This is actually similar to javascript's "OR" operator(not to be confused with operand).
I believe what Raccoon is asking for is something that does "first non-null/0/false" assignment.
var %a = $false||0||$null|hi
//this is true
if (%a === hi) return $true
edit: nvm. after rereading the OP, it seems he wants to assign an alternative value if none is specified