Closures and Upvalues

Lua functions can reference variables from an enclosing scope even after that scope has returned, this is a closure. The captured variable is called an upvalue.

A closure example

local function makeCounter()
  local count = 0
  return function()
    count = count + 1
    return count
  end
end

local counter1 = makeCounter()
local counter2 = makeCounter()

print(counter1()) -- 1
print(counter1()) -- 2
print(counter2()) -- 1, a separate count upvalue from counter1's

Each call to makeCounter creates a fresh count variable, the inner function closes over that specific instance, not a shared global. counter1 and counter2 don't interfere with each other.

Why this matters

Closures are how you build private state without a class: count isn't reachable from outside the returned function, there's no way to read or reset it except by calling counter1() again. This pattern shows up constantly, for example a memoized/cached computation.

local function memoize(fn)
  local cache = {}
  return function(n)
    if cache[n] == nil then
      cache[n] = fn(n)
    end
    return cache[n]
  end
end

local slowSquare = function(n) return n * n end
local fastSquare = memoize(slowSquare)

print(fastSquare(5)) -- computes and caches
print(fastSquare(5)) -- returns the cached value