ResizeObserver Polyfill

A polyfill for the Resize Observer API.

Implementation is based on the MutationObserver (no polling unless DOM changes) with a fall back to a continuous dirty checking cycle if the first one is not supported. Doesn't modify observed elements. Handles CSS transitions/animations, <textarea> resizes and can possibly observe changes caused by dynamic CSS pseudo-classes, e.g. by :hover.

Compliant with the spec and the native implementation. The size is 2.6kb when minified and gzipped.

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

Installation

From NPM:

npm install resize-observer-polyfill --save-dev

From Bower:

bower install resize-observer-polyfill --save-dev

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+ , native since v54
• 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.

import ResizeObserver from 'resize-observer-polyfill';

const ro = new ResizeObserver((entries, observer) => {
    for (const entry of entries) {
        const {left, top, width, height} = entry.contentRect;

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

ro.observe(document.body);

Though you always can extend the global object if you need it.

import ResizeObserver from 'resize-observer-polyfill';

window.ResizeObserver = ResizeObserver;

Package's main file is a ES5 UMD module and it will be dynamically substituted by the ES6 version for those bundlers that are aware of the jnext:main field, e.g. for Rollup.

Note: global versions (index.global and dist/ResizeObserver.global) are deprecated and will be removed in the next major release.

Limitations

• CSS changes caused by dynamic pseudo-classes, e.g. :hover and :focus, are not tracked. As a workaround you can add a short transition which would trigger the transitionend event when an element receives one of the former classes (example).
• Delayed transitions will receive only one notification with the latest dimensions of an element.

Building and testing

To build polyfill. Creates UMD bundle in the dist folder:

npm run build

To run a code style test:

npm run test:lint

Running unit tests:

npm run test:spec

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

npm run test:spec:custom

Testing against a native implementation:

npm run test:spec:native

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