This is Airbnb's Ruby Style Guide.

It was inspired by GitHub's guide and RuboCop's guide.

Airbnb also maintains a JavaScript Style Guide.

1. Whitespace
   1. Indentation
   2. Inline
   3. Newlines
2. Line Length
3. Commenting
   1. File/class-level comments
   2. Function comments
   3. Block and inline comments
   4. Punctuation, spelling, and grammar
   5. TODO comments
   6. Commented-out code
4. Methods
   1. Method definitions
   2. Method calls
5. Conditional Expressions
   1. Conditional keywords
   2. Ternary operator
6. Syntax
7. Naming
8. Classes
9. Exceptions
10. Collections
11. Strings
12. Regular Expressions
13. Percent Literals
14. Rails
    1. Scopes
15. Be Consistent
16. Translation

• Use soft-tabs with a two-space indent.^[link]

• Indent when as deep as case. ^[link]

  case
  when song.name == 'Misty'
    puts 'Not again!'
  when song.duration > 120
    puts 'Too long!'
  when Time.now.hour > 21
    puts "It's too late"
  else
    song.play
  end
  
  kind = case year
         when 1850..1889 then 'Blues'
         when 1890..1909 then 'Ragtime'
         when 1910..1929 then 'New Orleans Jazz'
         when 1930..1939 then 'Swing'
         when 1940..1950 then 'Bebop'
         else 'Jazz'
         end

• Align function parameters either all on the same line or one per line.^[link]

  # bad
  def self.create_translation(phrase_id, phrase_key, target_locale,
                              value, user_id, do_xss_check, allow_verification)
    ...
  end
  
  # good
  def self.create_translation(phrase_id,
                              phrase_key,
                              target_locale,
                              value,
                              user_id,
                              do_xss_check,
                              allow_verification)
    ...
  end
  
  # good
  def self.create_translation(
    phrase_id,
    phrase_key,
    target_locale,
    value,
    user_id,
    do_xss_check,
    allow_verification
  )
    ...
  end

• Indent succeeding lines in multi-line boolean expressions.^[link]

  # bad
  def is_eligible?(user)
    Trebuchet.current.launch?(ProgramEligibilityHelper::PROGRAM_TREBUCHET_FLAG) &&
    is_in_program?(user) &&
    program_not_expired
  end
  
  # good
  def is_eligible?(user)
    Trebuchet.current.launch?(ProgramEligibilityHelper::PROGRAM_TREBUCHET_FLAG) &&
      is_in_program?(user) &&
      program_not_expired
  end

• Never leave trailing whitespace. ^[link]

• When making inline comments, include a space between the end of the code and the start of your comment. ^[link]

  # bad
  result = func(a, b)# we might want to change b to c
  
  # good
  result = func(a, b) # we might want to change b to c

• Use spaces around operators; after commas, colons, and semicolons; and around { and before }. ^[link]

  sum = 1 + 2
  a, b = 1, 2
  1 > 2 ? true : false; puts 'Hi'
  [1, 2, 3].each { |e| puts e }

• Never include a space before a comma. ^[link]

  result = func(a, b)

• Do not include space inside block parameter pipes. Include one space between parameters in a block. Include one space outside block parameter pipes. ^[link]

  # bad
  {}.each { | x,  y |puts x }
  
  # good
  {}.each { |x, y| puts x }

• Do not leave space between ! and its argument.^[link]

  !something

• No spaces after (, [ or before ], ). ^[link]

  some(arg).other
  [1, 2, 3].length

• Omit whitespace when doing string interpolation.^[link]

  # bad
  var = "This #{ foobar } is interpolated."
  
  # good
  var = "This #{foobar} is interpolated."

• Don't use extra whitespace in range literals.^[link]

