_☕ CoffeeScript

CoffeeScript is a little language that compiles into JavaScript. Think of it as JavaScript's less ostentatious kid brother — the same genes, roughly the same height, but a different sense of style. Apart from a handful of bonus goodies, statements in CoffeeScript correspond one-to-one with their equivalent in JavaScript, it's just another way of saying it.

Disclaimer: CoffeeScript is just for fun and seriously alpha. I'm sure that there are still plenty of holes in the lexer and leaks in the syntax. There is no guarantee, explicit or implied, of its suitability for any purpose. That said, it compiles into clean JavaScript (the good parts) that can use existing JavaScript libraries seamlessly, and passes through JSLint without warnings. The compiled output is quite readable — pretty-printed, with comments preserved intact.

Table of Contents

Mini Overview
Installation and Usage
Punctuation Primer
Functions and Invocation
Assignment
Objects and Arrays
Lexical Scoping and Variable Safety
Conditionals, Ternaries, and Conditional Assignment
Everything is an Expression
Aliases
While Loops
Array Comprehensions
Slicing Arrays with Ranges
Inheritance, and Calling Super from a Subclass
Embedded JavaScript
Switch/When/Else
Try/Catch/Finally
Multiline Strings
Resources
Contributing
Change Log

Mini Overview

CoffeeScript on the left, compiled JavaScript output on the right.

# Assignment:
number: 42
opposite_day: true

# Conditions:
number: -42 if opposite_day

# Functions:
square: x => x * x

# Arrays:
list: [1, 2, 3, 4, 5]

# Objects:
math: {
  root:   Math.sqrt
  square: square
  cube:   x => x * square(x)
}

# Array comprehensions:
cubed_list: math.cube(num) for num in list

var __a, __b, __c, __d, cubed_list, list, math, num, number, opposite_day, square;
// Assignment:
number = 42;
opposite_day = true;
// Conditions:
if (opposite_day) {
  number = -42;
}
// Functions:
square = function square(x) {
  return x * x;
};
// Arrays:
list = [1, 2, 3, 4, 5];
// Objects:
math = {
  root: Math.sqrt,
  square: square,
  cube: function cube(x) {
    return x * square(x);
  }
};
// Array comprehensions:
__a = list;
__d = [];
for (__b=0, __c=__a.length; __b<__c; __b++) {
  num = __a[__b];
  __d[__b] = math.cube(num);
}
cubed_list = __d;

Installation and Usage

The CoffeeScript compiler is written in pure Ruby, and is available as a Ruby Gem.

gem install coffee-script

Installing the gem provides the coffee command, which can be used to compile CoffeeScript .coffee files into JavaScript, as well as debug them. In conjunction with Narwhal, the coffee command also provides direct evaluation and an interactive REPL. When compiling to JavaScript, coffee writes the output as .js files in the same directory by default, but output can be customized with the following options:

Examples:

coffee path/to/script.coffee
coffee --interactive
coffee --watch --lint experimental.coffee
coffee --print app/scripts/*.coffee > concatenation.js

Language Reference

This reference is structured so that it can be read from top to bottom, if you like. Later sections use ideas and syntax previously introduced. Familiarity with JavaScript is assumed. In all of the following examples, the source CoffeeScript is provided on the left, and the direct compilation into JavaScript is on the right.

Punctuation Primer You don't need to use semicolons ; to terminate expressions, ending the line will do just as well. All other whitespace is not significant. Instead of using curly braces { } to delimit a block of code, use a period . to mark the end of a block, for functions, if-statements, switch, and try/catch.

Functions and Invocation Functions are defined by a list of parameters, an arrow, and the function body. The empty function looks like this: =>.

square: x => x * x
cube:   x => square(x) * x

var cube, square;
square = function square(x) {
  return x * x;
};
cube = function cube(x) {
  return square(x) * x;
};

Assignment Use a colon : to assign, as in JSON. Equal signs are only needed for mathy things.

greeting: "Hello CoffeeScript"
difficulty: 0.5

var difficulty, greeting;
greeting = "Hello CoffeeScript";
difficulty = 0.5;

Declarations of new variables are pushed up to the top of the current scope, so that assignments may always be used within expressions.

