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Replace <img /> elements with <img-2> to automatically pre-cache images and improve page performance.
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README.md

Img2

Replace <img /> elements with <img-2></img-2> to automatically pre-cache images and improve page performance. Displaying even a small number of high-quality images on a web page can be difficult to do without causing jank or slowing down the initial load of the page. This is why clever developers employ techniques with JavaScript to pre-cache images and lazy load them as they become visible on the user's screen.

Img2 makes this super easy, just swap out your <img /> elements with <img-2></img-2> and let it do the work for you.

Img2 will automatically:

  1. Only render initial images which are visible to the user
  2. Pre-cache all other images off the main thread with a Web Worker
  3. Lazy load images as they enter the user's viewport instantly from the cache
  4. Display a blurred preview image while the user waits for initial images

Live Demo


Install

npm install --save img-2

Usage

You can include Img2 into your project in various ways:

As a ES6 Module

import "img-2"

Via <script> as ES6

<script src="dist/img-2.js"></script>

Via <script> as ES5

<script src="dist/img-2.es5.js"></script>

Then you simply use the <img-2></img-2> element in place of an <img /> element.

    <body>
        <h1>Cat Photos</h1>
        <img-2 src="https://notreal.com/cat_1920x1080.jpg" width="400" height="267" src-preview="https://notreal.com/cat_10x10.jpg" alt="An amazing picture of a cat"></img-2>
    </body>

Attributes

There are currently five attributes available on Img2, three of which are required.

src

The full-size source image to be pre-cached and lazy loaded.

width & height

Both of these are required to figure out the position of the image on screen to then determine if the image should be loaded right away or lazily loaded.

src-preview

A really small representation of the full-size image (e.g. 10px by 10px). This image will be displayed as a blurred preview while the full-size image is downloading if the image hasn't already been pre-cached.

alt

The alt text for the image, just maps on to the alt attribute of the <img /> element used in the component.

Supported platforms &

Platform Support Chrome Chrome for Android Firefox Safari iOS Safari Edge IE 11
Supported
Polyfill(s) Required - -

Img-2 uses the Intersection Observer to detect when an image is in the users visible viewport. For Safari and IE 11 you'll need to load the Intersection Observer polyfill.

<script src="https://polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"></script>

Img-2 uses Custom Elements and Shadow DOM so for FireFox, Edge and IE11 you'll need to use the webcomponents-loader from webcomponentsjs.

<script src="bower_components/webcomponentsjs/webcomponents-loader.js"></script>

If you need to support IE11 which doesn't support ES6 you'll want to conditionally load img-2.es5.js.

var supportsES6 = function() {
    try {
        new Function("(a = 0) => a");
        return true;
    }
    catch (err) {
        return false;
    }
}();

var $script = document.createElement("script");
$script.setAttribute("defer", "");
$script.src = (supportsES6 === true) ? "dist/img-2.js" : "dist/img-2.es5.js";
document.head.appendChild($script);

Take a look at index.html in the root of this repo for further examples.

Contributing

Any contributions are welcome, feel free to submit a pull request for review.

To Do

  1. Add srcset support
  2. Consider A11y
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