  # bad
  (0 ... coll).each do |item|
  
  # good
  (0...coll).each do |item|

• Add a new line after if conditions spanning multiple lines to help differentiate between the conditions and the body. ^[link]

  if @reservation_alteration.checkin == @reservation.start_date &&
     @reservation_alteration.checkout == (@reservation.start_date + @reservation.nights)
  
    redirect_to_alteration @reservation_alteration
  end

• Add a new line after conditionals, blocks, case statements, etc.^[link]

  if robot.is_awesome?
    send_robot_present
  end
  
  robot.add_trait(:human_like_intelligence)

• Don’t include newlines between areas of different indentation (such as around class or module bodies). ^[link]

  # bad
  class Foo
  
    def bar
      # body omitted
    end
  
  end
  
  # good
  class Foo
    def bar
      # body omitted
    end
  end

• Include one, but no more than one, new line between methods.^[link]

  def a
  end
  
  def b
  end

• Use a single empty line to break between statements to break up methods into logical paragraphs internally. ^[link]

  def transformorize_car
    car = manufacture(options)
    t = transformer(robot, disguise)
  
    car.after_market_mod!
    t.transform(car)
    car.assign_cool_name!
  
    fleet.add(car)
    car
  end

• End each file with a newline. Don't include multiple newlines at the end of a file. ^[link]

• Keep each line of code to a readable length. Unless you have a reason not to, keep lines to fewer than 100 characters. (rationale) ^[link]

Though a pain to write, comments are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are very important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names that you must then explain through comments.

When writing your comments, write for your audience: the next contributor who will need to understand your code. Be generous — the next one may be you!

—Google C++ Style Guide

Portions of this section borrow heavily from the Google C++ and Python style guides.

Every class definition should have an accompanying comment that describes what it is for and how it should be used.

A file that contains zero classes or more than one class should have a comment at the top describing its contents.

# Automatic conversion of one locale to another where it is possible, like
# American to British English.
module Translation
  # Class for converting between text between similar locales.
  # Right now only conversion between American English -> British, Canadian,
  # Australian, New Zealand variations is provided.
  class PrimAndProper
    def initialize
      @converters = { :en => { :"en-AU" => AmericanToAustralian.new,
                               :"en-CA" => AmericanToCanadian.new,
                               :"en-GB" => AmericanToBritish.new,
                               :"en-NZ" => AmericanToKiwi.new,
                             } }
    end

  ...

  # Applies transforms to American English that are common to
  # variants of all other English colonies.
  class AmericanToColonial
    ...
  end

  # Converts American to British English.
  # In addition to general Colonial English variations, changes "apartment"
  # to "flat".
  class AmericanToBritish < AmericanToColonial
    ...
  end

All files, including data and config files, should have file-level comments.

# List of American-to-British spelling variants.
#
# This list is made with
# lib/tasks/list_american_to_british_spelling_variants.rake.
#
# It contains words with general spelling variation patterns:
#   [trave]led/lled, [real]ize/ise, [flav]or/our, [cent]er/re, plus
# and these extras:
#   learned/learnt, practices/practises, airplane/aeroplane, ...

sectarianizes: sectarianises
neutralization: neutralisation
...

Every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments should be descriptive ("Opens the file") rather than imperative ("Open the file"); the comment describes the function, it does not tell the function what to do. In general, these comments do not describe how the function performs its task. Instead, that should be left to comments interspersed in the function's code.

Every function should mention what the inputs and outputs are, unless it meets all of the following criteria:

• not externally visible
• very short
• obvious

You may use whatever format you wish. In Ruby, two popular function documentation schemes are TomDoc and YARD. You can also just write things out concisely:

# Returns the fallback locales for the_locale.
# If opts[:exclude_default] is set, the default locale, which is otherwise
# always the last one in the returned list, will be excluded.
#
# For example:
#   fallbacks_for(:"pt-BR")
#     => [:"pt-BR", :pt, :en]
#   fallbacks_for(:"pt-BR", :exclude_default => true)
#     => [:"pt-BR", :pt]
def fallbacks_for(the_locale, opts = {})
  ...
end

