What is the purpose of a plus symbol before a variable?

Question

this really sounds like a simple question but I had no luck searching. what does the +d in

function addMonths(d, n, keepTime) { 
    if (+d) {
         


        

Answer 1

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.