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These are standards that Rails applications should have in terms of code style and performance. To check those standards RuboCop is used.

Rubocop

  1. Add these three gems to your Gemfile to support RuboCop in your project.

     gem 'rubocop', require: false
 gem 'rubocop-rails', require: false
 gem 'rubocop-performance', require: false

  2. Create a file called .rubocop.yml in your root folder of your project

  3. Add the following code to the file which we created called .rubocop.yml, currently the given config file will only check the styles.

     AllCops:
   DisabledByDefault: true 																																																						
 Style/Alias:
   Description: 'Use alias_method instead of alias.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#alias-method'
   Enabled: true 																																											
 Style/ArrayJoin:
       Description: 'Use Array#join instead of Array#*.'
       StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#array-join'
       Enabled: true 																														
 Style/AsciiComments:
   Description: 'Use only ascii symbols in comments.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#english-comments'
   Enabled: true 																															
 Style/Attr:
   Description: 'Checks for uses of Module#attr.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#attr'
   Enabled: true 																															
 Style/CaseEquality:
   Description: 'Avoid explicit use of the case equality operator(===).'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-case-equality'
   Enabled: true 																															
 Style/CharacterLiteral:
   Description: 'Checks for uses of character literals.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-character-literals'
   Enabled: true 																															
 Style/ClassAndModuleChildren:
   Description: 'Checks style of children classes and modules.'
   Enabled: true
   EnforcedStyle: nested 																												
 Style/ClassVars:
   Description: 'Avoid the use of class variables.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-class-vars'
   Enabled: true 																																			
 Style/CollectionMethods:
   Enabled: true
   PreferredMethods:
     find: detect
     inject: reduce
     collect: map
     find_all: select 																																		
 Style/ColonMethodCall:
   Description: 'Do not use :: for method call.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#double-colons'
   Enabled: true 																																
 Style/CommentAnnotation:
   Description: >-
                  Checks formatting of special comments
                  (TODO, FIXME, OPTIMIZE, HACK, REVIEW).
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#annotate-keywords'
   Enabled: true 																															
 Style/PreferredHashMethods:
   Description: 'Checks use of `has_key?` and `has_value?` Hash methods.'
   StyleGuide: '#hash-key'
   Enabled: true 																																							
 Style/Documentation:
   Description: 'Document classes and non-namespace modules.'
   Enabled: true 																																
 Style/DoubleNegation:
   Description: 'Checks for uses of double negation (!!).'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-bang-bang'
   Enabled: true 																																				
 Style/EachWithObject:
   Description: 'Prefer `each_with_object` over `inject` or `reduce`.'
   Enabled: true 																																
 Style/EmptyLiteral:
   Description: 'Prefer literals to Array.new/Hash.new/String.new.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#literal-array-hash'
   Enabled: true 																															
 Style/EvenOdd:
   Description: 'Favor the use of Fixnum#even? && Fixnum#odd?'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#predicate-methods'
   Enabled: true 																															
 Style/FormatString:
   Description: 'Enforce the use of Kernel#sprintf, Kernel#format or String#%.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#sprintf'
   Enabled: true 																																
 Style/GlobalVars:
   Description: 'Do not introduce global variables.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#instance-vars'
   Reference: 'http://www.zenspider.com/Languages/Ruby/QuickRef.html'
   Enabled: true 																																
 Style/GuardClause:
   Description: 'Check for conditionals that can be replaced with guard clauses'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-nested-conditionals'
   Enabled: true 																															
 Style/IfUnlessModifier:
   Description: >-
                  Favor modifier if/unless usage when you have a
                  single-line body.
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#if-as-a-modifier'
   Enabled: true 																															
 Style/IfWithSemicolon:
   Description: 'Do not use if x; .... Use the ternary operator instead.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-semicolon-ifs'
   Enabled: true 																															
 Style/InlineComment:
   Description: 'Avoid inline comments.'
   Enabled: true 																															
 Style/Lambda:
   Description: 'Use the new lambda literal syntax for single-line blocks.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#lambda-multi-line'
   Enabled: true 																																	
 Style/LambdaCall:
   Description: 'Use lambda.call(...) instead of lambda.(...).'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#proc-call'
   Enabled: true 																																
 Style/LineEndConcatenation:
   Description: >-
                  Use \ instead of + or << to concatenate two string literals at
                  line end.
   Enabled: true 																															
 Style/ModuleFunction:
   Description: 'Checks for usage of `extend self` in modules.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#module-function'
   Enabled: true 																															
 Style/MultilineBlockChain:
   Description: 'Avoid multi-line chains of blocks.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#single-line-blocks'
   Enabled: true 																															
 Style/NegatedIf:
   Description: >-
                  Favor unless over if for negative conditions
                  (or control flow or).
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#unless-for-negatives'
   Enabled: true 																																	
 Style/NegatedWhile:
