debug.getupvalue

Gets an upvalue from a function by index.

Syntax

debug.getupvalue(func: function | number, index: number) -> any

Parameters

Parameter	Type	Description
`func`	`function` or `number`	The function or stack level
`index`	`number`	The upvalue index (1-based)

Returns

Type	Description
`any`	The upvalue at the specified index

Description

debug.getupvalue retrieves an upvalue (captured variable from an outer scope) from a function. Upvalues are variables that a closure "closes over" from its enclosing environment.

Example

local counter = 0
local prefix = "Count: "

local function increment()
    counter = counter + 1
    return prefix .. counter
end

-- Upvalue order is compiler-dependent
for index in ipairs(debug.getupvalues(increment)) do
    print(index, debug.getupvalue(increment, index))
end

-- After calling the function
increment()

Use Cases

Inspect closures: See what variables a function has captured
Debugging: Examine the state of captured variables
Modification: Read before modifying with setupvalue

debug.getupvalues - Get all upvalues
debug.setupvalue - Modify an upvalue