The final place to have comments is in tricky parts of the code. If you're going to have to explain it at the next code review, you should comment it now. Complicated operations get a few lines of comments before the operations commence. Non-obvious ones get comments at the end of the line.

def fallbacks_for(the_locale, opts = {})
  # dup() to produce an array that we can mutate.
  ret = @fallbacks[the_locale].dup

  # We make two assumptions here:
  # 1) There is only one default locale (that is, it has no less-specific
  #    children).
  # 2) The default locale is just a language. (Like :en, and not :"en-US".)
  if opts[:exclude_default] &&
      ret.last == default_locale &&
      ret.last != language_from_locale(the_locale)
    ret.pop
  end

  ret
end

On the other hand, never describe the code. Assume the person reading the code knows the language (though not what you're trying to do) better than you do.

Related: do not use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments. ^[link]

# bad
=begin
comment line
another comment line
=end

# good
# comment line
# another comment line

Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.

Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.

Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.

Use TODO comments for code that is temporary, a short-term solution, or good-enough but not perfect.

TODOs should include the string TODO in all caps, followed by the full name of the person who can best provide context about the problem referenced by the TODO, in parentheses. A colon is optional. A comment explaining what there is to do is required. The main purpose is to have a consistent TODO format that can be searched to find the person who can provide more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO, it is almost always your name that is given.

  # bad
  # TODO(RS): Use proper namespacing for this constant.

  # bad
  # TODO(drumm3rz4lyfe): Use proper namespacing for this constant.

  # good
  # TODO(Ringo Starr): Use proper namespacing for this constant.

• Never leave commented-out code in our codebase. ^[link]

• Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters.^[link]

  def some_method
    # body omitted
  end
  
  def some_method_with_parameters(arg1, arg2)
    # body omitted
  end

• Do not use default positional arguments. Use keyword arguments (if available - in Ruby 2.0 or later) or an options hash instead.^[link]

  # bad
  def obliterate(things, gently = true, except = [], at = Time.now)
    ...
  end
  
  # good
  def obliterate(things, gently: true, except: [], at: Time.now)
    ...
  end
  
  # good
  def obliterate(things, options = {})
    options = {
      :gently => true, # obliterate with soft-delete
      :except => [], # skip obliterating these things
      :at => Time.now, # don't obliterate them until later
    }.merge(options)
  
    ...
  end

• Avoid single-line methods. Although they are somewhat popular in the wild, there are a few peculiarities about their definition syntax that make their use undesirable. ^[link]

  # bad
  def too_much; something; something_else; end
  
  # good
  def some_method
    # body
  end

Use parentheses for a method call:

• If the method returns a value. ^[link]

  # bad
  @current_user = User.find_by_id 1964192
  
  # good
  @current_user = User.find_by_id(1964192)

• If the first argument to the method uses parentheses.^[link]

  # bad
  put! (x + y) % len, value
  
  # good
  put!((x + y) % len, value)

• Never put a space between a method name and the opening parenthesis.^[link]

  # bad
  f (3 + 2) + 1
  
  # good
  f(3 + 2) + 1

• Omit parentheses for a method call if the method accepts no arguments.^[link]

  # bad
  nil?()
  
  # good
  nil?

• If the method doesn't return a value (or we don't care about the return), parentheses are optional. (Especially if the arguments overflow to multiple lines, parentheses may add readability.) ^[link]

  # okay
  render(:partial => 'foo')
  
  # okay
  render :partial => 'foo'

In either case:

• If a method accepts an options hash as the last argument, do not use { } during invocation. ^[link]

  # bad
  get '/v1/reservations', { :id => 54875 }
  
  # good
  get '/v1/reservations', :id => 54875

• Never use then for multi-line if/unless. ^[link]

  # bad
  if some_condition then
    ...
  end
  
  # good
  if some_condition
    ...
  end

• Never use do for multi-line while or until.^[link]

