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 checks for trailing comma in array and hash literals.

Example: EnforcedStyleForMultiline: consistent_comma

# bad
a = [1, 2,]

# good
a = [
  1, 2,
  3,
]

# good
a = [
  1,
  2,
]

Example: EnforcedStyleForMultiline: comma

# bad
a = [1, 2,]

# good
a = [
  1,
  2,
]

Example: EnforcedStyleForMultiline: no_comma (default)

# bad
a = [1, 2,]

# good
a = [
  1,
  2
]

Checks for unnecessary require statement.

The following features are unnecessary require statement because they are already loaded.

ruby -ve 'p $LOADED_FEATURES.reject { |feature| %r|/| =~ feature }' ruby 2.2.8p477 (2017-09-14 revision 59906) [x86_64-darwin13] ["enumerator.so", "rational.so", "complex.so", "thread.rb"]

This cop targets Ruby 2.2 or higher containing these 4 features.

Example:

# bad
require 'unloaded_feature'
require 'thread'

# good
require 'unloaded_feature'

This cop checks for rescue blocks targeting the Exception class.

Example:

# bad

begin
  do_something
rescue Exception
  handle_exception
end

Example:

# good

begin
  do_something
rescue ArgumentError
  handle_exception
end

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("-")