# Echo.js [](https://travis-ci.org/toddmotto/echo)
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](http://toddmotto.com/labs/echo). 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.
```html
```
## .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](http://underscorejs.org/#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.
```js
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:
```js
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 `` 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:
```html