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; unlike warnings, they can not be ignored.

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 Setting a read-only field of a global variable.
131 Unused implicitly defined global variable.
142 Setting an undefined field of a global variable.
143 Accessing an undefined field of a global variable.
211 Unused local variable.
212 Unused argument.
213 Unused loop variable.
214 Used 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.
561 Cyclomatic complexity of a function is too high.
571 A numeric for loop goes from #(expr) down to 1 or less without negative step.
581 Negation of a relational operator- operator can be flipped.
582 Error prone negation: negation has a higher priority than equality.
611 A line consists of nothing but whitespace.
612 A line contains trailing whitespace.
613 Trailing whitespace in a string.
614 Trailing whitespace in a comment.
621 Inconsistent indentation (SPACE followed by TAB).
631 Line is too long.

Global variables (1xx)

For each file, Luacheck builds list of defined globals and fields 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 or field is set, mutated or accessed, Luacheck produces a warning.

Read-only globals

By default, most standard globals and fields are marked as read-only, so that setting them produces a warning. Custom read-only globals and fields can be added using --read-globals CLI option or read_globals config option, or using a custom set of globals. See Custom sets of globals

Globals and fields that are not read-only by default:

  • _G
  • _ENV (treated as a global by Luacheck)
  • package.path
  • package.cpath
  • package.loaded
  • package.preload
  • package.loaders
  • package.searchers

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 (2xx) and values (3xx)

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 hint” (214)

If a function argument starts with an underscore _, it recevies an “unused hint”, meaning that it’s intended to be left unused. If it is used, a 214 warning is generated.

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 (4xx)

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 (5xx)

Unreachable code

Luacheck detects unreachable code. It also detects it if end of a loop block is unreachable, which means that the loop can be executed at most once:

for i = 1, 100 do
   -- Break statement is outside the `if` block,
   -- so that the loop always stops after the first iteration.
   if cond(i) then f() end break

Unused labels

Labels that are not used by any goto statements are reported as unused.

Unbalanced assignments

If an assignment has left side and right side with different lengths, the assignment is unbalanced and Luacheck warns about it.

An exception is initializing several local variables in a single statement while leaving some uninitialized:

local a, b, c = nil -- Effectively sets `a`, `b`, and `c` to nil, no warning.

Empty blocks

Luacheck warns about empty do end blocks and empty if branches (then else, then elseif, and then end).

Empty statements

In Lua 5.2+ semicolons are considered statements and can appear even when not following normal statements. Such semicolons produce Luacheck warnings as they are completely useless.

Cyclomatic complexity

If a limit is set using --max-cyclomatic-complexity CLI option or corresponding config or inline options, Luacheck warns about functions with too high cyclomatic complexity.

Reversed numeric for loops

Iterating a table in reverse using a numeric for loop going from #t to 1 requires a negative loop step. Luacheck warns about loops going from #(some expression) to 1 or a smaller constant when the loop step is not negative:

-- Warning for this loop:
-- numeric for loop goes from #(expr) down to 1 but loop step is not negative
for i = #t, 1 do

-- This loop is okay.
for i = #t, 1, -1 do

Error-prone and Unnecessary Negations

Negation has a higher priority than relational operators; (not x == 3) is interpreted as (not x) == 3, rather than not (x == 3).

Negating the output of a relational operator is unnecessary; each one has another operator that can be used directly:

not (x == y) => x ~= y not (x ~= y) => x == y not (x > y) => x <= y not (x >= y) => x < y not (x < y) => x >= y not (x <= y) => x > y

These replacements work for all numbers, but can fail with metatables or NaN’s.

Formatting issues (6xx)

Whitespace issues

Luacheck warns about trailing whitespace and inconsistent indentation (SPACE followed by TAB).

Some examples of trailing whitespace Luacheck finds:

-- Whitespace example.



  • Any tabs or spaces after either ) would be considered trailing.
  • Any tabs or spaces after the . in the comment would be considered trailing
  • Any tabs or spaces on the empty line between the two print statements would also be considered a form of trailing whitespace.

Trailing whitespace in any of these forms is useless, can be a nuisance to developers navigating around a file, and is forbidden in many formatting styles.

Line length limits

Luacheck warns about lines that are longer then some limit. Default limit is 120 characters. It’s possible to change this limit using --max-line-length CLI option or disable the check completely with --no-max-line-length; there are similar config and inline options.

Additionally, separate limits can be set for three different type of lines:

  • “String” lines have their line ending inside a string, typically a long string using [[...]] syntax.
  • “Comment” lines have their line ending inside a long comment (--[[...]]), or end with a short comment using normal --... syntax.
  • “Code” lines are all other lines.

These types of lines are limited using CLI options named --[no-]max-string-line-length, --[no-]max-comment-line-length, and --[no-]max-code-line-length, with similar config and inline options.