Conditionals in javascript with multiple popunder ad network

i am fairly new in javascript. I have 2 popunder network. I have to create a script that will select from two ad networks and output only one popunder if the other network has no ad output the ad on the other instead.

1. Popunder network 1
2. Popunder network 2

example:

if(there is an ad on popunder network 1){
// output popunder 1
}
else{
// output popunder network 2
}