This will log NaN.
The operator + is known as the unary operator. When an element is prefixed by the unary operator, the JavaScript interpreter will try to convert that element into a number. If the conversion fails, this will result in NaN.
Under the hood, the unary operator attempts to call valueOf() and toString() to convert the element into a number.
For a deep dive on the unary operator and the conversion process for different types, I encourage you to read The unary + operator.