Operators

An operator is a symbol for performing an operation or conditional evaluation.

Logical

Logical operators return values depending on the boolean values of the given arguments. If an argument isn't false or nil, then the operator evaluates it as true. Unlike many other languages, Luau considers both zero and the empty string as true. The following table summarizes how logical operators behave in conditionals.

and

The binary operator and returns one of the two arguments. If the first argument evaluates to true, then it returns the second argument. Otherwise, it returns the first argument.

print(4 and 5) -- 5
print(nil and 12) -- nil
print(false and 12) -- false
print(false and true) -- false
print(false and false) -- false
print(true and false) -- false
print(true and true) -- true

You can use and to test multiple conditions in control structures such as if statements and while loops. For example, the following if—then statement tests that two conditions are both true:

local pasta = true
local tomatoSauce = true

if pasta == true and tomatoSauce == true then
	print("We have spaghetti dinner")
else
	print("Something is missing...")
end
-- Output: We have spaghetti dinner

or

The binary operator or returns one of the two arguments. If the first argument evaluates to true, then it returns the first argument. Otherwise, it returns the second argument.

local y = x or 1
print(y) -- 1 because x doesn't exist and is therefore nil

local d = false
local e = d or 1
print(e) -- 1 because d is false

print(4 or 5) -- 4
print(nil or 12) -- 12
print(false or 12) -- 12
print(false or true) -- true
print(false or false) -- false
print(true or false) -- true
print(true or true) -- true

You can use or to perform complex logical tests in control structures. For example, the following if—then statement tests whether two conditions are true or a third condition is true:

local pasta = false
local tomatoSauce = true
local garlicBread = true

if (pasta == true and tomatoSauce == true) or garlicBread == true then
	print("We have either spaghetti dinner OR garlic bread")
else
	print("Something is missing...")
end

-- Output: We have either spaghetti dinner OR garlic bread

not

The unary operator not returns the opposite boolean value of the argument. If the argument is false or nil, then it returns true. Otherwise, it returns false.

print(not true) -- false
print(not false) -- true
print(not nil) -- true
print(not "text") -- false
print(not 0) -- false

You can use the not operator to trigger a conditional or loop if a variable is false or nil.

local nilVariable  -- Variable is declared but has no value, so it's nil
local falseVariable = false  -- Variable is declared with value of false

if not nilVariable then
	print(nilVariable)  -- Outputs "nil" because nil isn't true
end

if not falseVariable then
	print(falseVariable)  -- Outputs "false" because false isn't true
end

You can also use the not operator to test for the opposite of an entire multi-condition statement. In the following code sample, the if—then conditional tests that it's neither true that three is greater than four nor is it true that five is greater than four.

local three = 3
local four = 4
local five = 5

if not (three > four or five < four) then
	print("Three is less than 4 and five is greater than 4.")
end

-- Output: Three is less than 4 and five is greater than 4.

Relational

Relational operators compare two parameters and return a boolean: true or false.

Arithmetic

Lua supports the usual binary operators along with exponentiation, modulus, and unary negation.

Compound Assignment

You can use compound assignment operators to set a variable equal to the result of an operation where the first parameter is the variable's current value.

The operation in a compound assignment occurs once. For example, if an expression generates a random index in a table, Luau uses the same index for both the operation and the assignment.

In the following examples, suppose local x = 3.

Miscellaneous

Miscellaneous operators include concatenation and length.