Best-practices and coding conventions for the CoffeeScript programming language

CoffeeScript Style Guide

This guide presents a collection of best-practices and coding conventions for the CoffeeScript programming language.

This guide is intended to be community-driven, and contributions are highly encouraged.

Please note that this is a work-in-progress: there is much more that can be specified, and some of the guidelines that have been specified may not be deemed to be idiomatic by the community (in which case, these offending guidelines will be modified or removed, as appropriate).


The details in this guide have been very heavily inspired by several existing style guides and other resources. In particular:

Table of Contents

Code layout

Tabs or Spaces?

Use spaces only, with 2 spaces per indentation level. Never mix tabs and spaces.

Maximum Line Length

Limit all lines to a maximum of 79 characters.

Blank Lines

Separate top-level function and class definitions with a single blank line.

Separate method definitions inside of a class with a single blank line. Use a single blank line also before the first method of a class.

Use a single blank line within the bodies of methods or functions in cases where this improves readability (e.g., for the purpose of delineating logical sections).

Trailing Whitespace

Do not include trailing whitespace on any lines.


UTF-8 is the preferred source file encoding.

Module Imports

If using a module system (CommonJS Modules, AMD, etc.), require statements should be placed on separate lines.

require 'lib/setup'
Backbone = require 'backbone'

These statements should be grouped in the following order:

  1. Standard library imports (if a standard library exists)
  2. Third party library imports
  3. Local imports (imports specific to this application or library)

Whitespace in Expressions and Statements

Avoid extraneous whitespace in the following situations:

  • Immediately inside parentheses, brackets or braces

       $('body') # Yes
       $( 'body' ) # No
  • Immediately before a comma

       console.log x, y # Yes
       console.log x , y # No

Additional recommendations:

  • Always surround these binary operators with a single space on either side

    • assignment: =

      foo = 'bar' # Yes
      foo='bar' # No
      • Except don’t use spaces around the = sign when used to indicate a default parameter value.
         test: (param=null) -> # Yes
         test: (param = null) -> # No
    • augmented assignment: +=, -=, etc.

    • comparisons: ==, <, >, <=, >=, unless, etc.

    • arithmetic operators: +, -, *, /, etc.

    • (Do not use more than one space around these operators)

         # Yes
         x = 1
         y = 1
         fooBar = 3
         # No
         x      = 1
         y      = 1
         fooBar = 3


If modifying code that is described by an existing comment, update the comment such that it accurately reflects the new code. (Ideally, improve the code to obviate the need for the comment, and delete the comment entirely.)

The first word of the comment should be capitalized, unless the first word is an identifier that begins with a lower-case letter.

If a comment is short, the period at the end can be omitted.

Block Comments

Block comments apply to the block of code that follows them.

Each line of a block comment starts with a # and a single space, and should be indented at the same level of the code that it describes.

Paragraphs inside of block comments are separated by a line containing a single #.

  # This is a block comment. Note that if this were a real block
  # comment, we would actually be describing the proceeding code.
  # This is the second paragraph of the same block comment. Note
  # that this paragraph was separated from the previous paragraph
  # by a line containing a single comment character.


Inline Comments

Inline comments are placed on the line immediately above the statement that they are describing. If the inline comment is sufficiently short, it can be placed on the same line as the statement (separated by a single space from the end of the statement).

All inline comments should start with a # and a single space.

The use of inline comments should be limited, because their existence is typically a sign of a code smell.

Do not use inline comments when they state the obvious:

  # No
  x = x + 1 # Increment x

However, inline comments can be useful in certain scenarios:

  # Yes
  x = x + 1 # Compensate for border

Naming Conventions

Use camelCase (with a leading lowercase character) to name all variables, methods, and object properties.

Use CamelCase (with a leading uppercase character) to name all classes. (This style is also commonly referred to as PascalCase, CamelCaps, or CapWords, among other alternatives.)

(The official CoffeeScript convention is camelcase, because this simplifies interoperability with JavaScript. For more on this decision, see here.)

