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";
}
Comments
var name = person && person.getName();
might work the same as
var name;
if (person) {
name = person.getName();
}
because, in the above example, "person" might have the value "undefined" (depending on your context, but in the most common example, for a parameter named "person" inside a function), in which case "name" is declared and gets assigned the value "undefined", but in the lower example, if "person" is not "truthy" (fails an "if" test), then the variable "name" does not get declared at all, in which case attempting to access its value it afterward is an error. Try it!
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