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

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

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 unused method arguments.

Example:

# bad

def some_method(used, unused, _unused_but_allowed)
  puts used
end

Example:

# good

def some_method(used, _unused, _unused_but_allowed)
  puts used
end

This cops checks the indentation of the first key in a hash literal where the opening brace and the first key are on separate lines. The other keys' indentations are handled by the AlignHash cop.

By default, Hash literals that are arguments in a method call with parentheses, and where the opening curly brace of the hash is on the same line as the opening parenthesis of the method call, shall have their first key indented one step (two spaces) more than the position inside the opening parenthesis.

Other hash literals shall have their first key indented one step more than the start of the line where the opening curly brace is.

This default style is called 'specialinsideparentheses'. Alternative styles are 'consistent' and 'align_braces'. Here are examples:

Example: EnforcedStyle: specialinsideparentheses (default)

# The `special_inside_parentheses` style enforces that the first key
# in a hash literal where the opening brace and the first key are on
# separate lines is indented one step (two spaces) more than the
# position inside the opening parentheses.

# bad
hash = {
  key: :value
}
and_in_a_method_call({
  no: :difference
                     })

# good
special_inside_parentheses
hash = {
  key: :value
}
but_in_a_method_call({
                       its_like: :this
                     })

Example: EnforcedStyle: consistent

# The `consistent` style enforces that the first key in a hash
# literal where the opening brace and the first key are on
# seprate lines is indented the same as a hash literal which is not
# defined inside a method call.

# bad
hash = {
  key: :value
}
but_in_a_method_call({
                       its_like: :this
                      })

# good
hash = {
  key: :value
}
and_in_a_method_call({
  no: :difference
})

Example: EnforcedStyle: align_braces

# The `align_brackets` style enforces that the opening and closing
# braces are indented to the same position.

# bad
and_now_for_something = {
                          completely: :different
}

# good
and_now_for_something = {
                          completely: :different
                        }

This cops checks the indentation of the first key in a hash literal where the opening brace and the first key are on separate lines. The other keys' indentations are handled by the AlignHash cop.

By default, Hash literals that are arguments in a method call with parentheses, and where the opening curly brace of the hash is on the same line as the opening parenthesis of the method call, shall have their first key indented one step (two spaces) more than the position inside the opening parenthesis.

Other hash literals shall have their first key indented one step more than the start of the line where the opening curly brace is.

This default style is called 'specialinsideparentheses'. Alternative styles are 'consistent' and 'align_braces'. Here are examples:

Example: EnforcedStyle: specialinsideparentheses (default)

# The `special_inside_parentheses` style enforces that the first key
# in a hash literal where the opening brace and the first key are on
# separate lines is indented one step (two spaces) more than the
# position inside the opening parentheses.

# bad
hash = {
  key: :value
}
and_in_a_method_call({
  no: :difference
                     })

# good
special_inside_parentheses
hash = {
  key: :value
}
but_in_a_method_call({
                       its_like: :this
                     })

Example: EnforcedStyle: consistent

# The `consistent` style enforces that the first key in a hash
# literal where the opening brace and the first key are on
# seprate lines is indented the same as a hash literal which is not
# defined inside a method call.

# bad
hash = {
  key: :value
}
but_in_a_method_call({
                       its_like: :this
                      })

# good
hash = {
  key: :value
}
and_in_a_method_call({
  no: :difference
})

Example: EnforcedStyle: align_braces

# The `align_brackets` style enforces that the opening and closing
# braces are indented to the same position.

# bad
and_now_for_something = {
                          completely: :different
}

# good
and_now_for_something = {
                          completely: :different
                        }

This cops checks for indentation that doesn't use the specified number of spaces.

See also the IndentationConsistency cop which is the companion to this one.

Example:

# bad
class A
 def test
  puts 'hello'
 end
end

# good
class A
  def test
    puts 'hello'
  end
end

Example: IgnoredPatterns: ['^\s*module']

# bad
module A
class B
  def test
  puts 'hello'
  end
end
end

# good
module A
class B
  def test
    puts 'hello'
  end
end
end

Use next to skip iteration instead of a condition at the end.

Example: EnforcedStyle: skipmodifierifs (default)

# bad
[1, 2].each do |a|
  if a == 1
    puts a
  end
end

# good
[1, 2].each do |a|
  next unless a == 1
  puts a
end

# good
[1, 2].each do |o|
  puts o unless o == 1
end

Example: EnforcedStyle: always

# With `always` all conditions at the end of an iteration needs to be
# replaced by next - with `skip_modifier_ifs` the modifier if like
# this one are ignored: `[1, 2].each { |a| return 'yes' if a == 1 }`

# bad
[1, 2].each do |o|
  puts o unless o == 1
end

# bad
[1, 2].each do |a|
  if a == 1
    puts a
  end
end

# good
[1, 2].each do |a|
  next unless a == 1
  puts a
end

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