The most popular JavaScript engine (V8) is written mostly in C++. Same with WebKit. Other JS engines are written in Rust, like Deno or Servo (used in parts of Firefox).
I'm not aware of a major Javascript engine which is written in C, except for QuickJS, which is not "major" in any sense of popularity but is an efficient engine for embedded use cases (and anything from Fabrice Bellard is always work a look).
I'm not aware of a major Javascript engine which is written in C, except for QuickJS, which is not "major" in any sense of popularity but is an efficient engine for embedded use cases (and anything from Fabrice Bellard is always work a look).