How do I change the background colour smoothly using vanilla js only?

Question

I'm assigning background colors to body randomly from an array upon a button being clicked. But it happens fast. I need it smooth like 1s or 2s. Help pls.

Answer 1

https://code.sololearn.com/WmwYcnqb7Gs7/?ref=app

Answer 2

Calviղ thanks a lot...