First, let's define a few terms. According to Mozilla's object-oriented JavaScript documentation, JavaScript is really a classless language. However, class syntax was added on top of the language, so in JavaScript, a class is sort of like a template for an object. A class is technically an object as well. A class definition pre-defines the methods and properties of an object generated from the class. Most experienced JavaScript developers will be familiar with a similar concept: object prototypes.

An object therefore, in this context, is simply an instance of a class! These classes are only slightly different from the prototype constructors you're probably familiar with, and instances of these classes are generated in the same way as with prototypal notation: using the new keyword.

var grIncident = new GlideRecord('incident');

A method is a function or subroutine that's declared as part of a class (and any objects that are instances of that class).

A constructor is a special type of function that runs when an object is declared as a new instance of a class using the new keyword. In ServiceNow's server-side scripts (script includes, specifically), this constructor function is usually called initialize. Any arguments passed into the class during instantiation (using the new keyword) will be passed into this function, which builds the object that will be returned.

Here's an example of what a very simple class might look like:

var MyClass = Class.create(); 
MyClass.prototype = { 
    initialize: function(initArg) { 
        this.localProperty = initArg; 
    }, 
    getLocalProperty: function() { 
        return this.localProperty; 
    }, 
    type: 'MyClass' 
}; 

Now let's break this down line-by-line, and see how it works.

On line 1, we declare our class and give it a name (MyClass) using a special ServiceNow API: Class.create(). JavaScript doesn't technically have a data type called Class; at least not in ES5, which is the version of JavaScript that executes on the server. Instead, this code relies on function/object prototype extensions. In order to simplify this, and provide syntax more similar to the backend Java (and more similar to what most object-oriented programming languages use), ServiceNow provides Class.create() as a simple way to generate a basic class-esque object.

On line 2 and onward, we take this basic class scaffolding that we generated on line one, and extend that prototype by adding some stuff to it (everything between the curly braces: { and }).

On line 3, we declare the initialize method. This is a special method of our class, which is called automatically whenever a new object is generated (instantiated) from our class using the new keyword like so:

var myObj = new MyClass('input'); 

Any arguments passed into this instantiation process (the string input in the preceding case), are passed into this initialize method.

Next, on line 4, we set a property in the scope of the instantiated object using the keyword this. In this case, any time we create an instance of this class, the property called localProperty will be initialized with the value that was passed into the constructor function (initArg).

On line 6, we declare another method called getLocalProperty. This is a method that can be called from objects created from our class, and which (on line 7) returns the value of the property that was set on initialization.

Finally, on line 9, we declare the type property of our class (and instantiated objects). This is just a string that you can access to determine what class child objects were created from.