Length of an array with string index

programming flash
1

You can find the length of an array through the property length (e.g. arrayInstance.length)

But I realize that I cannot use this property if I specify the array index through strings
for example:

 
var arrayInstance = new Array();
arrayInstance["cat"] = "Garfield";
arrayInstance["dog"] = Jazzy Dog";
arrayInstance["mouse"] = "Mickey Mouse";
trace(arrayInstance.length)  //returns 0 !!