Objects and Arrays Object and Array literals look very similar to their JavaScript cousins. When you spread out each assignment on a separate line, the commas are optional. In this way, assigning object properties looks the same as assigning local variables.

song: ["do", "re", "mi", "fa", "so"]
ages: {
  max: 10
  ida: 9
  tim: 11
}

var ages, song;
song = ["do", "re", "mi", "fa", "so"];
ages = {
  max: 10,
  ida: 9,
  tim: 11
};

Lexical Scoping and Variable Safety The CoffeeScript compiler takes care to make sure that all of your variables are properly declared within lexical scope — you never need to write var yourself.

num: 1
change_numbers: =>
  num: 2
  new_num: 3
new_num: change_numbers()

var change_numbers, new_num, num;
num = 1;
change_numbers = function change_numbers() {
  var new_num;
  num = 2;
  return (new_num = 3);
};
new_num = change_numbers();

Notice how the variables are declared with var the first time they appear. The second reference of num, within the function, is not redeclared because num is still in scope. As opposed to the second occurrence of new_num, in the last line.

Although suppressed within this documentation for clarity, all CoffeeScript output is wrapped in an anonymous function: (function(){ ... })(); This safety wrapper, combined with the automatic generation of the var keyword, make it exceedingly difficult to pollute the global namespace by accident.

Conditionals, Ternaries, and Conditional Assignment If/else statements can be written without the use of parentheses and curly brackets. As with functions and other block expressions, conditionals are closed with periods. No period is necessary when using the single-line postfix form, with the if at the end.

CoffeeScript will compile if statements using the ternary operator when possible, to make it easier to use the result as an expression.

mood: greatly_improved if singing

if happy and knows_it
  claps_hands()
  cha_cha_cha()

date: if friday then sue else jill

expensive ||= do_the_math()

var date, mood;
if (singing) {
  mood = greatly_improved;
}
if (happy && knows_it) {
  claps_hands();
  cha_cha_cha();
}
date = friday ? sue : jill;
expensive = expensive || do_the_math();

The conditional assignment operators are available: ||=, which only assigns a value to a variable if the variable's current value is falsy, and &&=, which only replaces the value of truthy variables.

Everything is an Expression (at least, as much as possible) You might have noticed how even though we don't add return statements to CoffeeScript functions, they nonetheless return their final value. The CoffeeScript compiler tries to make sure that all statements in the language can be used as expressions. Watch how the return gets pushed down into each possible branch of execution, in the function below.

grade: student =>
  if student.excellent_work
    "A+"
  else if student.okay_stuff
    if student.tried_hard then "B" else "B-"
  else
    "C"

eldest: if 24 > 21 then "Liz" else "Ike"

var eldest, grade;
grade = function grade(student) {
  if (student.excellent_work) {
    return "A+";
  } else if (student.okay_stuff) {
    return student.tried_hard ? "B" : "B-";
  } else {
    return "C";
  }
};
eldest = 24 > 21 ? "Liz" : "Ike";

The same mechanism is used to push down assignment through switch statements, and if-elses (although the ternary operator is preferred). Another part of manipulating assignment statements is CoffeeScript's declaration of new variables at the top of the current scope. This allows assignment to be used as a piece of an expression.

six: (one: 1) + (two: 2) + (three: 3)

var one, six, three, two;
six = (one = 1) + (two = 2) + (three = 3);

Aliases Because the == operator frequently causes undesirable coercion, is intransitive, and has a different meaning than in other languages, CoffeeScript compiles == into ===, and != into !==. In addition, is compiles into ===, and isnt into !==.

You can use not as an alias for !.

For logic, and compiles to &&, and or into ||.

Instead of a newline or semicolon, then can be used to separate conditions from expressions, in while, if/else, and switch/when statements.

As in YAML, on and yes are the same as boolean true, while off and no are boolean false.

For single-line statements, unless can be used as the inverse of if.

launch() if ignition is on

volume: 10 if band isnt spinal_tap

let_the_wild_rumpus_begin() unless answer is no

if car.speed < speed_limit then accelerate()

var volume;
if (ignition === true) {
  launch();
}
if (band !== spinal_tap) {
  volume = 10;
}
if (!(answer === false)) {
  let_the_wild_rumpus_begin();
}
car.speed < speed_limit ? accelerate() : null;

While Loops The only low-level loop that CoffeeScript provides is the while loop.

while demand > supply
  sell()
  restock()

while supply > demand then buy()

while (demand > supply) {
  sell();
  restock();
}
while (supply > demand) {
  buy();
}

Other JavaScript loops, such as for loops and do-while loops can be mimicked by variations on while, but the hope is that you won't need to do that with CoffeeScript, either because you're using each (forEach) style iterators, or...

