Got Error when using alertify.js with Angular 6

[chathurikaCDS]

I try to use alertify.js in my angular 6 project.So I followed this material https://signl.uk/tutorial/index/notify-service-with-angular-5 .

I did those steps except add the alertifyjs css files and JS into the cli file. What I do for it I import CSS file into my Style.css file and JS file import into angular.json file. After run and click button what I want to show alert message then I got error alertify is not defined .Can anyone help find my mistake and solved this.