Window: crossOriginIsolated property

The crossOriginIsolated read-only property of the Window interface returns a boolean value that indicates whether the website is in a cross-origin isolation state. That state mitigates the risk of side-channel attacks and unlocks a few capabilities:

• SharedArrayBuffer can be created and sent via a Window.postMessage() or a MessagePort.postMessage() call.
• Performance.now() offers better precision.
• Performance.measureUserAgentSpecificMemory() can be accessed.

A website is in a cross-origin isolated state, when the response header Cross-Origin-Opener-Policy has the value same-origin and the Cross-Origin-Embedder-Policy header has the value require-corp or credentialless.

const myWorker = new Worker("worker.js");

if (window.crossOriginIsolated) {
  const buffer = new SharedArrayBuffer(16);
  myWorker.postMessage(buffer);
} else {
  const buffer = new ArrayBuffer(16);
  myWorker.postMessage(buffer);
}