  # bad
  while x > 5 do
    ...
  end
  
  until x > 5 do
    ...
  end
  
  # good
  while x > 5
    ...
  end
  
  until x > 5
    ...
  end

• The and, or, and not keywords are banned. It's just not worth it. Always use &&, ||, and ! instead. ^[link]

• Modifier if/unless usage is okay when the body is simple, the condition is simple, and the whole thing fits on one line. Otherwise, avoid modifier if/unless. ^[link]

  # bad - this doesn't fit on one line
  add_trebuchet_experiments_on_page(request_opts[:trebuchet_experiments_on_page]) if request_opts[:trebuchet_experiments_on_page] && !request_opts[:trebuchet_experiments_on_page].empty?
  
  # okay
  if request_opts[:trebuchet_experiments_on_page] &&
       !request_opts[:trebuchet_experiments_on_page].empty?
  
    add_trebuchet_experiments_on_page(request_opts[:trebuchet_experiments_on_page])
  end
  
  # bad - this is complex and deserves multiple lines and a comment
  parts[i] = part.to_i(INTEGER_BASE) if !part.nil? && [0, 2, 3].include?(i)
  
  # okay
  return if reconciled?

• Never use unless with else. Rewrite these with the positive case first.^[link]

  # bad
  unless success?
    puts 'failure'
  else
    puts 'success'
  end
  
  # good
  if success?
    puts 'success'
  else
    puts 'failure'
  end

• Avoid unless with multiple conditions.^[link]

    # bad
    unless foo? && bar?
      ...
    end
  
    # okay
    if !(foo? && bar?)
      ...
    end

• Avoid unless with comparison operators if you can use if with an opposing comparison operator.^[link]

    # bad
    unless x == 10
      ...
    end
  
    # good
    if x != 10
      ...
    end
  
    # bad
    unless x < 10
      ...
    end
  
    # good
    if x >= 10
      ...
    end
  
    # ok
    unless x === 10
      ...
    end

• Don't use parentheses around the condition of an if/unless/while. ^[link]

  # bad
  if (x > 10)
    ...
  end
  
  # good
  if x > 10
    ...
  end

• Avoid the ternary operator (?:) except in cases where all expressions are extremely trivial. However, do use the ternary operator(?:) over if/then/else/end constructs for single line conditionals.^[link]

  # bad
  result = if some_condition then something else something_else end
  
  # good
  result = some_condition ? something : something_else

• Use one expression per branch in a ternary operator. This also means that ternary operators must not be nested. Prefer if/else constructs in these cases.^[link]

  # bad
  some_condition ? (nested_condition ? nested_something : nested_something_else) : something_else
  
  # good
  if some_condition
    nested_condition ? nested_something : nested_something_else
  else
    something_else
  end

• Avoid multiple conditions in ternaries. Ternaries are best used with single conditions. ^[link]

• Avoid multi-line ?: (the ternary operator), use if/then/else/end instead. ^[link]

  # bad
  some_really_long_condition_that_might_make_you_want_to_split_lines ?
    something : something_else
  
  # good
  if some_really_long_condition_that_might_make_you_want_to_split_lines
    something
  else
    something_else
  end

• Avoid the use of nested conditionals for flow of control. (More on this.) ^[link]

  Prefer a guard clause when you can assert invalid data. A guard clause is a conditional statement at the top of a function that returns as soon as it can.

  The general principles boil down to:

  • Return immediately once you know your function cannot do anything more.
  • Reduce nesting and indentation in the code by returning early. This makes the code easier to read and requires less mental bookkeeping on the part of the reader to keep track of else branches.
  • The core or most important flows should be the least indented.

  # bad
  def compute
    server = find_server
    if server
      client = server.client
      if client
        request = client.make_request
        if request
           process_request(request)
        end
      end
    end
  end
  
  # good
  def compute
    server = find_server
    return unless server
    client = server.client
    return unless client
    request = client.make_request
    return unless request
    process_request(request)
  end

  Prefer next in loops instead of conditional blocks.

