Completely flew over my head to use ternary operators here!

Conditional operator

What does the ? and : mean in Javascript?

The second part (true or false) is unnecesary. It's already assesed in the parentheses

-1, needs more ?:.

I see this sort of thing a lot in code, and it's redundant. You should just return the original conditional check. Consider it as if it were the folowing:

if (isTrue)
  return true;
else
  return false;

The above would be better written:

return isTrue;

Same idea with what is written in this solution.

Thx, for the hint...

The expected result is correct. To see the problem of yor code try console.log(30000000300000000+7). This produces 3000000030000010 instead of 300000030000007.

There is a way to resolve the problem without using an accumulator with such enormous numbers.