There are libraries for this, but I don't think they can be used on Codewars.
If you're running node locally, you can find libraries on https://www.npmjs.com/

Complexity
Since you have to multiply every term of one polynomial with each term of the other polynomial,
the best you can do is O(m*n) where m and n are the number of terms in the two polynomials.
You can only get O(n^2) if both polynomials have the same number of terms.