Web Workers: Run Heavy Computation Without Freezing the UI
JavaScript is single-threaded. Heavy computation on the main thread blocks everything — animations, input response, scrolling. Web Workers run JS in a background thread.
When to Use Workers
Use a Web Worker for tasks taking more than ~16ms:
- Parsing large JSON or CSV files
- Image processing
- Cryptographic operations
- Complex data transformations over large datasets
Basic Setup
// worker.ts
self.addEventListener('message', (event: MessageEvent) => {
const { data, operation } = event.data;
if (operation === 'sort') {
const sorted = data.sort((a: number, b: number) => a - b);
self.postMessage({ result: sorted });
}
});
// main.ts
const worker = new Worker(new URL('./worker.ts', import.meta.url), { type: 'module' });
worker.addEventListener('message', (event) => setData(event.data.result));
function sortInBackground(data: number[]) {
worker.postMessage({ operation: 'sort', data });
// Returns immediately — UI stays responsive
}
Comlink: The Clean API
// worker.ts
import { expose } from 'comlink';
expose({ async parseCSV(csv: string) { return parse(csv); } });
// main.ts
import { wrap } from 'comlink';
const api = wrap<typeof api>(new Worker('./worker.ts'));
const result = await api.parseCSV(csvString); // runs in worker
Transferable Objects
For large ArrayBuffers, transfer ownership instead of copying:
const buffer = new ArrayBuffer(100 * 1024 * 1024);
worker.postMessage({ buffer }, [buffer]); // zero-copy transfer
Web Workers, optimistic updates, code splitting — the performance patterns that separate fast apps from slow ones are part of the Ship Fast Skill Pack.
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