what is the role of the plus sign(+) in the following function which is excerpt from jquery source code

Question

function now(){
    return +new Date;
}

questions :

1. what does the plus sign mean?
2. when can you create a new object with a constructor function but without the following parentheses, such as new Date but not new Date()

great thanks!

Answer 1

1 . The plus sign is the unary + operator.

That expression is equivalent to cast the Date object to number:

function now(){
    return Number(new Date);
}

2 . If you don't add the parenthesis, the new operator will call the object type (Date) parameterlessly

Answer 2

1. Using the plus sign will convert the date into a number (the number of milliseconds since 1 Jan 1970)

2. You can do this whenever there are no parameters - although you may wish to include them still for readability.