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Deno is a simple, modern and secure runtime for JavaScript and TypeScript that uses V8 and is built in Rust.

So it's probably a rust wrapper around a C++ project (unless they reimplement V8 in rust... )

Yeah the communication with V8 and all the API are in rust, V8 is still in c++

Yup, it uses rusty_v8 which are Rust bindings for V8's C++ API - https://github.com/denoland/rusty_v8