index 0 in the second level javascript arrays -

can please know why if second level entry in multidimensional javascript array returned undefined if level length equal 1 ?
example :

arr = array(   array(1)  ) 

the following return unlikely result : undefined, while it's suppose 1 (or @ least that's expect be)

console.log(arr[0][0]) 

but when have more record in level (length>1) :

arr = array(   array(1,2)  ) 

and try : console.log(arr[0][0]), returns 1 time.
explanation issue or why happening appreciated
thanks

you should consult documentation of array constructor:

arraylength
if argument passed array constructor integer between 0 , 232-1 (inclusive), returns new javascript array length set number.

array(1) not create array element 1, creates array of length 1 one element: undefined.

to have less ambiguous constructor, use array literal:

[[1]] 

this 1 array contains 1 array contains value 1.