Plan to throw one away
When I had interviewed for the job, the interviewer had explained that I was needed to work on their JavaScript engine. The company was a web development tools startup, and they had spotted that it would be a good idea to have JavaScript code running on the server as well as on the client, so that you could share your business logic and validation code between the client-side and server-side parts of your application. There are now lots of products in this space but twenty years ago was probably too soon for this to be a success.
Anyway, the interviewer told me that the JavaScript engine was nearly done, but there were one or two features that were missing, and it was running a bit slowly, so I might have to do a bit of optimization. ( Sure, I said, that sounded like it would be fun … )