Adding Decimal128

I feel stupid. This is from my Atlas function which runs after an object change trigger:

const double = BSON.Decimal128.fromString("2500");
console.log(double + double);
// => "25002500"

Please embarras me with what I am missing here.
I assume this is due to Javascript converting a Decimal128 into String and that I need some special function to perform any arithmetic on Decimal128. Is there any documentation that I’m not googling for correctly about how to do this in Atlas functions?