prop	Type	Default	Description
afterLoad	Function		Function called after the image has been completely loaded.
beforeLoad	Function		Function called right before the placeholder is replaced with the image element.
delayMethod	String	throttle	Method from lodash to use to delay the scroll/resize events. It can be throttle or debounce.
delayTime	Number	300	Time in ms sent to the delayMethod.
effect	String		Name of the effect to use. Please, read next section with an explanation on how to use them.
placeholder	ReactClass	<span>	React element to use as a placeholder.
placeholderSrc	String		Image src to display while the image is not visible or loaded.
threshold	Number	100	Threshold in pixels. So the image starts loading before it appears in the viewport.
visibleByDefault	Boolean	false	Whether the image must be visible from the beginning.
wrapperClassName	String		In some occasions (for example, when using a placeholderSrc) a wrapper span tag is rendered. This prop allows setting a class to that element.
...			Any other image attribute

Using effects

LazyLoadImage includes several effects ready to be used, they are useful to add visual candy to your application, but are completely optional in case you don't need them or want to implement you own effect.

They rely on CSS and the corresponding CSS file must be imported:

import React from 'react';
import { LazyLoadImage } from 'react-lazy-load-image-component';
import 'react-lazy-load-image-component/src/effects/blur.css';

const MyImage = ({ image }) => (
  <LazyLoadImage
    alt={image.alt}
    effect="blur"
    src={image.src} />
);

The current available effects are:

• blur: renders a blurred image based on placeholderSrc and transitions to a non-blurred one when the image specified in the src is loaded.

• black-and-white: renders a black and white image based on placeholderSrc and transitions to a colorful image when the image specified in the src is loaded.

• opacity: renders a blank space and transitions to full opacity when the image is loaded.

LazyLoadComponent usage

import React from 'react';
import { LazyLoadComponent } from 'react-lazy-load-image-component';
import { ArticleContent, ArticleComments } from 'my-app';

const Article = ({ articleId }) => (
  <div>
    <ArticleContent id={articleId} />
    <LazyLoadComponent>
      <ArticleComments id={articleId} />
    </LazyLoadComponent>
  </div>
);

export default Article;

Props

Prop	Type	Default	Description
afterLoad	Function		Function called after the component has been rendered.
beforeLoad	Function		Function called right before the component is rendered.
delayMethod	String	throttle	Method from lodash to use to delay the scroll/resize events. It can be throttle or debounce.
delayTime	Number	300	Time in ms sent to the delayMethod from lodash.
placeholder	ReactClass	<span>	React element to use as a placeholder.
threshold	Number	100	Threshold in pixels. So the component starts loading before it appears in the viewport.
visibleByDefault	Boolean	false	Whether the component must be visible from the beginning.

Using trackWindowScroll HOC to improve performance

When you have many elements to lazy load in the same page, you might get poor performance because each one is listening to the scroll/resize events. In that case, it's better to wrap the deepest common parent of those components with a HOC to track those events (trackWindowScroll).

For example, if we have an App which renders a Gallery, we would wrap the Gallery component with the HOC.

import React from 'react';
import { LazyLoadImage, trackWindowScroll }
  from 'react-lazy-load-image-component';

const Gallery = ({ images, scrollPosition }) => (
  <div>
    {images.map((image) =>
      <LazyLoadImage
        key={image.key}
        alt={image.alt}
        height={image.height}
        // Make sure to pass down the scrollPosition,
        // this will be used by the component to know
        // whether it must track the scroll position or not
        scrollPosition={scrollPosition}
        src={image.src}
        width={image.width} />
    )}
  </div>
);
// Wrap Gallery with trackWindowScroll HOC so it receives
// a scrollPosition prop to pass down to the images
export default trackWindowScroll(Gallery);

You must set the prop scrollPosition to the lazy load components. This way, they will know the scroll/resize events are tracked by a parent component and will not subscribe to them.

Props

LazyLoadImage

Prop	Type	Default	Description
scrollPosition	Object		Object containing x and y with the curent window scroll position. Required.
afterLoad	Function		Function called after the image has been rendered.
beforeLoad	Function		Function called right before the image is rendered.
placeholder	ReactClass	<span>	React element to use as a placeholder.
threshold	Number	100	Threshold in pixels. So the image starts loading before it appears in the viewport. Defaults to 100.
visibleByDefault	Boolean	false	Whether the image must be visible from the beginning.
...			Any other image attribute

Component wrapped with trackWindowScroll (in the example, Gallery)

Prop	Type	Default	Description
delayMethod	String	throttle	Method from lodash to use to delay the scroll/resize events. It can be throttle or debounce.
delayTime	Number	300	Time in ms sent to the delayMethod from lodash.

Notice you can do the same replacing LazyLoadImage with LazyLoadComponent.

When to use visibleByDefault?

The prop visibleByDefault makes the LazyLoadImage to behave like a normal <img>. Why is it useful, then?

Imagine you are going to lazy-load an image you have already loaded in the same page. In that case, there is no need to lazy-load it because it's already stored in the cache of the user's browser. You can directly display it.

Maybe the following code snippet will make it more clear:

import React from 'react';
import { LazyLoadImage, trackWindowScroll }
  from 'react-lazy-load-image-component';

const Gallery = ({ images, scrollPosition }) => (
  <div>
    // We are loading landscape.jpg here
    <img src="/landscape.jpg" alt="Beautiful landscape" />
    {images.map((image) =>
      <LazyLoadImage
        key={image.key}
        alt={image.alt}
        scrollPosition={scrollPosition}
        src={image.src}
        // If the image we are creating here has the same src than before,
        // we can directly display it with no need to lazy-load.
        visibleByDefault={image.src === '/landscape.jpg'} />
    )}
  </div>
);

export default trackWindowScroll(Gallery);

GitHub