resize-observer-polyfill

What is resize-observer-polyfill?

The resize-observer-polyfill package provides a polyfill for the ResizeObserver API, which allows you to be notified when an element's content rectangle has changed its size, and thus react accordingly. This is particularly useful for responsive designs and element-specific layout updates.

What are resize-observer-polyfill's main functionalities?

Observing element size changes

This feature allows you to observe changes in the size of an element. The callback function is executed whenever the size of the observed element changes, providing you with the new dimensions and allowing you to perform any necessary adjustments.

var ro = new ResizeObserver(entries => {
  for (let entry of entries) {
    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.getElementById('myElement'));

Other packages similar to resize-observer-polyfill

element-resize-detector

element-resize-detector is an npm package that offers a way to detect when an element has been resized. It differs from resize-observer-polyfill in that it uses scroll events and other tricks to detect size changes instead of the native ResizeObserver API, which can be useful for broader browser support.

css-element-queries

css-element-queries provides a way to attach event listeners to elements for various purposes, including element resize detection. It includes a ResizeSensor class that can be used to detect when an element's size changes. This package offers additional functionalities like element query conditions, which go beyond the scope of resize-observer-polyfill.

README

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 latter one is not supported. Handles non-delayed 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 spec. The size is 3.0kb 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

Local export

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) => {
    for (const entry of entries) {
        const cr = entry.contentRect;

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

ro.observe(document.body);

As a pre-built ES5 UMD version if ES6 is not your thing: resize-observer-polyfill/dist/ResizeObserver.

Global export

Use following versions if you want polyfill to extend global object.

ES6 module:

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

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

UMD version: resize-observer-polyfill/dist/ResizeObserver.global.

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

Configuration

ResizeObserver class additionally implements following static accessor property:

continuousUpdates

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

NOTE: changes made to this property 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 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.