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CoffeeScript Fly-by

CoffeeScript is a little language that compiles into JavaScript.
- coffeescript.org

CoffeeScript was inspired by Ruby and Python. It is not directly interpreted by browsers and can therefore be compiled into JavaScript. NodeJS accepts CoffeScript directly.

Regarding websites this can be done either on the client-side like shown below or to increase client-side performance on the server-side.

<script src="//jashkenas.github.com/coffee-script/extras/coffee-script.js"></script>
<script type="text/coffeescript">
  # Some CoffeeScript
</script>

In general CoffeeScript provides the following benefits:

  • It reduces to amount of code you write by a third to a half by taking syntactic advantage of white space and line breaks.
  • It introduces high level functions like array comprehensions, prototype aliases and classes that further reduce code size.
  • It hides the quirky behaving parts (as documented here) of JavaScript.
  • It compiles into readable JavaScript code.
  • CoffeeScript tends to run faster than handwritten JavaScript.
    This is because CoffeeScripts equivalents of forEach() and map() are compiled into faster for() loop constructs.

Local Installation

sudo apt-add-repository ppa:chris-lea/node.js
sudo apt-get update
sudo apt-get install nodejs
sudo npm install -g coffee-script

Usage in Bash Scripts

#!/usr/bin/env coffee

# array: filename, absolute path, arguments
global.process.argv

# log to console
console.log 'Some text'

# include another Coffee/JS file
require './relativePath.coffee'

More in the NodeJS documentation.

CoffeeScript Syntax

Comments

# Single line comment

###
  Multi line comment
###

Variables

Variables can not be declared explicitly. They are automatically attached to the current scope. Thus variables can not be shadowed.

Variables can be attached to the global scope in the following way: TODO

If you'd like to create top-level variables for other scripts to use, attach them as properties on window, or on the exports object in CommonJS. The existential operator (covered below), gives you a reliable way to figure out where to add them; if you're targeting both CommonJS and the browser: exports ? this

Functions

Named functions

f1 = -> a * b
f2 = ->
  # additional code
  a * b
f3 = (a, b, c) -> # same here
f1 => a * b
# binds to current scope

Splats

f1 = (a...) ->
  # `a` is an array holding all given arguments

TODO: "real" array p. 15

Destructuring Assignments

o = {a: 1, b: 2}
a = [1, 2]
{c, d} = o
{e, f} = a
[g, h] = o
[i, j] = a

Return value

f1 = -> 2
f2 = -> return 2
f3 = ->
  return 2
  return 3
# all return 2

Default values

f1 = (a = 2) -> a * b
f2 = (a, b = 2) -> a * b
f3 = (a = 2, b...) -> a * b[0]

Invocation

f1(a, b) == f1 a, b
f1(f2(a, b)) == f1 f2(a, b) == f1 f2 a, b
f1(f2(a), b) == f1 f2(a), b
f1 != f1()
# f1 only returns the function
c = [a, b]
f1 c... == f1 a, b
f1 ->
  Math.max arguments...
f1 1, 2 # == 2

Anonymous functions

-> a * b
() -> a * b
=> a * b
() => a * b
# same options concerning arguments, default values and splats

Object Literals

o1 = {a: 1, b: 2}
o2 = {a: 1, b: 2,}
o3 = a: 1, b: 2
o4 =
  a: 1
  b: 2
# o1 == o2 == o3 == o4

Arrays

a1 = [a, b]
a2 = [a, b,]
a3 =
  a
  b
# a1 == a2 == a3

Ranges

a = [1..3] # [1, 2, 3]
b = [1...3] # [1, 2]

Slices

[1 ,2 ,3][0..1] # [1, 2]
hello = "Hello there"[0..4] # "Hello"
"Hello there"[5..9] = "friend" # "Hello friend"

Flow control

if true
  "Yep, true"
if true then "Yep, true"
unless false
  "Yep, false"
unless false then "Yep, false"
if true is true
  "Yep, true"
if true is true then "Yep, true"
if true is !false
  "Yep, true"
if true is not false
  "Yep, true"
if true isnt false
  "Yep, true"
if 2 in [1, 2, 3] then true

"Reversed" if/unless

"Yep, true" if true
# the same for all constructs above

Logical Operators

0, "", null, undefined == false

  • and == &&
  • or == ||
  • is == ==
  • isnt == !=
  • not == !
  • or= == if not (exprLeft) then (exprLeft) = (exprRight)

Existential Operator

  • (expr)? returns true if (expr) is neither null nor undefined.
  • (obj)?.(prop) checks if the property is available. If not does not try to access it.
  • (func)?() checks if the function is available. If not does not try to call it.
  • ?= == if (exprLeft) is in [undefined, null] then (exprLeft) = (exprRight)

String interpolation

a = 2
b = "Two: #{a}" # "Two: 2"
c = "Two: #{if a is 2 then "Two"}" # "Two: Two"

Comprehensions

Arrays: "in"

a = 0
for number in [1, 2, 3]
  a += number
for number, index in [1, 2, 3]
  a += number * index
a = 0
for number in [1, 2, 3] when number > 2
  a += number
# same as above

reversed

a = 0
a += number for number in [1, 2, 3]
a += number * index for number, index in [1, 2, 3]

Objects: "of"

a = 0
s = ""
o = a: 1, b: 2, b: 3
for letter of o
  s += letter
for letter, number of o
  a += number
s = ""
o = a: 1, b: 2, b: 3
for letter of o when letter isnt "a"
  s += letter

reversed

a = 0
s = ""
o = a: 1, b: 2, b: 3
s += letter for letter of o
a += number for letter, number of o
s += letter for letter of o when letter isnt "a"

Loops

The only "low-level" loop that CoffeeScript provides is "while".

"while" Loop

a = b = 0
c = while a < 3
  a++
d = while b < 3 then b++ when b > 2
# c == d == [0, 1, 2]

Aliases

@var == this.var
User::prop == User.prototype.prop

Classes

Constructors

Constructors get invoked on instantiation. If the constructor arguments are prefixed by an @ they will automatically get set as instance properties.

class A
  constructor: (@instanceProp, argument) ->
    # constructor code

Instance Property

class A
  instanceProp1: 2
  instanceProp2: ->
    @instanceProp1 = 4 # value change
    @instanceProp3 = 2 # new instance property
A::instanceProp4 = 2 # new instance property

In order to bind the context of a instance method to the class use the fat arrow:

class A
  instanceProp1 = 2
  instanceProp2: (a) =>
    @instanceProp1 = a

Static Property

On the class level static properties can be accessed/declared using this., the corresponding @-alias or with ClassName.propName. Within methods only with ClassName.propName.

class A
  this.staticProp1: 2
  @staticProp2: 2
  A.staticProp3: 2

  instanceProp: ->
    A.staticProp1 = 3

Inheritance and Super

When shadowing methods super() calls the corresponding method of the parent.

If the child does not define a constructor function the constructor of the parent gets called on instantiation.

class A
  instanceProp1: 2
  constructor: ->
    @instanceProp1 += 2
  instanceProp2: (a) ->
    @instanceProp1 += a

class B extends A
  instanceProp2: ->
    super(2)

b = new B
b.instanceProp2()
# b.instanceProp1 ==
#  2 (initial value)
#  + 2 (parents constructor)
#  + 2 (method call)

Dynamic Inheritance

New instance properties are automatically inhertited even by existing instances of the children.

class A
  instanceProp: 2

class B extends A

b = new B
A::newInstanceProp = 2
# b dynamically inherits `newInstanceProp`:
# b.newInstanceProp == 2