You can import libraries. I believe the example solutions for the markdown previewer, clock, and calculator all import libraries. The point is to do the front end and functionality, not to make a specific algorithmic library.
My first time through the various challenges, yes, I used various libraries. In the example of the Pomodoro Timer, I Revisited the challenge later, and wrote my own es6 timer class. Both approaches have value. Yes learn to use the libraries and Frameworks, but you willing to revisit challenges to get a deeper understanding of what those Frameworks and libraries give you.
Thanks for your advice .I will certainly try a different method when i will revist these challenges.