You can do the code compression which is just removing all the white space that not needed but that then makes editing harder, some compression tools will rename functions to remove lengthy name conventions, this also has problems with following the script. IMHO you should only do this if its going to save you a sizeable chunk of data space and speed up transfer of the script to the client.
JSLint, "strict"; and the other stuff... I tried using a couple of online tools and to be honest, they are more bother than they are worth as your able to write code on your computer and with the browsers console log, see all the errors that it is detecting and correct them without having some tool to tell you that something is wrong and I don't ever use "strict"; in any of my scripts because it stops you from using certain functions that are really useful, IMHO its a retarded approach by a standards organisation and by some stuffed shirt that is trying to impose compiled programming standards on something that has for 30 odd years never needed restrictions while justifying their job. Totally pointless if you ask me, I am sure that theirs plenty of people out there who would find some way of disagreeing and giving reasons why you should use "strict";, only thing I can say is if you want to limit yourself, go ahead and use it.