  # bad
  [0, 1, 2, 3].each do |item|
    if item > 1
      puts item
    end
  end
  
  # good
  [0, 1, 2, 3].each do |item|
    next unless item > 1
    puts item
  end

  See also the section "Guard Clause", p68-70 in Beck, Kent. Implementation Patterns. Upper Saddle River: Addison-Wesley, 2008, which has inspired some of the content above.

• Never use for, unless you know exactly why. Most of the time iterators should be used instead. for is implemented in terms of each (so you're adding a level of indirection), but with a twist - for doesn't introduce a new scope (unlike each) and variables defined in its block will be visible outside it.^[link]

  arr = [1, 2, 3]
  
  # bad
  for elem in arr do
    puts elem
  end
  
  # good
  arr.each { |elem| puts elem }

• Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multiline chaining is always ugly). Always use do...end for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoid do...end when chaining.^[link]

  names = ["Bozhidar", "Steve", "Sarah"]
  
  # good
  names.each { |name| puts name }
  
  # bad
  names.each do |name| puts name end
  
  # good
  names.each do |name|
    puts name
    puts 'yay!'
  end
  
  # bad
  names.each { |name|
    puts name
    puts 'yay!'
  }
  
  # good
  names.select { |name| name.start_with?("S") }.map { |name| name.upcase }
  
  # bad
  names.select do |name|
    name.start_with?("S")
  end.map { |name| name.upcase }

  Some will argue that multiline chaining would look okay with the use of {...}, but they should ask themselves if this code is really readable and whether the block's content can be extracted into nifty methods.

• Use shorthand self assignment operators whenever applicable.^[link]

  # bad
  x = x + y
  x = x * y
  x = x**y
  x = x / y
  x = x || y
  x = x && y
  
  # good
  x += y
  x *= y
  x **= y
  x /= y
  x ||= y
  x &&= y

• Avoid semicolons except for in single line class definitions. When it is appropriate to use a semicolon, it should be directly adjacent to the statement it terminates: there should be no space before the semicolon.^[link]

  # bad
  puts 'foobar'; # superfluous semicolon
  puts 'foo'; puts 'bar' # two expressions on the same line
  
  # good
  puts 'foobar'
  
  puts 'foo'
  puts 'bar'
  
  puts 'foo', 'bar' # this applies to puts in particular

• Use :: only to reference constants(this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Do not use :: for regular method invocation.^[link]

  # bad
  SomeClass::some_method
  some_object::some_method
  
  # good
  SomeClass.some_method
  some_object.some_method
  SomeModule::SomeClass::SOME_CONST
  SomeModule::SomeClass()

• Avoid return where not required. ^[link]

  # bad
  def some_method(some_arr)
    return some_arr.size
  end
  
  # good
  def some_method(some_arr)
    some_arr.size
  end

• Don't use the return value of = in conditionals^[link]

  # bad - shows intended use of assignment
  if (v = array.grep(/foo/))
    ...
  end
  
  # bad
  if v = array.grep(/foo/)
    ...
  end
  
  # good
  v = array.grep(/foo/)
  if v
    ...
  end

• Use ||= freely to initialize variables. ^[link]

  # set name to Bozhidar, only if it's nil or false
  name ||= 'Bozhidar'

• Don't use ||= to initialize boolean variables. (Consider what would happen if the current value happened to be false.)^[link]

  # bad - would set enabled to true even if it was false
  enabled ||= true
  
  # good
  enabled = true if enabled.nil?

• Use .call explicitly when calling lambdas. ^[link]

  # bad
  lambda.(x, y)
  
  # good
  lambda.call(x, y)

• Avoid using Perl-style special variables (like $0-9, $, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Prefer long form versions such as $PROGRAM_NAME.^[link]

