Date as a constructor and as a function

Question

var a = Date;
Object.defineProperty(window, "Date", { get: function() { console.log(this); return a; } });

(new Date);
Date();

How can I know, when Date is being called as a constructor(with "new"), and when it's being called as a function?

Update
these answers don't work for this case.
this is always equal to a window

Notice that the getter you defined is always invoked on the global object. Only the function that is called can really distinguish between constructor and function calls. — Bergi, Jun 09 '16 at 01:25
@Bergi sorry, need is a strong word. Isn't it convention though? — Jay, Jun 09 '16 at 01:32
@Wade: I don't think it's a convention, it's just that most of the constructor invocations you see are with arguments and you need the parentheses for those. — Bergi, Jun 09 '16 at 01:34
Per [*ECMA-262 §20.3.2*](http://www.ecma-international.org/ecma-262/6.0/#sec-date-constructor): "*When Date is called as a function rather than as a constructor, it returns a String representing the current time (UTC).*". When called as a constructor, it must return an object. When called as a function, it will return a string. If you replace the global *Date* property with some other function, all bets are off. — RobG, Jun 09 '16 at 01:37
@Bergi oh, everyone I've ever worked with has defined it to be convention haha. Since yeah you have to have parentheses when you have arguments, it makes sense to use an empty set of parentheses, so that it mimics how function calls work. That is, you always use parentheses — Jay, Jun 09 '16 at 01:37
Just as a note, I think in ES2015 you can check if (new^), which returns whether it was called with the new keyword — Jay, Jun 09 '16 at 02:01

score -2 · Answer 1 · answered Jun 09 '16 at 01:27

-2

Date is a Constructor function. It's just like any JavaScript Constructor. You will alway need to use new when you use it.

answered Jun 09 '16 at 01:27

andre mcgruder

Very untrue. You NEVER strictly have to use new unless explicitly defined inside of the function. – Jay Jun 09 '16 at 02:00

1 Answers1