JSC is a multi-tier JIT with the last stage ultimately being LLVM, so if you want to be pedantic, Bun relies on LLVM’s optimizer which is written in C++.

The transpiling itself is written in Zig, which is the part that has the performance improvement. If Bun relied on JavaScript and JSC for the heavy lifting, it would be no faster than the other JS bundlers.

edit: no longer LLVM: https://webkit.org/blog/5852/introducing-the-b3-jit-compiler...

But bundling doesn't