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react-lazy

react-lazy

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Lazy loader container element that triggers load when element comes into view. Provides fallback for SEO and no-JS by using a noscript element. This means your images and/or content can be crawled by search engines that are not JavaScript aware. Also means this component supports universal rendering (isomorphic JS).

Works for both vertical and horizontal scrolling, which is also unlike some other solutions.

View demo

Why lazy load content such as images?

You want to save your bandwidth and/or server load. As a side effect you may also gain some performance benefits on client side, especially on mobile devices. However the main benefit (and main purpose) for you should always be the reduction of bandwidth/server load.

Another side effect of lazy loading is that user may see content flashing as it comes into view; sometimes with a lot of delay as it depends on connectivity.

Usage

A sample for targetting a single image. You are encouraged to give the container lazy element an explicit size, as can be seen in the CSS below.

/* sample of traditional CSS centered thumbnail styling */
.image-link {
    display: inline-block;
    margin: 5px;
    position: relative;
    text-align: center;
}
 
.image-link--100px {
    height: 100px;
    line-height: 98px;
    width: 100px;
}
 
.image-link__image {
    line-height: 1;
    max-height: 100%;
    max-width: 100%;
    vertical-align: middle;
}
import { Lazy } from 'react-lazy'
 
...
 
    <Lazy component="a" href="/" className="image-link image-link--100px">
        <img alt="My Lazy Loaded Image" className="image-link__image" src="my-lazy-loaded-image.png" />
    </Lazy>

Output HTML

<a href="/" class="image-link image-link--100px">
    <!--[if IE 9]><!--><noscript><!--<![endif]-->
        <img alt="My Lazy Loaded Image" class="image-link__image" src="my-lazy-loaded-image.png" />
    <!--[if IE 9]><!--></noscript><!--<![endif]-->
</a>

Why IE conditional comments?

You probably develop your site in a way that your scripts don't really run on Internet Explorer 8. Maybe you see just enough trouble to make things render and work just enough that IE8 user can browse around without things being unusably wrong. The above syntax ensures that IE9+, modern browsers and crawlers do notice the noscript tag, but IE8 and below will not see it, therefore causing a non-lazy load of the content immediately upon first browser render.

In other words, if you want to have minimal support in legacy browsers when using this component... you can have that! This component does not support lazy loading in any form in Internet Explorer 8 and older.

Other features

cushion

You can apply "cushion" around elements so they are loaded slighly before coming into the actual viewport:

// element content appear if it is in viewport or within 100px radius of it
<Lazy cushion={100}>...</Lazy>

imgWrapperComponent

Allows you to toggle a render where given component is rendered around all contained img elements, while still rendering all the other children as usual.

<Lazy component="ul" className="thumbnail-list" imgWrapperComponent={MyThumbnailPlaceholder}>
    {imagesWithProps.map((props, index) =>
        <li key={index} className="thumbnail-list__item"><img {...props} /></li>
    )}
</Lazy>

Will result in HTML like:

<ul class="thumbnail-list">
    <li class="thumbnail-list__item">
        <div class="my-thumbnail-placeholder">
            <!--[if IE 9]><!--><noscript><!--<![endif]-->
                <img alt="My Image" class="my-thumbnail" src="my-image.png" />
            <!--[if IE 9]><!--></noscript><!--<![endif]-->
        </div>
    </li>
</ul>

Which will change to a DOM tree like this when coming into viewport:

<ul class="thumbnail-list">
    <li class="thumbnail-list__item">
        <img alt="My Image" class="my-thumbnail" src="my-image.png" />
    </li>
</ul>

You can also have Lazy containers inside Lazy containers.

Note that component given to imgWrapperComponent will have any of it's own children overwritten.

onLoad

You can also get notified on just before lazy load switch render happens:

<Lazy onLoad={yourCustomFunction}>...</Lazy>

checkElementsInViewport

Finally, you can also manually trigger checking for elements in viewport, which can be useful if you toggle element resize (which won't cause resize or scroll events). Or you can use setInterval if you want to be very lazy.

import { checkElementsInViewport } from 'react-lazy'
 
// now you're being a very lazy dev... 
setInterval(checkElementsInViewport, 250)

Developing

npm install
npm run build
npm test

Note! This component uses jsdom in it's tests. This means you may need to install stuff, especially on Windows. The following is copied from node-jsdom's readme:

Contextify

Contextify is a dependency of jsdom, used for running <script> tags within the page. In other words, it allows jsdom, which is run in Node.js, to run strings of JavaScript in an isolated environment that pretends to be a browser environment instead of a server. You can see how this is an important feature.

Unfortunately, doing this kind of magic requires C++. And in Node.js, using C++ from JavaScript means using "native modules." Native modules are compiled at installation time so that they work precisely for your machine; that is, you don't download a contextify binary from npm, but instead build one locally after downloading the source from npm.

Getting C++ compiled within npm's installation system can be tricky, especially for Windows users. Thus, one of the most common problems with jsdom is trying to use it without the proper compilation tools installed. Here's what you need to compile Contextify, and thus to install jsdom:

Windows

  • The latest version of Node.js for Windows

  • A copy of Visual Studio Express 2013 for Windows Desktop

  • A copy of Python 2.7, installed in the default location of C:\Python27

  • Set your system environment variable GYP_MSVS_VERSION like so (assuming you have Visual Studio 2013 installed):

    setx GYP_MSVS_VERSION 2013
  • Restart your command prompt window to ensure required path variables are present.

There are some slight modifications to this that can work; for example other Visual Studio versions often work too. But it's tricky, so start with the basics!

Mac

  • XCode needs to be installed
  • "Command line tools for XCode" need to be installed
  • Launch XCode once to accept the license, etc. and ensure it's properly installed

Linux

You'll need various build tools installed, like make, Python 2.7, and a compiler toolchain. How to install these will be specific to your distro, if you don't already have them.