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. Until it reaches 1.0, there are no guarantees that the syntax won't change between versions. 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 pretty-printed and quite readable.

Latest Version: 0.9.3

## Mini Overview

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

    # Assignment:
    number   = 42
    opposite = true

    # Conditions:
    number = -42 if opposite

    # Functions:
    square = (x) -> x * x

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

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

    # Splats:
    race = (winner, runners...) ->
      print winner, runners

    # Existence:
    alert "I knew it!" if elvis?

    # Array comprehensions:
    cubes = math.cube num for num in list

    var _a, _b, _c, _d, cubes, list, math, num, number, opposite, race, square;
    var __slice = Array.prototype.slice;
    number = 42;
    opposite = true;
    if (opposite) {
      number = -42;
    }
    square = function(x) {
      return x * x;
    };
    list = [1, 2, 3, 4, 5];
    math = {
      root: Math.sqrt,
      square: square,
      cube: function(x) {
        return x * square(x);
      }
    };
    race = function(winner) {
      var runners;
      runners = __slice.call(arguments, 1);
      return print(winner, runners);
    };
    if (typeof elvis !== "undefined" && elvis !== null) {
      alert("I knew it!");
    }
    cubes = (function() {
      _a = []; _c = list;
      for (_b = 0, _d = _c.length; _b < _d; _b++) {
        num = _c[_b];
        _a.push(math.cube(num));
      }
      return _a;
    })();

For a longer CoffeeScript example, check out Underscore.coffee, a port of the Underscore.js library of helper functions. Underscore.coffee can pass the entire Underscore.js test suite. The CoffeeScript version is faster than the original for a number of methods (in general, due to the speed of CoffeeScript's array comprehensions), and after being minified and gzipped, is only 241 bytes larger than the original JavaScript version. Additional examples are included in the source repository, inside the examples folder.

## Installation and Usage

The CoffeeScript compiler is written in pure CoffeeScript, using a small DSL on top of the Jison parser generator, and is available as a Node.js utility. The core compiler however, does not depend on Node, and can be run in other server-side-JavaScript environments, or in the browser (see "Try CoffeeScript", above).

To install, first make sure you have a working copy of the latest tagged version of Node.js, currently 0.1.102 or higher. Then clone the CoffeeScript source repository from GitHub, or download the latest release: 0.9.3. To install the CoffeeScript compiler system-wide under `/usr/local`, open the directory and run:

    sudo bin/cake install

Alternatively, if you already have the Node Package Manager installed, you can use that to grab the latest CoffeeScript:

    sudo npm install coffee-script

Both of these provide the `coffee` command, which will execute CoffeeScripts under Node.js by default, but is also used to compile CoffeeScript `.coffee` files into JavaScript, or to run an 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:

`-c, --compile` | Compile a `.coffee` script into a `.js` JavaScript file of the same name.
`-i, --interactive` | Launch an interactive CoffeeScript session to try short snippets. More pleasant if wrapped with rlwrap.
`-o, --output [DIR]` | Write out all compiled JavaScript files into the specified directory. Use in conjunction with `--compile` or `--watch`.
`-w, --watch` | Watch the modification times of the coffee-scripts, recompiling as soon as a change occurs.
`-p, --print` | Instead of writing out the JavaScript as a file, print it directly to stdout.
`-l, --lint` | If the `jsl` (JavaScript Lint) command is installed, use it to check the compilation of a CoffeeScript file. (Handy in conjunction with `--watch`)
`-s, --stdio` | Pipe in CoffeeScript to STDIN and get back JavaScript over STDOUT. Good for use with processes written in other languages. An example: `cat src/cake.coffee | coffee -sc`
`-e, --eval` | Compile and print a little snippet of CoffeeScript directly from the command line. For example: `coffee -e "puts num for num in [10..1]"`
`-r, --require` | Load a library before compiling or executing your script. Can be used to hook in to the compiler (to add Growl notifications, for example).
`--no-wrap` | Compile the JavaScript without the top-level function safety wrapper. (Used for CoffeeScript as a Node.js module.)
`-t, --tokens` | Instead of parsing the CoffeeScript, just lex it, and print out the token stream: `[IDENTIFIER square] [ASSIGN =] [PARAM_START (]` ...
`-n, --nodes` | Instead of compiling the CoffeeScript, just lex and parse it, and print out the parse tree:
    Expressions
      Assign
        Value "square"
        Code "x"
          Op *
            Value "x"
            Value "x"

Examples:

    coffee -c 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.

