I will lower the range to 32 bit in JS.

Try not to use floating point arithmetic.

Fix applied. Thanks @RileyHunter

Confirming this is a bug in the reference solution. When the first polynomial string ends with the variable and the second starts with it the variable is incorrectly detected as being multiple characters, e.g.

Testing: -1+4a and a-9a^3: expected '-36a^4+9a^3+4a^2-a' to equal '-NaN'

This is because it's scanning the concatenated string -1+4aa-9a^3 and finding the "variable" aa, then parsing breaks down from there. I've fixed this in a fork here:

https://www.codewars.com/kumite/65383e33e681c41613bcb605?sel=65383e33e681c41613bcb605

Just waiting for approval

Thank you, your advices has a sense. Foreach here would be perfect choice.
Reduce is also an option, but as with a map case, there will no further chaining after reduce, so it would be not so much neccessery.

Solution with map+join is also a good case, so we can get rid of result variable. But in this case we additionally need to handle empty string scenario: return ......map().join('') || '';