Logical Operators
The logical operators in Lua are and, or, and not. Like the control structures (see Control Structures), all logical operators consider both false and null as false and anything else as true.
The negation operator not always returns false or true. The conjunction operator and returns its first argument if this value is false or null; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from null and false; otherwise, or returns its second argument. Both and and or use short-cut evaluation; that is, the second operand is evaluated only if necessary. Here are some examples:
10 or 20 --> 10
10 or error() --> 10
null or "a" --> "a"
null and 10 --> null
false and error() --> false
false and --> false
false or null --> null
10 and 20 --> 20
(In this guide, -->
indicates the result of the preceding expression.)