Sorry for a late answer.

"Bread and butter" may or may not work, depending on your definition of "formal." It is a somewhat dated expression. (My father used it many decades ago, and I can't recall a single, unforced usage in my last decade of using American English.)

Since it is a metaphor, "bread and butter" is comprised of smaller, everyday words, and might be considered as less formal. Also, since it is dated (but not incorrect), it could be misunderstood by a younger examiner.

A better phrase might be:

core competency

a defining capability or advantage that distinguishes an enterprise from its competitors.

a defined level of competence in a particular job or academic program. (Source: Oxford)

If the interviewer asks, "How do you feel about web development in JavaScript?"

You could reply:

That's one of my core competencies. I used every day in my past job.