Do thread pool callbacks run on the main thread in Node.js?
Yes. The work runs on a thread pool thread, but the callback always runs on the main thread via the event loop. This is important: the JavaScript callback is never on the thread pool thread, so it does not need thread safety.
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More FAQs in How the libuv Thread Pool Works in Node.js
A pool of 4 threads (by default) in libuv that handles operations which cannot be done asynchronously by the OS, like certain file system operations, DNS lookups, and crypto work. These run off the main thread so they do not block the event loop.
Because the OS cannot handle all operations asynchronously. File system operations, some DNS lookups, and certain crypto work are blocking. The thread pool runs these on separate threads so the main thread's event loop stays free.
File system operations (fs module), DNS lookups (dns.lookup), and some crypto operations (crypto.pbkdf2). Network I/O does not use the thread pool; it uses the OS's async capabilities directly.
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