Echo.js 
Echo is a standalone JavaScript lazy-loading image micro-library. Echo is fast, 2KB, and uses HTML5 data-* attributes for simple. Check out a demo. Echo works in IE8+.
bower install echojs
npm install echo-js
Using Echo.js is simple, to add an image directly into the page simply add a data-echo attribute to the img tag. Alternatively if you want to use Echo to lazy load background images simply add a `data-echo-background' attribute to the element with the image URL.
<body>
<img src="img/blank.gif" alt="Photo" data-echo="img/photo.jpg">
<script src="dist/echo.js"></script>
<script>
echo.init({
offset: 100,
throttle: 250,
unload: false,
callback: function (element, op) {
console.log(element, 'has been', op + 'ed')
}
});
// echo.render(); is also available for non-scroll callbacks
</script>
</body>
.init() (options)
The init() API takes a few options
offset
Type: Number, String Default: 0
The offset option allows you to specify how far below, above, to the left, and to the right of the viewport you want Echo to begin loading your images. If you specify 0, Echo will load your image as soon as it is visible in the viewport, if you want to load 1000px below or above the viewport, use 1000.
offsetVertical
Type: Number, String Default: offset's value
The offsetVertical option allows you to specify how far above and below the viewport you want Echo to begin loading your images.
offsetHorizontal
Type: Number, String Default: offset's value
The offsetHorizontal option allows you to specify how far to the left and right of the viewport you want Echo to begin loading your images.
offsetTop
Type: Number, String Default: offsetVertical's value
The offsetTop option allows you to specify how far above the viewport you want Echo to begin loading your images.
offsetBottom
Type: Number, String Default: offsetVertical's value
The offsetBottom option allows you to specify how far below the viewport you want Echo to begin loading your images.
offsetLeft
Type: Number, String Default: offsetVertical's value
The offsetLeft option allows you to specify how far to left of the viewport you want Echo to begin loading your images.
offsetRight
Type: Number, String Default: offsetVertical's value
The offsetRight option allows you to specify how far to the right of the viewport you want Echo to begin loading your images.
throttle
Type: Number, String Default: 250
The throttle is managed by an internal function that prevents performance issues from continuous firing of window.onscroll events. Using a throttle will set a small timeout when the user scrolls and will keep throttling until the user stops. The default is 250 milliseconds.
debounce
Type: Boolean Default: true
By default the throttling function is actually a debounce function so that the checking function is only triggered after a user stops scrolling. To use traditional throttling where it will only check the images every throttle milliseconds, set debounce to false.
unload
Type: Boolean Default: false
This option will tell echo to unload loaded images once they have scrolled beyond the viewport (including the offset area).
callback
Type: Function
The callback will be passed the element that has been updated and what the update operation was (ie load or unload). This can be useful if you want to add a class like loaded to the element. Or do some logging.
echo.init({
callback: function(element, op) {
if(op === 'load') {
element.classList.add('loaded');
} else {
element.classList.remove('loaded');
}
}
});
.render()
Echo's callback render() can be used to make Echo poll your images when you're not scrolling, for instance if you've got a filter layout that swaps images but does not scroll, you need to call the internal functions without scrolling. Use render() for this:
echo.render();
Using render() is also throttled, which means you can bind it to an onresize event and it will be optimised for performance in the same way onscroll is.
Manual installation
Drop your files into your required folders, make sure you're using the file(s) from the dist folder, which is the compiled production-ready code. Ensure you place the script before the closing </body> tag so the DOM tree is populated when the script runs.
Configuring Echo
Add the image that needs to load when it's visible inside the viewport in a data-echo attribute:
<img src="img/blank.gif" alt="Photo" data-echo="img/photo.jpg">
Contributing
In lieu of a formal style guide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint and test your code using Gulp.
License
MIT license