• When a method block takes only one argument, and the body consists solely of reading an attribute or calling one method with no arguments, use the &: shorthand. ^[link]

  # bad
  bluths.map { |bluth| bluth.occupation }
  bluths.select { |bluth| bluth.blue_self? }
  
  # good
  bluths.map(&:occupation)
  bluths.select(&:blue_self?)

• Prefer some_method over self.some_method when calling a method on the current instance.^[link]

  # bad
  def end_date
    self.start_date + self.nights
  end
  
  # good
  def end_date
    start_date + nights
  end

  In the following three common cases, self. is required by the language and is good to use:

  1. When defining a class method: def self.some_method.
  2. The left hand side when calling an assignment method, including assigning an attribute when self is an ActiveRecord model: self.guest = user.
  3. Referencing the current instance's class: self.class.

• When defining an object of any mutable type meant to be a constant, make sure to call freeze on it. Common examples are strings, arrays, and hashes. (More on this.)^[link]

  The reason is that Ruby constants are actually mutable. Calling freeze ensures they are not mutated and are therefore truly constant and attempting to modify them will raise an exception. For strings, this allows older versions of Ruby below 2.2 to intern them.

  # bad
  class Color
    RED = 'red'
    BLUE = 'blue'
    GREEN = 'green'
  
    ALL_COLORS = [
      RED,
      BLUE,
      GREEN,
    ]
  
    COLOR_TO_RGB = {
      RED => 0xFF0000,
      BLUE => 0x0000FF,
      GREEN => 0x00FF00,
    }
  end
  
  # good
  class Color
    RED = 'red'.freeze
    BLUE = 'blue'.freeze
    GREEN = 'green'.freeze
  
    ALL_COLORS = [
      RED,
      BLUE,
      GREEN,
    ].freeze
  
    COLOR_TO_RGB = {
      RED => 0xFF0000,
      BLUE => 0x0000FF,
      GREEN => 0x00FF00,
    }.freeze
  end

• Use snake_case for methods and variables. ^[link]

• Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) ^[link]

• Use SCREAMING_SNAKE_CASE for other constants.^[link]

• The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e. Array#empty?).^[link]

• The names of potentially "dangerous" methods (i.e. methods that modify self or the arguments, exit!, etc.) should end with an exclamation mark. Bang methods should only exist if a non-bang method exists. (More on this.) ^[link]

• Name throwaway variables _. ^[link]

  version = '3.2.1'
  major_version, minor_version, _ = version.split('.')

• Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance. ^[link]

  class Parent
    @@class_var = 'parent'
  
    def self.print_class_var
      puts @@class_var
    end
  end
  
  class Child < Parent
    @@class_var = 'child'
  end
  
  Parent.print_class_var # => will print "child"

  As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.

• Use def self.method to define singleton methods. This makes the methods more resistant to refactoring changes. ^[link]

  class TestClass
    # bad
    def TestClass.some_method
      ...
    end
  
    # good
    def self.some_other_method
      ...
    end

• Avoid class << self except when necessary, e.g. single accessors and aliased attributes. ^[link]

  class TestClass
    # bad
    class << self
      def first_method
        ...
      end
  
      def second_method_etc
        ...
      end
    end
  
    # good
    class << self
      attr_accessor :per_page
      alias_method :nwo, :find_by_name_with_owner
    end
  
    def self.first_method
      ...
    end
  
    def self.second_method_etc
      ...
    end
  end

• Indent the public, protected, and private methods as much the method definitions they apply to. Leave one blank line above and below them.^[link]

  class SomeClass
    def public_method
      # ...
    end
  
    private
  
    def private_method
      # ...
    end
  end

• Don't use exceptions for flow of control. ^[link]

  # bad
  begin
    n / d
  rescue ZeroDivisionError
    puts "Cannot divide by 0!"
  end
  
  # good
  if d.zero?
    puts "Cannot divide by 0!"
  else
    n / d
  end

• Avoid rescuing the Exception class. ^[link]

