use-strict

I have never seen the use of this directive before, but almost all the more mature jQuery plugins use it. I would like to know when and how to use it and what its purpose is.

Apparently Visual Studio does not recognize the Directive 'use strict', as I entered the code below that assigns value to a v ... itor". How to make Visual Studio recognize 'use strict' and point out errors in the code, as for example in this case above?