List of warnings

Warnings produced by Luacheck are categorized using three-digit warning codes. Warning codes can be displayed in CLI output using --codes CLI option or codes config option. Errors also have codes starting with zero.

Code Description
011 A syntax error.
021 An invalid inline option.
022 An unpaired inline push directive.
023 An unpaired inline pop directive.
111 Setting an undefined global variable.
112 Mutating an undefined global variable.
113 Accessing an undefined global variable.
121 Setting a read-only global variable.
122 Mutating a read-only global variable.
131 Unused implicitly defined global variable.
211 Unused local variable.
212 Unused argument.
213 Unused loop variable.
221 Local variable is accessed but never set.
231 Local variable is set but never accessed.
232 An argument is set but never accessed.
233 Loop variable is set but never accessed.
241 Local variable is mutated but never accessed.
311 Value assigned to a local variable is unused.
312 Value of an argument is unused.
313 Value of a loop variable is unused.
314 Value of a field in a table literal is unused.
321 Accessing uninitialized local variable.
331 Value assigned to a local variable is mutated but never accessed.
341 Mutating uninitialized local variable.
411 Redefining a local variable.
412 Redefining an argument.
413 Redefining a loop variable.
421 Shadowing a local variable.
422 Shadowing an argument.
423 Shadowing a loop variable.
431 Shadowing an upvalue.
432 Shadowing an upvalue argument.
433 Shadowing an upvalue loop variable.
511 Unreachable code.
512 Loop can be executed at most once.
521 Unused label.
531 Left-hand side of an assignment is too short.
532 Left-hand side of an assignment is too long.
541 An empty do end block.
542 An empty if branch.
551 An empty statement.
611 A line consists of nothing but whitespace.
612 A line contains trailing whitespace.
621 Inconsistent indentation (SPACE followed by TAB).

Global variables

For each file, Luacheck builds list of defined globals which can be used there. By default only globals from Lua standard library are defined; custom globals can be added using --globals CLI option or globals config option, and version of standard library can be selected using --std CLI option or std config option. When an undefined global is set, mutated or accessed, Luacheck produces a warning.

Read-only globals

By default, all standard globals except _G and package are marked as read-only, so that setting or mutating them produces a warning. Custom read-only globals can be added using --read-globals CLI option or read_globals config option.

Implicitly defined globals

Luacheck can be configured to consider globals assigned under some conditions to be defined implicitly. When -d/--allow_defined CLI option or allow_defined config option is used, all assignments to globals define them; when -t/--allow_defined_top CLI option or allow_defined_top config option is used, assignments to globals in the top level function scope (also known as main chunk) define them. A warning is produced when an implicitly defined global is not accessed anywhere.


Files can be marked as modules using -m/--module CLI option or module config option to simulate semantics of the deprecated module function. Globals implicitly defined inside a module are considired part of its interface, are not visible outside and are not reported as unused. Assignments to other globals are not allowed, even to defined ones.

Unused variables and values

Luacheck generates warnings for all unused local variables except one named _. It also detects variables which are set but never accessed or accessed but never set.

Unused values and uninitialized variables

For each value assigned to a local variable, Luacheck computes set of expressions where it could be used. Warnings are produced for unused values (when a value can’t be used anywhere) and for accessing uninitialized variables (when no values can reach an expression). E.g. in the following snippet value assigned to foo on line 1 is unused, and variable bar is uninitialized on line 9:

local foo = expr1()
local bar

if condition() then
   foo = expr2()
   bar = expr3()
   foo = expr4()

return foo, bar

Secondary values and variables

Unused value assigned to a local variable is secondary if its origin is the last item on the RHS of assignment, and another value from that item is used. Secondary values typically appear when result of a function call is put into locals, and only some of them are later used. For example, here value assigned to b is secondary, value assigned to c is used, and value assigned to a is simply unused:

local a, b, c = f(), g()

return c

A variable is secondary if all values assigned to it are secondary. In the snippet above, b is a secondary variable.

Warnings related to unused secondary values and variables can be removed using -s/--no-unused-secondaries CLI option or unused_secondaries config option.

Shadowing declarations

Luacheck detects declarations of local variables shadowing previous declarations, unless the variable is named _. If the previous declaration is in the same scope as the new one, it is called redefining.

Note that it is not necessary to define a new local variable when overwriting an argument:

local function f(x)
   local x = x or "default" -- bad

local function f(x)
   x = x or "default" -- good

Control flow and data flow issues

The following control flow and data flow issues are detected:

  • Unreachable code and loops that can be executed at most once (e.g. due to an unconditional break);
  • Unused labels;
  • Unbalanced assignments;
  • Empty blocks.
  • Empty statements (semicolons without preceding statements).

Whitespace issues

Luacheck detects some common whitespace issues, such as trailing whitespace.