🧵 Worker Threads the browser is a multicore runtime

"JavaScript is single-threaded" is only true of the main thread. Workers are real OS threads. Watch the spinner and the pulsing dot — they are your jank detector.

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UI thread health — if these freeze, the page is blocked

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The job: count primes below 6,000,000 (deliberately brutal)

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The workers are created from a blob: URL — the worker's source code is a string in this very page. No extra files.