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

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) {


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.

It is a unary "+" operator which yields a numeric expression. It would be the same as d*1, I believe.

As explained in other answers it converts the variable to a number. Specially useful when d can be either a number or a string function that evaluates to a number.

Example (using the addMonths function in the question):


then the +d will evaluate to a number in all cases. Thus avoiding the need to check for the type and take different code paths depending on whether d is a number, a function or a string that can be converted to a number.