For constants, use all uppercase with underscores:


Methods and variables that are intended to be “private” should begin with a leading underscore:

_privateMethod: ->


(These guidelines also apply to the methods of a class.)

When declaring a function that takes arguments, always use a single space after the closing parenthesis of the arguments list:

foo = (arg1, arg2) -> # Yes
foo = (arg1, arg2)-> # No

Use parentheses when declaring functions that take no arguments:

bar = () -> # Yes
bar = -> # No

When calling functions, omit the parentheses when it’s possible without sacrificing readability. Keeping in mind that “readability” can be subjective, the following examples demonstrate cases where parentheses have been omitted or included in a way that is readable in our opinion.

foo() # Call functions without arguments like this.

baz 12

brush.ellipse x: 10, y: 20

foo(4).bar 8

coolEl = $ '#cool'

print (inspect value) # Use parentheses to make nested function calls more readable.

new Tag(new Value(a, b), new Arg(c))

Do not omit the parentheses, if that would just add another pair of them around the expression. Mathematical expressions usually are more readable without omitting argument parentheses.

obj.value(10, 20) / obj.value(20, 10 + 1) * 5

For chaining, use parentheses for function arguments instead of wrapping the whole function call in parentheses, since this keeps readable even if the chain is long.

$('#selektor').addClass 'klass' # Yes
($ '#selektor').addClass 'klass' # No

In cases where method calls are being chained and the code does not fit on a single line, each call should be placed on a separate line and indented by one level (i.e., two spaces), with a leading ..

  .map((x) -> x * x)
  .filter((x) -> x < 11)
  .reduce((x, y) -> x + y)


Use string interpolation instead of string concatenation:

"this is an #{adjective} string" # Yes
"this is an " + adjective + " string" # No

Prefer single quoted strings ('') instead of double quoted ("") strings, unless features like string interpolation are being used for the given string.


Favor unless over if not for negative conditions.

Instead of using unless...else, use if...else:

  # Yes
  if true

  # No
  unless false

Multi-line if/else clauses should use indentation:

  # Yes
  if true

  # No
  if true then ...
  else ...

Looping and Comprehensions

Take advantage of comprehensions whenever possible:

  # Yes
  result = ( for item in array)

  # No
  results = []
  for item in array

To filter:

result = (item for item in array when is "test")

To iterate over the keys and values of objects:

object = one: 1, two: 2
alert("#{key} = #{value}") for key, value of object

Extending Native Objects

Do not modify native objects.

For example, do not modify Array.prototype to introduce Array#forEach.


Do not suppress exceptions.


Use annotations when necessary to describe a specific action that must be taken against the indicated block of code.

Write the annotation on the line immediately above the code that the annotation is describing.

The annotation keyword should be followed by a colon and a space, a descriptive note and a ticket number in the issue tracker for taking the required action. The ticket MUST be created when annotations are used. Generally avoiding hacks should be preferred over adding annotations for them.

If multiple lines are required by the description, indent subsequent lines with two spaces:

  # TODO: Ensure that the value returned by this call falls within a certain
  #   range, or throw an exception.

Do not use annotations like FIXME to justify broken functionality.

  # FIXME: The client's current state should *not* affect payload processing. # No

Annotation types:

  • TODO: describe missing functionality that should be added at a later date
  • OPTIMIZE: describe code that is inefficient and may become a bottleneck
  • HACK: describe the use of a questionable (or ingenious) coding practice


and is preferred over &&.

or is preferred over ||.

is is preferred over ==.

not is preferred over !.

Prefer shorthand notation (::) for accessing an object’s prototype:

Array::slice # Yes
Array.prototype.slice # No

Prefer @property over

return @property # Yes
return # No

However, avoid the use of standalone @:

return this # Yes
return @ # No

Avoid return where not required, unless the explicit return increases clarity.

Use splats (...) when working with functions that accept variable numbers of arguments:

console.log args... # Yes

(a, b, c, rest...) -> # Yes

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