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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