in the MDN documentation i have seen it many times for example

p.then(onFulfilled[, onRejected]);

this method takes two functions callbacks that I understand but why in the syntax example provided by MDN there is brackets like its an array and to add to my confusion there is a comma in the beginning or maybe this doesn't mean an array at all and means something else i know it might be something really small or trivial but i cant find relative information regarding this thank you so much for helping