I will be changing jobs soon, and I would like to make life as easy as possible for whoever inherits the codebases in the future. There is unlikely to be overlap time where a new hire could ramp up whilst I'm still available. There are multiple codebases, varying in size from 4KLOC - 130KLOC.
What kinds of resources are most helpful for new hires? Documentation? Slide decks? Comments? Test coverage? Structure / quality of the code? Something else? All of the above? What value weights correspond to each of these?
What aspects of other codebases you have encountered in the past have made learning easy or difficult? Are there any language specific issues to be concerned with? I welcome a wide variety of answers since different things will work for different people.