How to address a step variable in JS expression?

There are steps to bridge any step data to JS and vice versa. See :

jSimpleSource | Convertigo Documentation

and

use jSimpleSource to “Source” some sequence data in a Javascript variable. Then you will be able to use this variable in any SequenceJS step in your Sequence.

Do the computation you want and at the end, you can use the jElement Step to output this javascript Variable as sequence data using the jElement Step.