Node is a non-blocking Javascript runtime. You can concurrently run an HTTP Server, read files from disk, send UDP datagrams, accept TCP connections from clients and still have room to execute Javascript code operations without blocking. Most of these operations are known as I/O, you send an input to a device, file or a socket and it replies back with an output. Node achieves non-blocking I/O with mostly a single thread executed asynchronously using a library called lib_uv.
The exception being DNS queries, which use a thread pool. This means when you use fetch or axios to make an HTTP request to a domain, the DNS resolution for that domain will most probably go through the thread pool, while the actual request itself will be sent asynchronously on the main thread.
In this episode of the backend engineering show I go through an article I wrote discussing NodeJS Asynchronous I/O.
https://medium.com/@hnasr/when-nodejs-i-o-blocks-327f8a36fbd4
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0:00 Intro
3:00 Part 1 Socket/IO
9:48 Part 2 File I/O
12:42 Part 3 DNS
16:22 Part 4 NodeJS Single Threaded
19:10 Part 5 NodeJS Thread Pool
21:23 Part 6 DNS lookup bottleneck in Node
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