   Description: 'Favor until over while for negative conditions.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#until-for-negatives'
   Enabled: true 																																
 Style/Next:
   Description: 'Use `next` to skip iteration instead of a condition at the end.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-nested-conditionals'
   Enabled: true 																																		
 Style/NilComparison:
   Description: 'Prefer x.nil? to x == nil.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#predicate-methods'
   Enabled: true 																																			
 Style/Not:
   Description: 'Use ! instead of not.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#bang-not-not'
   Enabled: true 																															
 Style/NumericLiterals:
   Description: >-
                  Add underscores to large numeric literals to improve their
                  readability.
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#underscores-in-numerics'
   Enabled: true 																																		
 Style/OneLineConditional:
   Description: >-
                  Favor the ternary operator(?:) over
                  if/then/else/end constructs.
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#ternary-operator'
   Enabled: true 																																		
 Style/PercentLiteralDelimiters:
   Description: 'Use `%`-literal delimiters consistently'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#percent-literal-braces'
   Enabled: true 																															
 Style/PerlBackrefs:
   Description: 'Avoid Perl-style regex back references.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-perl-regexp-last-matchers'
   Enabled: true 																															
 Style/Proc:
   Description: 'Use proc instead of Proc.new.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#proc'
   Enabled: true 																															
 Style/RaiseArgs:
   Description: 'Checks the arguments passed to raise/fail.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#exception-class-messages'
   Enabled: true 																																	
 Style/RegexpLiteral:
   Description: 'Use / or %r around regular expressions.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#percent-r'
   Enabled: true 																															
 Style/Sample:
   Description: >-
     Use `sample` instead of `shuffle.first`,
     `shuffle.last`, and `shuffle[Fixnum]`.
   Reference: 'https://github.com/JuanitoFatas/fast-ruby#arrayshufflefirst-vs-arraysample-code'
   Enabled: true 																																		
 Style/SingleLineBlockParams:
   Description: 'Enforces the names of some block params.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#reduce-blocks'
   Enabled: true 																																		
 Style/SingleLineMethods:
   Description: 'Avoid single-line methods.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-single-line-methods'
   Enabled: true 																																						
 Style/SignalException:
   Description: 'Checks for proper usage of fail and raise.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#fail-method'
   Enabled: true 																																		
 Style/SpecialGlobalVars:
   Description: 'Avoid Perl-style global variables.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-cryptic-perlisms'
   Enabled: true 																															
 Style/StringLiterals:
   Description: 'Checks if uses of quotes match the configured preference.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#consistent-string-literals'
   EnforcedStyle: double_quotes
   Enabled: true 																																
 Style/TrailingCommaInArguments:
   Description: 'Checks for trailing comma in argument lists.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-trailing-array-commas'
   EnforcedStyleForMultiline: comma
   SupportedStylesForMultiline:
     - comma
     - consistent_comma
     - no_comma
   Enabled: true 																																			
 Style/TrailingCommaInArrayLiteral:
   Description: 'Checks for trailing comma in array literals.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-trailing-array-commas'
   EnforcedStyleForMultiline: comma
   SupportedStylesForMultiline:
     - comma
     - consistent_comma
     - no_comma
   Enabled: true 																															
 Style/TrailingCommaInHashLiteral:
   Description: 'Checks for trailing comma in hash literals.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#no-trailing-array-commas'
   EnforcedStyleForMultiline: comma
   SupportedStylesForMultiline:
     - comma
     - consistent_comma
     - no_comma
   Enabled: true 																																				
 Style/TrivialAccessors:
   Description: 'Prefer attr_* methods to trivial readers/writers.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#attr_family'
   Enabled: true 																															
 Style/VariableInterpolation:
   Description: >-
                  Don't interpolate global, instance and class variables
                  directly in strings.
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#curlies-interpolate'
   Enabled: true 																																	
 Style/WhenThen:
   Description: 'Use when x then ... for one-line cases.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#one-line-cases'
   Enabled: true 																																							
 Style/WhileUntilModifier:
   Description: >-
                  Favor modifier while/until usage when you have a
                  single-line body.
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#while-as-a-modifier'
   Enabled: true 																																
 Style/WordArray:
   Description: 'Use %w or %W for arrays of words.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#percent-w'
   Enabled: true

  4. Now in the terminal we can check the number of offences which we did in our code base by running the command

     $ rubocop

  5. Alternatively we can pass RuboCop a list of files and directories to check

     $ rubocop app spec lib/something.rb

  6. You can also run RuboCop in an auto-correct mode, where it will try to automatically fix the problems it found in your code

     $ rubocop -a

  7. Additionally to check coffee and scss files, add this two gems to your file

     gem 'scss_lint', require: false
 gem 'coffeelint'

  8. Now run a command to check the sass files

     $	scss-lint

    Also you can run the command to check specific files using

     $	scss-lint app/assets/stylesheets/

  9. To check coffee lint use the command

     $	bundle exec rake coffeelint