Functions and Scope

Functions in Lua are values, they can be stored in variables, passed as arguments, and returned from other functions, just like tables or numbers.

Defining and calling functions

Lua

local function add(a, b)
  return a + b
end

print(add(2, 3)) -- 5

-- Multiple return values
local function minMax(numbers)
  local lo, hi = numbers[1], numbers[1]
  for _, n in ipairs(numbers) do
    if n < lo then lo = n end
    if n > hi then hi = n end
  end
  return lo, hi
end

local lowest, highest = minMax({4, 1, 9, 2})
print(lowest, highest) -- 1  9

local function add(a, b)
  return a + b
end

print(add(2, 3)) -- 5

-- Multiple return values
local function minMax(numbers)
  local lo, hi = numbers[1], numbers[1]
  for _, n in ipairs(numbers) do
    if n < lo then lo = n end
    if n > hi then hi = n end
  end
  return lo, hi
end

local lowest, highest = minMax({4, 1, 9, 2})
print(lowest, highest) -- 1  9

Lua functions can return multiple values at once, something many other languages need a tuple or object to fake.

Scope with local vs. global

Lua

xp = 100        -- global! visible everywhere, including other files

local function levelUp()
  local bonus = 10 -- local: only visible inside this function
  xp = xp + bonus
end

levelUp()
print(xp) -- 110
print(bonus) -- nil, bonus does not exist outside levelUp

xp = 100        -- global! visible everywhere, including other files

local function levelUp()
  local bonus = 10 -- local: only visible inside this function
  xp = xp + bonus
end

levelUp()
print(xp) -- 110
print(bonus) -- nil, bonus does not exist outside levelUp

WARNING

Forgetting local silently creates a global variable, visible to your entire program (and, if you're embedded in a game or app, potentially to other scripts too). Get in the habit of typing local in front of every variable and function unless you deliberately want it global.

Functions as arguments

Lua

local function forEach(list, callback)
  for _, item in ipairs(list) do
    callback(item)
  end
end

forEach({"a", "b", "c"}, function(item)
  print("Item: " .. item)
end)

local function forEach(list, callback)
  for _, item in ipairs(list) do
    callback(item)
  end
end

forEach({"a", "b", "c"}, function(item)
  print("Item: " .. item)
end)

Passing an anonymous function(...) ... end directly as an argument is idiomatic Lua, this is exactly how callback-style APIs (like button click handlers in game engines) are typically used.

Functions and Scope

Defining and calling functions

Scope with local vs. global

Functions as arguments

📝 Functions & Scope Quiz