This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks that the ABC size of methods is not higher than the configured maximum. The ABC size is based on assignments, branches (method calls), and conditions. See http://c2.com/cgi/wiki?AbcMetric

Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

This cop makes sure that certain binary operator methods have their sole parameter named other.

Example:

# bad
def +(amount); end

# good
def +(other); end

This cop checks for comments put on the same line as some keywords. These keywords are: begin, class, def, end, module.

Note that some comments (such as :nodoc: and rubocop:disable) are allowed.

Example:

# bad
if condition
  statement
end # end if

# bad
class X # comment
  statement
end

# bad
def x; end # comment

# good
if condition
  statement
end

# good
class X # :nodoc:
  y
end

This cops checks if empty lines around the bodies of modules match the configuration.

Example: EnforcedStyle: empty_lines

# good

module Foo

  def bar
    # ...
  end

end

Example: EnforcedStyle: emptylinesexcept_namespace

# good

module Foo
  module Bar

    # ...

  end
end

Example: EnforcedStyle: emptylinesspecial

# good
module Foo

  def bar; end

end

Example: EnforcedStyle: noemptylines (default)

# good

module Foo
  def bar
    # ...
  end
end

This cop checks for redundant return expressions.

Example:

def test
  return something
end

def test
  one
  two
  three
  return something
end

It should be extended to handle methods whose body is if/else or a case expression with a default branch.

Check for uses of braces or do/end around single line or multi-line blocks.

Example: EnforcedStyle: linecountbased (default)

# bad - single line block
items.each do |item| item / 5 end

# good - single line block
items.each { |item| item / 5 }

# bad - multi-line block
things.map { |thing|
  something = thing.some_method
  process(something)
}

# good - multi-line block
things.map do |thing|
  something = thing.some_method
  process(something)
end

Example: EnforcedStyle: semantic

# Prefer `do...end` over `{...}` for procedural blocks.

# return value is used/assigned
# bad
foo = map do |x|
  x
end
puts (map do |x|
  x
end)

# return value is not used out of scope
# good
map do |x|
  x
end

# Prefer `{...}` over `do...end` for functional blocks.

# return value is not used out of scope
# bad
each { |x|
  x
}

# return value is used/assigned
# good
foo = map { |x|
  x
}
map { |x|
  x
}.inspect

Example: EnforcedStyle: bracesforchaining

# bad
words.each do |word|
  word.flip.flop
end.join("-")

# good
words.each { |word|
  word.flip.flop
}.join("-")

This cop checks for redundant return expressions.

Example:

def test
  return something
end

def test
  one
  two
  three
  return something
end

It should be extended to handle methods whose body is if/else or a case expression with a default branch.

This cop checks for redundant uses of self.

The usage of self is only needed when:

• Sending a message to same object with zero arguments in presence of a method name clash with an argument or a local variable.

• Calling an attribute writer to prevent an local variable assignment.

Note, with using explicit self you can only send messages with public or protected scope, you cannot send private messages this way.

Note we allow uses of self with operators because it would be awkward otherwise.

Example:

# bad
def foo(bar)
  self.baz
end

# good
def foo(bar)
  self.bar  # Resolves name clash with the argument.
end

def foo
  bar = 1
  self.bar  # Resolves name clash with the local variable.
end

def foo
  %w[x y z].select do |bar|
    self.bar == bar  # Resolves name clash with argument of the block.
  end
end

This cop checks for usage of comparison operators (==, >, <) to test numbers as zero, positive, or negative. These can be replaced by their respective predicate methods. The cop can also be configured to do the reverse.

The cop disregards #nonzero? as it its value is truthy or falsey, but not true and false, and thus not always interchangeable with != 0.

The cop ignores comparisons to global variables, since they are often populated with objects which can be compared with integers, but are not themselves Interger polymorphic.

Example: EnforcedStyle: predicate (default)

# bad

foo == 0
0 > foo
bar.baz > 0

# good

foo.zero?
foo.negative?
bar.baz.positive?

Example: EnforcedStyle: comparison

# bad

foo.zero?
foo.negative?
bar.baz.positive?

