1.

If Node.js is single threaded then how does it handle concurrency?

Answer»

The main loop is single-threaded and all async calls are managed by libuv library.

For example:

const CRYPTO = REQUIRE("crypto");const start = Date.now();function logHashTime() { crypto.pbkdf2("a", "b", 100000, 512, "sha512", () => {console.log("Hash: ", Date.now() - start); });}logHashTime();logHashTime();logHashTime();logHashTime();

This gives the output:

Hash: 1213Hash: 1225Hash: 1212Hash: 1222

This is because libuv sets up a thread pool to HANDLE such concurrency. How MANY threads will be there in the thread pool depends upon the number of cores but you can override this.


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