Why [] symbols are used in parameters of general notation of any thing in JavaScript for eg 'new RegExp(pattern[, flags])'??

Question

Another example 'Date.UTC(year[, month[, day[, hour[, minute[, second[, millisecond]]]]]])'
its an example from MDN page.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/UTC

Answer 1

It is telling you that the parameter(s) within the '[' and ']' is/are optional, meaning you don't necessarily provide any argument for it/them, and the method/function can and will still work, with a defaulted behaviour of course.