array question

[pelegk1]

i have did this :
var arr1=new Aray();
arr1[3]=5;

when i will check alert(arr1[2]) i will get the value "NaN"
how can i check in advance that the arr1[2] isn't initialized yet
??
thanks inadvance
Peleg

[jeffmott]

if (arr1[2]) {The variable could have been initialized to 0. This line should beif (arr1[2] != undefined)

[pelegk1]

that gets first
arr1[2]=3
arr1[15]=1
and i dont know the exect length in advance
and dont want to waste time on init it