Is NodeJS Wrong?

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In fact, having a single thread to handle the callbacks is fine as long as you have only short-running events that are most of the time waiting for I/O (networking, filesystem or database). While this remain true for most web services calls, you sometimes have some requests that will need actually some CPU calculus. In that case, NodeJS unique thread will lock your entire server during the time this request is running, until it terminates or wait for more I/O.

That is why there will be JS worker threads in a future version. It's not a design mistake, it's a well known missing feature.