Common questions

Why would you use onready var versus regular var ? 
 
 
 var is assigned when the script instance is created, if this is a node then it might not be added to the tree yet, so you can't use get_parent and similar on it. 
 
 onready var is a shortcut for assigning vars in _ready() and is assigned a value after a node and siblings enter the tree. Only useful for nodes. 
 
 Reference