How to prepend to postgresql json array

I have a plpgsql function which at some point has a text variable _x (which is guaranteed to not be NULL) and another variable _y which is a JSONB array of text (which might be empty.)

How can I prepend the value of _x to the array _y?

I was trying to do it like this:

SELECT jsonb_build_array(_x, VARIADIC array_agg(value)) FROM jsonb_array_elements_text(_y) INTO _y 

which is wrong. I was trying to use info about VARIADIC from this answer.

I am using PostgreSQL 9.6