javascript: sending the name of an array as a variable

Hello,
I am predefining a series of arrays and I want to send the name of the array as a variable. But the problem is that the variable is not recognized as an array...

ie
[arrayName is equal to "array1"]

function thisfunction(arrayName)
{

var array1 = new array(...);
var array2 = new array(...);

alert(arrayName.length);

}

This does not give me the length of the array.