For the first rule, it is a tirck, ARRAY IS KIND OF OBJECT
var arr = [];
arr[-1]=-1;//add an property named "-1" on arr OBJECT
arr[1]=1; //put a value whose index is 1 into arr ARRAY
for(var k in arr)alert("arr[" + k + "]=" + arr[k]);
alert("length = " + arr.length);
for(var i=0;i<arr.length;i++)alert("arr[" + i + "]=" +arr[i]);