Where do I find the Paragraph.prototype or how else can I extend it?

Question

I wanted to extend the Paragraph (type? class? object?) and add some repeating functions to it like:

Paragraph.prototype.isEmpty = function() {
    return this.getText().trim().length === 0;
};

ReferenceError: "Paragraph" is not defined. (line 23, file "Code")

or

DocumentApp.Paragraph.prototype.isEmpty = function() {
    return this.getText().trim().length === 0;
};

TypeError: Cannot read property "prototype" from undefined. (line 23, file "Code")

but none of these work. I put this code in a Code.gs file and tried to use it but the compiler says there is no such thing.

What is this thing actually and how can I add something to it?