BeRo did build a javascript engine from scratch on his own: <a href="https:&#x2F;&#x2F;github.com&#x2F;BeRo1985&#x2F;besen" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;BeRo1985&#x2F;besen</a><p>From the copyright headers he worked on it starting in 2009 and had the first public release in 2014 (actually 2010, see comment below). He certainly worked on other stuff in between too. This is also not a toy implementation. While I have no idea how it compares to state of the art engines, it does feature a whole bunch of optimizations and a JIT compiler.