The && and || operators are called short-circuit operators. They will return the value of the second operand based on the value of the first operand.
The && operator is useful for checking for null objects before accessing their attributes. For example...
var name = person && person.getName();This code is the same as
if(person) {
var name = person.getName();
}The || operator is used for setting default values.
var name = persons_name || "John Doe";The equalant code is
if(persons_name) {
var name = persons_name;
} else {
var name = "John Doe";
}