Javascript dynamic nested arrays

Tonight is not my night and it may be time to step away from the computer. I cannot for the life of me remember how to do this. I am trying to dynamically create an empty nested array based on the the length of a word. For example if the length was 7 I would want the array to look like this

Rows = [
    ['', '', '', '', '', '', ''],
    ['', '', '', '', '', '', ''],
    ['', '', '', '', '', '', ''],
    ['', '', '', '', '', '', ''],
    ['', '', '', '', '', '', ''],
    ['', '', '', '', '', '', '']
]

and if the length was 3 I would want my array to be dynamically created like this

Rows = [
    ['', '', ''],
    ['', '', ''],
    ['', '', ''],
    ['', '', ''],
    ['', '', ''],
    ['', '', '']
]