How does the length method work in a two dimensional array?

An array is just a collection of objects, so the length method calculates how many objects are in the collection you call it on:

[[1,2],['a','b;]].length //2

the above example is 2, because there are two objects in the collection the array [1,2] and ['a', 'b']. Notice that it doesn't matter what type of objects or the depth of those objects:

[[1,2],[3],5,{key: "value"}].length //4

If you view arrays as containers, length calculates how many objects are directly in that container, if your array is like a box, the length method calculates how many things are directly in that box, any small box within your array is counted once no matter how much is in it.

It returns the amount of rows.

A multi-dimensional array is a array that contains arrays (which in turn may include more arrays). The length of arrays is simply the number of values in the array. But these "values" can actually be arrays, because in JavaScript (as in many modern languages), arrays are treated as values.

I hope this helps.