Many of the examples can be run (where it makes sense) by pressing the "run" button towards the bottom right. You can also paste examples into "Try CoffeeScript" in the toolbar, and play with them from there.

Significant Whitespace CoffeeScript uses Python-style significant whitespace: You don't need to use semicolons `;` to terminate expressions, ending the line will do just as well. Semicolons can still be used to fit multiple expressions onto a single line. Instead of using curly braces `{ }` to delimit blocks of code (like functions, if-statements, switch, and try/catch), use indentation.

You don't need to use parentheses to invoke a function if you're passing arguments:
`print "coffee"`. Implicit parentheses wrap forwards to the end of the line, or block expression.

You can use newlines to break up your expression into smaller pieces, as long as CoffeeScript can determine that the line hasn't finished yet, because it ends with an operator or a dot ... seen most commonly in jQuery-chaining style JavaScript.

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

Objects and Arrays Object and Array literals look very similar to their JavaScript cousins. When you spread out each property on a separate line, the commas are optional. Implicit objects may be created with indentation instead of brackets, winding up looking quite similar to YAML.

    song = ["do", "re", "mi", "fa", "so"]

    singers = {Jagger: "Rock", Elvis: "Roll"}

    matrix = [
      1, 0, 1
      0, 0, 1
      1, 1, 0
    ]

    kids =
      brother:
        name: "Max"
        age:  11
      sister:
        name: "Ida"
        age:  9

    var kids, matrix, singers, song;
    song = ["do", "re", "mi", "fa", "so"];
    singers = {
      Jagger: "Rock",
      Elvis: "Roll"
    };
    matrix = [1, 0, 1, 0, 0, 1, 1, 1, 0];
    kids = {
      brother: {
        name: "Max",
        age: 11
      },
      sister: {
        name: "Ida",
        age: 9
      }
    };

In JavaScript, you can't use reserved words, like `class`, as properties of an object, without quoting them as strings. CoffeeScript notices and quotes them for you, so you don't have to worry about it (say, when using jQuery).

    $('.account').css class: 'active'

    $('.account').css({
      "class": 'active'
    });

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.

    outer = 1
    changeNumbers = ->
      inner = -1
      outer = 10
    inner = changeNumbers()

    var changeNumbers, inner, outer;
    outer = 1;
    changeNumbers = function() {
      var inner;
      inner = -1;
      return (outer = 10);
    };
    inner = changeNumbers();

Notice how all of the variable declarations have been pushed up to the top of the closest scope, the first time they appear. outer is not redeclared within the inner function, because it's already in scope; inner within the function, on the other hand, should not be able to change the value of the external variable of the same name, and therefore has a declaration of its own.

This behavior is effectively identical to Ruby's scope for local variables. Because you don't have direct access to the `var` keyword, it's impossible to shadow an outer variable on purpose, you may only refer to it. So be careful that you're not reusing the name of an external variable accidentally, if you're writing a deeply nested function.

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.

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: `root = exports ? this`

If, Else, Unless, and Conditional Assignment If/else statements can be written without the use of parentheses and curly brackets. As with functions and other block expressions, multi-line conditionals are delimited by indentation. There's also a handy postfix form, with the `if` or `unless` at the end.

CoffeeScript can compile if statements into JavaScript expressions, using the ternary operator when possible, and closure wrapping otherwise. There is no explicit ternary statement in CoffeeScript — you simply use a regular if statement inline.

    mood = greatlyImproved if singing

    if happy and knowsIt
      clapsHands()
      chaChaCha()
    else
      showIt()

    date = if friday then sue else jill

    options or= defaults

    var date, mood, options;
    if (singing) {
      mood = greatlyImproved;
    }
    if (happy && knowsIt) {
      clapsHands();
      chaChaCha();
    } else {
      showIt();
    }
    date = friday ? sue : jill;
    options || (options = defaults);

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`.

As a shortcut for `this.property`, you can use `@property`.

