this._helloDialog in OpenUI5 walkthrough - sapui5

Question

I am new to JavaScript and OpenUI5.
I was going through the walkthrough demo on the openUi5 website OpenUI5 walkthrough demo
I came through the below code:
sap.ui.define([
"sap/ui/core/UIComponent",
"sap/ui/model/json/JSONModel",
"sap/ui/demo/wt/controller/HelloDialog"
], function(UIComponent, JSONModel, HelloDialog) {
"use strict";
return UIComponent.extend("sap.ui.demo.wt.Component", {
metadata: {
manifest: "json"
},
init: function() {
// call the init function of the parent
UIComponent.prototype.init.apply(this, arguments);
// set data model
var oData = {
recipient: {
name: "World"
}
};
var oModel = new JSONModel(oData);
this.setModel(oModel);
// set dialog
this._helloDialog = new HelloDialog(this.getRootControl());
},
openHelloDialog: function() {
this._helloDialog.open();
}
});
});
I have doubt in the line this._helloDialog = new HelloDialog(this.getRootControl());
If _helloDialog is not defined and we are using strict mode, then why does the system not throw message that _helloDialog is undefined?

Answer 1

_helloDialog is a property of this (the controller), and properties do not need to be initialized when creating an object.
"use strict"
var example = {};
example.newProperty = "i am a new property"; //This is absolutely correct
undefinedVariable = 1; // This is going to throw an error
Strict mode prevents you from implicitly creating global variables (as undefinedVariable = 1; would do). But it is not going to prevent adding a property to an object.
If you are interested on preventing the creation of properties, I suggest reading Freeze vs Seal