Array Comprehensions For your looping needs, CoffeeScript provides array comprehensions similar to Python's. They replace (and compile into) for loops, with optional guard clauses and the value of the current array index. Unlike for loops, array comprehensions are expressions, and can be returned and assigned. They should be able to handle most places where you otherwise would use a loop, each/forEach, map, or select/filter.

# Eat lunch.
lunch: food.eat() for food in ['toast', 'cheese', 'wine']

# Zebra-stripe a table.
highlight(row) for row, i in table when i % 2 is 0

var __a, __b, __c, __d, __e, __f, __g, __h, food, i, lunch, row;
// Eat lunch.
__a = ['toast', 'cheese', 'wine'];
__d = [];
for (__b=0, __c=__a.length; __b<__c; __b++) {
  food = __a[__b];
  __d[__b] = food.eat();
}
lunch = __d;
// Zebra-stripe a table.
__e = table;
__h = [];
for (__f=0, __g=__e.length; __f<__g; __f++) {
  row = __e[__f];
  i = __f;
  __h[__f] = i % 2 === 0 ? highlight(row) : null;
}
__h;

If you're not iterating over an actual array, you can use a range to specify the start and end of an array comprehension: coundown(i) for i in [10..1].

Slicing Arrays with Ranges CoffeeScript borrows Ruby's range syntax for extracting slices of arrays. With two dots (3..5), the range is inclusive: the first argument is the index of the first element in the slice, and the second is the index of the last one. Three dots signify a range that excludes the end.

numbers: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

three_to_six: numbers[3..6]

numbers_copy: numbers[0...numbers.length]

var numbers, numbers_copy, three_to_six;
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
three_to_six = numbers.slice(3, 6 + 1);
numbers_copy = numbers.slice(0, numbers.length);

Inheritance, and Calling Super from a Subclass JavaScript's prototypal inheritance has always been a bit of a brain-bender, with a whole family tree of libraries that provide a cleaner syntax for classical inheritance on top of JavaScript's prototypes: Base2, Prototype.js, JS.Class, etc. The libraries provide syntactic sugar, but the built-in inheritance would be completely usable if it weren't for a couple of small exceptions: it's awkward to call super (the prototype object's implementation of the current function), and it's awkward to correctly set the prototype chain. CoffeeScript provides extends to help with prototype setup, and converts super() calls into calls against the immediate ancestor's method of the same name.

Animal: =>
Animal.prototype.move: meters =>
  alert(this.name + " moved " + meters + "m.")

Snake: name => this.name: name
Snake extends Animal
Snake.prototype.move: =>
  alert("Slithering...")
  super(5)

Horse: name => this.name: name
Horse extends Animal
Horse.prototype.move: =>
  alert("Galloping...")
  super(45)

sam: new Snake("Sammy the Python")
tom: new Horse("Tommy the Palomino")

sam.move()
tom.move()

var Animal, Horse, Snake, sam, tom;
Animal = function Animal() {
};
Animal.prototype.move = function move(meters) {
  return alert(this.name + " moved " + meters + "m.");
};
Snake = function Snake(name) {
  return (this.name = name);
};
Snake.__superClass__ = Animal.prototype;
Snake.prototype = new Animal();
Snake.prototype.constructor = Snake;
Snake.prototype.move = function move() {
  alert("Slithering...");
  return Snake.__superClass__.move.call(this, 5);
};
Horse = function Horse(name) {
  return (this.name = name);
};
Horse.__superClass__ = Animal.prototype;
Horse.prototype = new Animal();
Horse.prototype.constructor = Horse;
Horse.prototype.move = function move() {
  alert("Galloping...");
  return Horse.__superClass__.move.call(this, 45);
};
sam = new Snake("Sammy the Python");
tom = new Horse("Tommy the Palomino");
sam.move();
tom.move();

Embedded JavaScript If you ever need to interpolate literal JavaScript snippets, you can use backticks to pass JavaScript straight through.

hi: `function() {
  return [document.title, "Hello JavaScript"].join(": ");
}`

var hi;
hi = function() {
return [document.title, "Hello JavaScript"].join(": ");
};

