What are Web Workers and when should you use them?

Web Workers run JavaScript in a separate background thread — preventing heavy computation from blocking the main thread (UI freezing).

// main.js
const worker = new Worker('worker.js');

// Send data to worker (structured clone — copies data)
worker.postMessage({ array: bigArray, threshold: 50 });

// Receive results
worker.onmessage = (e) => {
  console.log('Result:', e.data.result);
  worker.terminate(); // clean up
};

worker.onerror = (e) => console.error('Worker error:', e.message);

// worker.js — completely separate context
self.onmessage = (e) => {
  const { array, threshold } = e.data;
  // Heavy computation — won't block UI
  const result = array.filter(x => x > threshold).reduce((a,b) => a+b, 0);
  self.postMessage({ result });
};

Limitations: No access to DOM, window, document. Communication only via postMessage. Data is copied (structured clone), not shared (except SharedArrayBuffer).

Use cases: Image/video processing, data parsing, crypto, ML inference, large sort/filter operations.