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 the cyclomatic complexity of methods is not higher than the configured maximum. The cyclomatic complexity is the number of linearly independent paths through a method. The algorithm counts decision points and adds one.

An if statement (or unless or ?:) increases the complexity by one. An else branch does not, since it doesn't add a decision point. The && operator (or keyword and) can be converted to a nested if statement, and ||/or is shorthand for a sequence of ifs, so they also add one. Loops can be said to have an exit condition, so they add one. Blocks that are calls to builtin iteration methods (e.g. `ary.map{...}) also add one, others are ignored.

def each_child_node(*types)               # count begins: 1
  unless block_given?                     # unless: +1
    return to_enum(__method__, *types)

  children.each do |child|                # each{}: +1
    next unless child.is_a?(Node)         # unless: +1

    yield child if types.empty? ||        # if: +1, ||: +1
                   types.include?(child.type)
  end

  self
end                                       # total: 6

Checks for blocks without a body. Such empty blocks are typically an oversight or we should provide a comment be clearer what we're aiming for.

Empty lambdas and procs are ignored by default.

NOTE: For backwards compatibility, the configuration that allows/disallows empty lambdas and procs is called AllowEmptyLambdas, even though it also applies to procs.

Example:

# bad
items.each { |item| }

# good
items.each { |item| puts item }

Example: AllowComments: true (default)

# good
items.each do |item|
  # TODO: implement later (inner comment)
end

items.each { |item| } # TODO: implement later (inline comment)

Example: AllowComments: false

# bad
items.each do |item|
  # TODO: implement later (inner comment)
end

items.each { |item| } # TODO: implement later (inline comment)

Example: AllowEmptyLambdas: true (default)

# good
allow(subject).to receive(:callable).and_return(-> {})

placeholder = lambda do
end
(callable || placeholder).call

proc { }

Proc.new { }

Example: AllowEmptyLambdas: false

# bad
allow(subject).to receive(:callable).and_return(-> {})

placeholder = lambda do
end
(callable || placeholder).call

proc { }

Proc.new { }