Javascript array best practice to use [] instead of new array?

Question

I read at many tutorials that the current best practices to create a new javascript array is to use

var arr = [] 

instead of

Answer 1

To create Array without Length

var arr = [];

To create Array with Length more dynamically

var arr;
( arr = [] ).length = 10;  // 10 is array length

To create Array with Length less dynamically

var arr = [];
arr.length = 10;