What is the difference between new ObjectId() and ObjectId() and ObjectId

It's more like a theoretical questions than an actual issue but I hope somebody have time to answer it:

Suppose that you have this definition at the beginning of a node.js file:

const ObjectId = mongoose.Types.ObjectId;

Which one do you prefer and why?

// 1 
new ObjectId;  

// 2
new ObjectId();

// 3
ObjectId();

The official document suggests to use new ObjectId (#1), for me new ObjectId() (#2) feel more natural but each of them generates a new ObjectId and I have seen examples for each of them in SO's questions and answers.

http://mongoosejs.com/docs/api.html#types-objectid-js

via Festo