Common JavaScript Manual/How read this book

Optional arguments
Optinal variables enclosed in square brackets. For example Array.join([separator])

Variable in description
In function description variables are bolded. For example Math.sqrt(num) - square root of num

Undefined number of arguments
Undefined number of arguments in function written as name... and function description name are all these variables in general, and name[n] either name. For example Array.concat(elements...) - returns array that containg elements from Array and elements if elements[n] is array then to the array that returns are added all elements from elements[n].