this.getView().byId vs sap.ui.getCore().byId

Hello!

I don't understand difference between subject method.

If I created XML-view, then work method this.getView().byId.

However, if I try to create JS-view, then this method does not work properly. It return unknow value. If I replace method with sap.ui.getCore().byId, then I received requested control by Id.

What's the difference?