That will only work for the most simple of functions. How would you even make such a prompt? "Plz explain this code for me?" It's going to come back with the same essential complexity except in English instead of Javascript, you still need to understand the surroundings. Often you need to approach the understanding from different angles. Are you going to ask it "What will happen given preconditions A, B and C". How many of those questions do you need?

Not to mention how slow this is, instead of just reading a piece of code or instantly query my already existing internal mental model of it, I have to type a prompt, wait for the response, read the response, hope it understood my intent, hope it understood the code, map it to my own understanding.