ResizeObserver Polyfill

A polyfill for the Resize Observer API.

Implements event based tracking of changes in the content dimensions of elements, i.e. no polling (in the general case) unless DOM is mutated or the resize event is triggered. Uses MutationsObserver and falls back to an infinite dirty checking cycle if the first one is not supported. Handles long running CSS transitions/animations and can optionally observe resizing of a <textarea> element along with changes caused by the :hover pseudo-class.

Compliant with the spec and the native implementation. Doesn't contain any publicly available methods except for those described in the spec. The size is 3.6kb when minified and gzipped.

Live demo (has style problems in IE10 and lower).

Installation

From NPM:

npm install --save resize-observer-polyfill

From Bower:

bower install --save resize-observer-polyfill

Or just grab one of the pre-built versions from dist.

Browser Support

Polyfill has been tested and known to work in the following browsers:

• Chrome 40+
• Firefox 37+
• Safari 9+, including mobile
• Opera 30+
• Edge 13+
• Internet Explorer 9+

NOTE: Internet Explorer 8 and its earlier versions are not supported.

Usage Examples

It's recommended to use this library in the form of the ponyfill, which doesn't inflict modifications of the global object.

With ES6 modules:

import ResizeObserver from 'resize-observer-polyfill';

const ro = new ResizeObserver((entries, observer) => {
    const entry = entries[0];
    const cr = entry.contentRect;

    console.log('Element:', entry.target);
    console.log(`Element size: ${cr.width}px x ${cr.height}px`);
    console.log(`Element padding: ${cr.top}px ; ${cr.left}px`);
});

ro.observe(document.body);

As a pre-built ES5 UMD module.

AMD:

define([
    'resize-observer-polyfill/dist/ResizeObserver'
], function (ResizeObserver) {
    // ...
});

CommonJS:

var ResizeObserver = require('resize-observer-polyfill/dist/ResizeObserver');

As a browsers' global object:

<script src="resize-observer-polyfill/dist/ResizeObserver.js"></script>
<script>
    (function () {
        var ro = new ResizeObserver(function () {});
    })();
</script>

Global exports

Use following versions in case if you need to export polyfill globally.

With ES6 modules:

import 'resize-observer-polyfill/index.global';

const ro = new ResizeObserver(() => {});

As UMD module: resize-observer-polyfill/dist/ResizeObserver.global.

You can also take the minified bundles: ResizeObserver.min.js or ResizeObserver.global.min.js.

Configuration

ResizeObserver class additionally implements following static accessor properties:

idleTimeout

After the mutations of DOM attributes, like class or style, the update cycle will be started which will run either until the dimensions of observed elements keep changing or the idleTimeout is reached. This approach is used to handle transitions and animations.

Default timeout value is 50 milliseconds and you can increase it to match the delay of transitions, e.g. if the delay is 500 milliseconds you can set ResizeObserver.idleTimeout = 500 to the corresponding value.

Note that even if transitions are meant to start "immediately" it's still better to leave this value as it is. And you can set this property to zero if you don't need transitions to be tracked.

continuousUpdates

By default resizing of a <textarea> element and changes caused by the CSS :hover pseudo-class are not tracked. To handle them you can set ResizeObserver.continuousUpdtaes = true which in turn will start a continuous update cycle with the interval of 100 milliseconds (using RAF, of course). Keep in mind that this is going to affect the performance.

NOTE: changes made to these properties will affect all existing and future instances of ResizeObserver.

Building and testing

First make sure that you have all dependencies installed by running npm install. Then you can execute following tasks either with a gulp CLI or with the npm run gulp command.

To build polyfill. This will create UMD bundles in the dist folder:

gulp build

To run a code style test:

gulp test:lint

Functional tests for all available browsers that are defined in karma.config.js file:

gulp test:spec

To test in a browser that is not present in karmas' config file:

gulp test:spec:custom

Testing against a native implementation:

gulp test:spec:native

NOTE: after you invoke spec:native and spec:custom commands head to the http://localhost:9876/debug.html page.