Where does "has" come from in "this.has" expression?

I was reading Nats-Jetstream documentation that I encountered the following code:

var createModel = require('jetstream').model;

var Shape = createModel('Shape', function() {
    this.has('x', Number);
    this.has('y', Number);
    this.has('width', Number);
    this.has('height', Number);
    this.has('type', Number);
});

I am not sure what does this.has mean? Where does the has come from? How does this code work?

The shape of the library is described in the docs. It has:

type JetStream : {
    model: (name: String, fn: ModelDefiner) => Model,

So the callback is a ModelDefiner, which is defined here:

type ModelDefiner : (this: Model) => void

So this is a Model, which is defined here:

type ModelObject : {
    has: (propertyName: String, PropertyType) => void,
    setScope: (scope: Scope, Callback<Error, void>) => void,
    getValues: () => Object,
    getAddSyncFragment: () => SyncFragment<"add">
}

-- A Model instance inherits from ModelObject.
type Model : ModelObject & {
    typeName: String,
    uuid: ObjectUUID,
    scope: Scope | null
}

As you can see above, a Model extends a ModelObject, and a ModelObject has a has method.

If you're curious how you would write code that could do this yourself, all you really need is to create an object with a has method, and .call the callback with it.

class Model {
  has(str, type) {
    console.log('has called with', str, type);
  };
}
const createModel = (str, callback) => {
  const model = new Model();
  callback.call(model);
};

var Shape = createModel('Shape', function() {
    this.has('x', Number);
    this.has('y', Number);
    this.has('width', Number);
    this.has('height', Number);
    this.has('type', Number);
});