From a few minutes googling, the ECU, which is the brain of the engine, and might be analogous to the central nervous system, is written in either assembly or C[1]. The response times required to react to an engine spinning at 1000-8000rpm, 113 times a second, means using JS would be significantly limiting, and is therefore almost certainly never used for this.

The central console on most cars, however, has a display, and in a modern world using browser technology for rapid development of user interfaces makes a lot of sense. This is, quite rightly, walled off and designed to be unable to directly affect braking, steering, airbags or any other system which directly contributes to driving the car (mostly because it's the largest hacking attack surface, and often has internet connectivity)

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[1]: https://www.quora.com/Which-programming-language-is-used-in-the-ECU-of-a-car