Bun.js is a Javascript runtime environment.
Context about Javascript runtime environment:
Before 2009 Javascript could only be run in browsers. Then a person named Ryan Dahl took Google Chrome's V8 engine and made Javascript to be used as backend in the servers.
Then several other technologies emerged like Deno.
Bun.js
Bun.js has been a hot topic lately on how it can replace Node.js.
While node.js used Chrome's V8 engine bun.js uses Javascriptcore by safari.
And bun.js uses Zig(ziglang.org).
It's founders claim it to be way faster than node js and deno. Developers are going crazy about bun but only time will tell how well it ages.
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