What is the purpose of a plus symbol before a variable?
javascript
Solution
The `+` operator returns the numeric representation of the object. So in your particular case, it would appear to be predicating the if on whether or not `d` is a non-zero number.
Reference here. And, as pointed out in comments, here.
Problem
What does the `+d` in ``` function addMonths(d, n, keepTime) { if (+d) { ``` mean?