Theoretically yes, may vary in practice
Every programming language you know is undoubtedly a skill. Of course, your proficiency in each skill could reach various levels (let's say from total beginner to master). But that is not the main question. Main question is does your employer needs your skills ?
I would give you one example: let's say you are working in a restaurant as a waiter. You are also reasonably proficient with C++ . But does this restaurant needs C++ ? Likely not. If you were skilled in cooking, you could occasionally double as restaurant chef, so you would have some ground to ask for a raise (otherwise restaurant could need to hire another person, which would cost more money then your raise) . But with C++ ... you would likely had to search for another employer to cash in that skill.
Now, in your case, you have been asked to do some coding in JavaScript which strictly speaking is not your job (especially as a data scientist) . But you have to ask yourself these questions: is this JS task so crucial for the company they would have to hire JS developer if you could not do it ? Is this task so hard that some of your colleagues could not learn some JS on the go and do it ? Does your employer really needs you in your primary role as a Data Scientist ?
With proper introspection you may find out that you were actually given some light JS coding just to keep you occupied and somewhat useful because you are not really shining as Data Scientist. Or you could find out that you are doing a tremendous job on both fronts and actually are exploited by the company. Depending on that, you may decide to ask for a raise, or to keep quiet and be thankful you have a job in a first place. In any case, what is important is to be true to yourself, and honestly assess the situation.