As a mere mortal - How hard would it be for a day programmer to built something cool like an JavaScript engine ?

I sometimes have crazy thoughts about the world ending.

How hard would it be to built your own javascript engine from scratch ?

There are many JS engines which are extremely small compared to the big guys used in browsers. Most of them tend to grab a subset of the language and implement that.

For example, the creators of nginx created their own specialized engine called nginScript that runs JS for their stuff https://www.nginx.com/resources/wiki/nginScript/

There is even a javascript interpreter written in javascript which can be a good starting point to learn a bit how it works https://github.com/jterrace/js.js/