The answer
When a callback does heavy synchronous computation, it blocks the event loop and all connections. This is the main scalability concern in Node.js, and why you must keep the main thread non-blocking by offloading CPU-heavy work.
How Node.js Handles Concurrency on a Single Thread
When does Node.js concurrency break down?
More FAQs in How Node.js Handles Concurrency on a Single Thread
Through the event loop and non-blocking I/O. When an async operation starts, the thread moves on. When it completes, its callback is queued, and the event loop processes callbacks one at a time. This lets one thread serve thousands of concurrent connections.
No. Node.js handles many concurrent connections. The JavaScript code runs on one thread, but I/O operations are delegated to libuv, so the thread is free to process other requests while waiting for I/O to complete.
For operations that cannot be done asynchronously, like certain file system operations. These go to libuv's thread pool (default 4 threads), so they run off the main thread without blocking the event loop.
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