  # bad
  begin
    # an exception occurs here
  rescue Exception
    # exception handling
  end
  
  # good
  begin
    # an exception occurs here
  rescue StandardError
    # exception handling
  end
  
  # acceptable
  begin
    # an exception occurs here
  rescue
    # exception handling
  end

• Don't specify RuntimeError explicitly in the two argument version of raise. Prefer error sub-classes for clarity and explicit error creation.^[link]

  # bad
  raise RuntimeError, 'message'
  
  # better - RuntimeError is implicit here
  raise 'message'
  
  # best
  class MyExplicitError < RuntimeError; end
  raise MyExplicitError

• Prefer supplying an exception class and a message as two separate arguments to raise, instead of an exception instance. ^[link]

  # bad
  raise SomeException.new('message')
  # Note that there is no way to do `raise SomeException.new('message'), backtrace`.
  
  # good
  raise SomeException, 'message'
  # Consistent with `raise SomeException, 'message', backtrace`.

• Avoid using rescue in its modifier form. ^[link]

  # bad
  read_file rescue handle_error($!)
  
  # good
  begin
    read_file
  rescue Errno:ENOENT => ex
    handle_error(ex)
  end

• Prefer map over collect.^[link]

• Prefer detect over find. The use of find is ambiguous with regard to ActiveRecord's find method - detect makes clear that you're working with a Ruby collection, not an AR object. ^[link]

• Prefer reduce over inject. ^[link]

• Prefer size over either length or count for performance reasons.^[link]

• Prefer literal array and hash creation notation unless you need to pass parameters to their constructors. ^[link]

  # bad
  arr = Array.new
  hash = Hash.new
  
  # good
  arr = []
  hash = {}
  
  # good because constructor requires parameters
  x = Hash.new { |h, k| h[k] = {} }

• Favor Array#join over Array#* for clarity. ^[link]

  # bad
  %w(one two three) * ', '
  # => 'one, two, three'
  
  # good
  %w(one two three).join(', ')
  # => 'one, two, three'

• Use symbols instead of strings as hash keys. ^[link]

  # bad
  hash = { 'one' => 1, 'two' => 2, 'three' => 3 }
  
  # good
  hash = { :one => 1, :two => 2, :three => 3 }

• Relatedly, use plain symbols instead of string symbols when possible.^[link]

  # bad
  :"symbol"
  
  # good
  :symbol

• Use Hash#key? instead of Hash#has_key? and Hash#value? instead of Hash#has_value?. According to Matz, the longer forms are considered deprecated. ^[link]

  # bad
  hash.has_key?(:test)
  hash.has_value?(value)
  
  # good
  hash.key?(:test)
  hash.value?(value)

• Use multi-line hashes when it makes the code more readable, and use trailing commas to ensure that parameter changes don't cause extraneous diff lines when the logic has not otherwise changed. ^[link]

  hash = {
    :protocol => 'https',
    :only_path => false,
    :controller => :users,
    :action => :set_password,
    :redirect => @redirect_url,
    :secret => @secret,
  }

• Use a trailing comma in an Array that spans more than 1 line^[link]

  # good
  array = [1, 2, 3]
  
  # good
  array = [
    "car",
    "bear",
    "plane",
    "zoo",
  ]

• Prefer string interpolation instead of string concatenation:^[link]

  # bad
  email_with_name = user.name + ' <' + user.email + '>'
  
  # good
  email_with_name = "#{user.name} <#{user.email}>"

  Furthermore, keep in mind Ruby 1.9-style interpolation. Let's say you are composing cache keys like this:

  CACHE_KEY = '_store'
  
  cache.write(@user.id + CACHE_KEY)

  Prefer string interpolation instead of string concatenation:

  CACHE_KEY = '%d_store'
  
  cache.write(CACHE_KEY % @user.id)

• Avoid using String#+ when you need to construct large data chunks. Instead, use String#<<. Concatenation mutates the string instance in-place and is always faster than String#+, which creates a bunch of new string objects.^[link]

  # good and also fast
  story = ''
  story << 'The Ugly Duckling'
  
  paragraphs.each do |paragraph|
    story << paragraph
  end

• Use \ at the end of the line instead of + or << to concatenate multi-line strings. ^[link]

  # bad
  "Some string is really long and " +
    "spans multiple lines."
  
  "Some string is really long and " <<
    "spans multiple lines."
  
