Where a regular worker belongs to one page, a SharedWorker is shared across all tabs/windows of an origin — one instance, connected through ports. Useful for a single shared connection (one WebSocket feeding every tab) or centralized state. Support is less universal than dedicated workers, so feature-detect.
<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>const sw = new SharedWorker('shared.js');
sw.port.start();
sw.port.postMessage('hi');
sw.port.onmessage = e => console.log(e.data);
// one instance serves every tab of this origin</pre></body></html>