Evaluating a function that returns nothing

Question

function foo() end

if foo() then end

I saw some code that did this and wondered: What is being evaluated by the if statement when foo() does not return anything.

Tried it in an interpreter and it evaluates to false, but is that a guarantee? Is it evaluating the last thing returned somewhere else?

Answer 1

If control reaches the end of a function without encountering a return statement, the function implicitly returns nothing. As the syntax for if requires a single expression, the zero-length multires expression of nothing is adjusted to a single nil value.

In a control structure (if, while, repeat), a condition expression equal to nil tests false (conversely, not nil is true).

See Lua 5.4:

• 2.1 – Values and Types
• 3.3.4 – Control Structures
• 3.4.11 – Function Definitions
• 3.4.12 – Lists of expressions, multiple results, and adjustment