Import external javascript to Ionic

I am facing some problem when try to import javascript library to my Ionic app, specifically import this chart into my ionic project

https://github.com/rendro/easy-pie-chart. 

Firstly I install the latest ionic, after that go to

/src/

open the assets folder, create a

/js/

folder inside assets copy .js inside this js folder goto

/src/index.html

Add path <script src="assets/js/N.js">
Go to typescript file and declare var N;

And another way like this video: https://www.youtube.com/watch?v=xWIzZ8wBqk0

But both of them didn’t worked. Do you have any else idea ? Thank you.

does anyone have any ideas?