What does this documentation syntax mean at MDN for a JavaScript function? [duplicate]

Answer 1

It's usually a convention for API documentations to use [] to denote optional parameters. However, the [] is not part of the syntax on usage. It's just documentation convention.

Answer 2

As already explained in other answers, the parameters inside of the "[]" are optional. Regarding the question as to why the "other parameters" (i.e. previousValue etc.) are not there, those are parameters to callback and not to reduce. So callback will receive those arguments on each run of reduce.