Mentoring is not teaching. This is improving. If a person knows Javascript and works with me, I would ask him to go through existing codebase and analyse how code is written against his current code. This will improve his code but will improve the overall codebase as eventually he has to start merging his code into working codebase.
Will ask him to do a lot of online coding puzzles not to learn the coding logic only but to learn to manage time. The problem with most new guys are they don't value time. They need to work within time. I see a lot of them stay late at night and finish things but can do the same without staying late. I believe in writing clean code and if my code-review tool says otherwise, I will not merge the code. Even if that means weeks of his work. In the end we need to enforce such things for better code instead of giving in for such short time pressures.
Lot more can be done, but I just mentioned few that I deemed as important.