Switch/When/Else Switch statements in JavaScript are rather broken. You can only do comparisons based on string equality, and need to remember to break at the end of every case statement to avoid accidentally falling through to the default case. CoffeeScript compiles switch statements into JavaScript if-else chains, allowing you to compare any object (via ===), preventing fall-through, and resulting in a returnable, assignable expression. The format is: switch condition, when clauses, else the default case.

switch day
  when "Tuesday"   then eat_breakfast()
  when "Wednesday" then go_to_the_park()
  when "Saturday"
    if day is bingo_day
      go_to_bingo()
      go_dancing()
  when "Sunday"    then go_to_church()
  else go_to_work()

if (day === "Tuesday") {
  eat_breakfast();
} else if (day === "Wednesday") {
  go_to_the_park();
} else if (day === "Saturday") {
  if (day === bingo_day) {
    go_to_bingo();
    go_dancing();
  }
} else if (day === "Sunday") {
  go_to_church();
} else {
  go_to_work();
}

Try/Catch/Finally Try/catch statements are just about the same as JavaScript (although they work as expressions).

try
  all_hell_breaks_loose()
  cats_and_dogs_living_together()
catch error
  print(error)
finally
  clean_up()

try {
  all_hell_breaks_loose();
  cats_and_dogs_living_together();
} catch (error) {
  print(error);
} finally {
  clean_up();
}

Multiline Strings Multiline strings are allowed in CoffeeScript.

moby_dick: "Call me Ishmael. Some years ago --
never mind how long precisely -- having little
or no money in my purse, and nothing particular
to interest me on shore, I thought I would sail
about a little and see the watery part of the
world..."

var moby_dick;
moby_dick = "Call me Ishmael. Some years ago -- \
never mind how long precisely -- having little \
or no money in my purse, and nothing particular \
to interest me on shore, I thought I would sail \
about a little and see the watery part of the \
world...";

Resources

Source Code
Bugs and Feature Requests

Contributing

Here's a wish list of things that would be wonderful to have in CoffeeScript:

• A JavaScript version of the compiler, perhaps using Alessandro Warth's OMeta.
• Ideas for alternate syntax to end blocks of expressions — the periods can look a little weird with deeply nested structure. (There's now a 'whitespace' branch — help add significant whitespace over there.)
• Test cases for any syntax errors you encounter that you think CoffeeScript should be able to compile properly.
• A tutorial that introduces CoffeeScript from the ground up for folks without knowledge of JavaScript.
• Integration with Processing.js's JavaScript API (this would depend on having a JavaScript version of the compiler).
• A lot of the code generation in nodes.rb gets into messy string manipulation. Techniques for cleaning this up across the board would be appreciated.

Change Log

0.1.6 Bugfix for running coffee --interactive and --run from outside of the CoffeeScript directory. Bugfix for nested function/if-statements.

0.1.5 Array slice literals and array comprehensions can now both take Ruby-style ranges to specify the start and end. JavaScript variable declaration is now pushed up to the top of the scope, making all assignment statements into expressions. You can use \ to escape newlines. The coffee-script command is now called coffee.

0.1.4 The official CoffeeScript extension is now .coffee instead of .cs, which properly belongs to C#. Due to popular demand, you can now also use = to assign. Unlike JavaScript, = can also be used within object literals, interchangeably with :. Made a grammatical fix for chained function calls like func(1)(2)(3)(4). Inheritance and super no longer use __proto__, so they should be IE-compatible now.

0.1.3 The coffee command now includes --interactive, which launches an interactive CoffeeScript session, and --run, which directly compiles and executes a script. Both options depend on a working installation of Narwhal. The aint keyword has been replaced by isnt, which goes together a little smoother with is. Quoted strings are now allowed as identifiers within object literals: eg. {"5+5": 10}. All assignment operators now use a colon: +:, -:, *:, etc.

0.1.2 Fixed a bug with calling super() through more than one level of inheritance, with the re-addition of the extends keyword. Added experimental Narwhal support (as a Tusk package), contributed by Tom Robinson, including bin/cs as a CoffeeScript REPL and interpreter. New --no-wrap option to suppress the safety function wrapper.

0.1.1 Added instanceof and typeof as operators.

0.1.0 Initial CoffeeScript release.