# good

foo == 0
0 > foo
bar.baz > 0

This cop checks for redundant uses of self.

The usage of self is only needed when:

• Sending a message to same object with zero arguments in presence of a method name clash with an argument or a local variable.

• Calling an attribute writer to prevent an local variable assignment.

Note, with using explicit self you can only send messages with public or protected scope, you cannot send private messages this way.

Note we allow uses of self with operators because it would be awkward otherwise.

Example:

# bad
def foo(bar)
  self.baz
end

# good
def foo(bar)
  self.bar  # Resolves name clash with the argument.
end

def foo
  bar = 1
  self.bar  # Resolves name clash with the local variable.
end

def foo
  %w[x y z].select do |bar|
    self.bar == bar  # Resolves name clash with argument of the block.
  end
end

This cop enforces the use the shorthand for self-assignment.

Example:

# bad
x = x + 1

# good
x += 1

This cop checks for missing top-level documentation of classes and modules. Classes with no body are exempt from the check and so are namespace modules - modules that have nothing in their bodies except classes, other modules, or constant definitions.

The documentation requirement is annulled if the class or module has a "#:nodoc:" comment next to it. Likewise, "#:nodoc: all" does the same for all its children.

Example:

# bad
class Person
  # ...
end

# good
# Description/Explanation of Person class
class Person
  # ...
end

This cop checks that quotes inside the string interpolation match the configured preference.

Example: EnforcedStyle: single_quotes (default)

# bad
result = "Tests #{success ? "PASS" : "FAIL"}"

# good
result = "Tests #{success ? 'PASS' : 'FAIL'}"

Example: EnforcedStyle: double_quotes

# bad
result = "Tests #{success ? 'PASS' : 'FAIL'}"

# good
result = "Tests #{success ? "PASS" : "FAIL"}"

This cop checks for extra/unnecessary whitespace.

Example:

# good if AllowForAlignment is true
name      = "RuboCop"
# Some comment and an empty line

website  += "/bbatsov/rubocop" unless cond
puts        "rubocop"          if     debug

# bad for any configuration
set_app("RuboCop")
website  = "https://github.com/bbatsov/rubocop"

Checks that operators have space around them, except for ** which should not have surrounding space.

Example:

# bad
total = 3*4
"apple"+"juice"
my_number = 38/4
a ** b

# good
total = 3 * 4
"apple" + "juice"
my_number = 38 / 4
a**b

This cop checks for non-ascii (non-English) characters in comments. You could set an array of allowed non-ascii chars in AllowedChars attribute (empty by default).

Example:

# bad
# Translates from English to 日本語。

# good
# Translates from English to Japanese

This cop checks that quotes inside the string interpolation match the configured preference.

Example: EnforcedStyle: single_quotes (default)

# bad
result = "Tests #{success ? "PASS" : "FAIL"}"

# good
result = "Tests #{success ? 'PASS' : 'FAIL'}"

Example: EnforcedStyle: double_quotes

# bad
result = "Tests #{success ? 'PASS' : 'FAIL'}"

# good
result = "Tests #{success ? "PASS" : "FAIL"}"

This cop checks for every useless assignment to local variable in every scope. The basic idea for this cop was from the warning of ruby -cw:

assigned but unused variable - foo

Currently this cop has advanced logic that detects unreferenced reassignments and properly handles varied cases such as branch, loop, rescue, ensure, etc.

Example:

# bad

def some_method
  some_var = 1
  do_something
end

Example:

# good

def some_method
  some_var = 1
  do_something(some_var)
end

This cop checks for numeric comparisons that can be replaced by a predicate method, such as receiver.length == 0, receiver.length > 0, receiver.length != 0, receiver.length < 1 and receiver.size == 0 that can be replaced by receiver.empty? and !receiver.empty.

Example:

# bad
[1, 2, 3].length == 0
0 == "foobar".length
array.length < 1
{a: 1, b: 2}.length != 0
string.length > 0
hash.size > 0

# good
[1, 2, 3].empty?
"foobar".empty?
array.empty?
!{a: 1, b: 2}.empty?
!string.empty?
!hash.empty?