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

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?

CodePudding user response：

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);
});

CodePudding user response：

When curious, it's always good to look at the source, which they've made available. First thing to check since this is a node project would be the package.json -- main is "index". All the exports are going to be defined in libs/index.js.

Looking at libs/index.js line 52, we see where model is defined as an export -- it's a function where you pass in a name and definition that get passed to ModelObject.model; if you scroll back up, you can see that that's included from libs/model_object.js.

Going over to libs/model_object, you can see what the passed definitions field (the lambda function you see in the example code) is used for: definitions.call(typeClass). typeClass is created earlier in the function. If you're unfamiliar with what call does, it lets a function be executed in another object -- the this you see in the passed function is going to refer to the typeClass object. Inside of libs/model_object.js, a has function is defined that adds ModelObjectProperties; this.has is being used to add properties to the ModelObject.

edit: CertainPerformance's answer is probably more clear, refer to that.