Run results for: ResizeObserver vs ResizeObserver polyfill

Test name	Executions per second
✓ ResizeObserver	6,474 Ops/sec
ResizeObserver polyfill	6,265 Ops/sec

Tests:

ResizeObserver
let value; const observer = new ResizeObserver(entries => { value = entries[0].contentRect.width; }) observer.observe(document.body) for (let i = 500; i <= 1920; i++) { window.resizeTo(i, 1000) }
ResizeObserver polyfill
class ResizeObserver { constructor(callback) { this.observables = []; this.boundCheck = this.check.bind(this); this.boundCheck(); this.callback = callback; } observe(el) { if (this.observables.some((observable) => observable.el === el)) { return; } const newObservable = { el: el, size: { height: el.clientHeight, width: el.clientWidth } } this.observables.push(newObservable); } unobserve(el) { this.observables = this.observables.filter((obj) => obj.el !== el); } disconnect() { this.observables = []; } check() { const changedEntries = this.observables.filter((obj) => { const currentHeight = obj.el.clientHeight; const currentWidth = obj.el.clientWidth; if (obj.size.height !== currentHeight || obj.size.width !== currentWidth) { obj.size.height = currentHeight; obj.size.width = currentWidth; return true; } }).map((obj) => obj.el); if (changedEntries.length > 0) { this.callback(changedEntries); } window.requestAnimationFrame(this.boundCheck); } } let value; const observer = new ResizeObserver(entries => { value = entries[0].contentRect.width; }) observer.observe(document.body) for (let i = 500; i <= 1920; i++) { window.resizeTo(i, 1000) }