  # good
  "Some string is really long and " \
    "spans multiple lines."

• Avoid using $1-9 as it can be hard to track what they contain. Named groups can be used instead. ^[link]

  # bad
  /(regexp)/ =~ string
  ...
  process $1
  
  # good
  /(?<meaningful_var>regexp)/ =~ string
  ...
  process meaningful_var

• Be careful with ^ and $ as they match start/end of line, not string endings. If you want to match the whole string use: \A and \z.^[link]

  string = "some injection\nusername"
  string[/^username$/]   # matches
  string[/\Ausername\z/] # don't match

• Use x modifier for complex regexps. This makes them more readable and you can add some useful comments. Just be careful as spaces are ignored.^[link]

  regexp = %r{
    start         # some text
    \s            # white space char
    (group)       # first group
    (?:alt1|alt2) # some alternation
    end
  }x

• Prefer parentheses over curly braces, brackets, or pipes when using %-literal delimiters for consistency, and because the behavior of %-literals is closer to method calls than the alternatives.^[link]

  # bad
  %w[date locale]
  %w{date locale}
  %w|date locale|
  
  # good
  %w(date locale)

• Use %w freely.^[link]

  STATES = %w(draft open closed)

• Use %() for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.^[link]

  # bad - no interpolation needed
  %(Welcome, Jane!)
  # should be 'Welcome, Jane!'
  
  # bad - no double-quotes
  %(This is #{quality} style)
  # should be "This is #{quality} style"
  
  # bad - multiple lines
  %(Welcome, Jane!\nPlease enjoy your stay at #{location}\nCheers!)
  # should be a heredoc.
  
  # good - requires interpolation, has quotes, single line
  %(Welcome, #{name}!)

• Use %r only for regular expressions matching more than one '/' character.^[link]

  # bad
  %r(\s+)
  
  # still bad
  %r(^/(.*)$)
  # should be /^\/(.*)$/
  
  # good
  %r(^/blog/2011/(.*)$)

• Avoid the use of %x unless you're going to invoke a command with backquotes in it (which is rather unlikely). ^[link]

  # bad
  date = %x(date)
  
  # good
  date = `date`
  echo = %x(echo `date`)

• When immediately returning after calling render or redirect_to, put return on the next line, not the same line. ^[link]

  # bad
  render :text => 'Howdy' and return
  
  # good
  render :text => 'Howdy'
  return
  
  # still bad
  render :text => 'Howdy' and return if foo.present?
  
  # good
  if foo.present?
    render :text => 'Howdy'
    return
  end

• When defining ActiveRecord model scopes, wrap the relation in a lambda. A naked relation forces a database connection to be established at class load time (instance startup). ^[link]

  # bad
  scope :foo, where(:bar => 1)
  
  # good
  scope :foo, -> { where(:bar => 1) }

If you're editing code, take a few minutes to look at the code around you and determine its style. If they use spaces around all their arithmetic operators, you should too. If their comments have little boxes of hash marks around them, make your comments have little boxes of hash marks around them too.

The point of having style guidelines is to have a common vocabulary of coding so people can concentrate on what you're saying rather than on how you're saying it. We present global style rules here so people know the vocabulary, but local style is also important. If code you add to a file looks drastically different from the existing code around it, it throws readers out of their rhythm when they go to read it. Avoid this.

—Google C++ Style Guide

This style guide is also available in other languages:

• Chinese (Simplified): 1c7/ruby-airbnb