Add WeekDays to Date
What is the expected behaviour if you add one weekday to a Saturday? Possible answers are: throw an exception, behavior is undefined, Monday, or Tuesday. Make a decision, and document it.

The use of var funcName = function(…) { … } is a bit unconventional compared to function funcName(…) { … }, but it's not wrong.

Other than that, I think that the code looks correct.  For a more efficient way to add large intervals, you could add a number of integral weeks (7 * Math.floor(days / 5)) and advance the remainder (days % 5) the slow way.