You can use `in` to test for array presence, and `of` to test for JavaScript object-key presence.

    launch() if ignition is on

    volume = 10 if band isnt SpinalTap

    letTheWildRumpusBegin() unless answer is no

    if car.speed < limit then accelerate()

    winner = yes if pick in [47, 92, 13]

    print inspect "My name is " + @name

    var volume, winner;
    if (ignition === true) {
      launch();
    }
    if (band !== SpinalTap) {
      volume = 10;
    }
    if (answer !== false) {
      letTheWildRumpusBegin();
    }
    if (car.speed < limit) {
      accelerate();
    }
    if ((47 === pick || 92 === pick || 13 === pick)) {
      winner = true;
    }
    print(inspect("My name is " + this.name));

Splats... The JavaScript arguments object is a useful way to work with functions that accept variable numbers of arguments. CoffeeScript provides splats `...`, both for function definition as well as invocation, making variable numbers of arguments a little bit more palatable.

    gold = silver = rest = "unknown"

    awardMedals = (first, second, others...) ->
      gold   = first
      silver = second
      rest   = others

    contenders = [
      "Michael Phelps"
      "Liu Xiang"
      "Yao Ming"
      "Allyson Felix"
      "Shawn Johnson"
      "Roman Sebrle"
      "Guo Jingjing"
      "Tyson Gay"
      "Asafa Powell"
      "Usain Bolt"
    ]

    awardMedals contenders...

    alert "Gold: " + gold
    alert "Silver: " + silver
    alert "The Field: " + rest

    var awardMedals, contenders, gold, rest, silver;
    var __slice = Array.prototype.slice;
    gold = (silver = (rest = "unknown"));
    awardMedals = function(first, second) {
      var others;
      others = __slice.call(arguments, 2);
      gold = first;
      silver = second;
      return (rest = others);
    };
    contenders = ["Michael Phelps", "Liu Xiang", "Yao Ming", "Allyson Felix", "Shawn Johnson", "Roman Sebrle", "Guo Jingjing", "Tyson Gay", "Asafa Powell", "Usain Bolt"];
    awardMedals.apply(this, contenders);
    alert("Gold: " + gold);
    alert("Silver: " + silver);
    alert("The Field: " + rest);

While, Until, and Loop The only low-level loop that CoffeeScript provides is the while loop. The main difference from JavaScript is that the while loop can be used as an expression, returning an array containing the result of each iteration through the loop.

    # Econ 101
    if this.studyingEconomics
      buy()  while supply > demand
      sell() until supply > demand

    # Nursery Rhyme
    num = 6
    lyrics = while num -= 1
      num + " little monkeys, jumping on the bed.
        One fell out and bumped his head."

    var _a, lyrics, num;
    if (this.studyingEconomics) {
      while (supply > demand) {
        buy();
      }
      while (!(supply > demand)) {
        sell();
      }
    }
    num = 6;
    lyrics = (function() {
      _a = [];
      while (num -= 1) {
        _a.push(num + " little monkeys, jumping on the bed.\
        One fell out and bumped his head.");
      }
      return _a;
    })();

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

Comprehensions (Arrays, Objects, and Ranges) 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 = eat food for food in ['toast', 'cheese', 'wine']

    # Naive collision detection.
    for roid in asteroids
      for roid2 in asteroids when roid isnt roid2
        roid.explode() if roid.overlaps roid2

    var _a, _b, _c, _d, _e, _f, _g, _h, _i, _j, food, lunch, roid, roid2;
    lunch = (function() {
      _a = []; _c = ['toast', 'cheese', 'wine'];
      for (_b = 0, _d = _c.length; _b < _d; _b++) {
        food = _c[_b];
        _a.push(eat(food));
      }
      return _a;
    })();
    _f = asteroids;
    for (_e = 0, _g = _f.length; _e < _g; _e++) {
      roid = _f[_e];
      _i = asteroids;
      for (_h = 0, _j = _i.length; _h < _j; _h++) {
        roid2 = _i[_h];
        if (roid !== roid2) {
          if (roid.overlaps(roid2)) {
            roid.explode();
          }
        }
      }
    }

If you know the start and end of your loop, or would like to step through in fixed-size increments, you can use a range to specify the start and end of your comprehension.

    countdown = num for num in [10..1]

    deliverEggs = ->
      for i in [0...eggs.length] by 12
        dozen = eggs[i...i+12]
        deliver new eggCarton dozen

    var _a, countdown, deliverEggs, num;
    countdown = (function() {
      _a = [];
      for (num = 10; num >= 1; num--) {
        _a.push(num);
      }
      return _a;
    })();
    deliverEggs = function() {
      var _b, _c, dozen, i;
      _b = []; _c = eggs.length;
      for (i = 0; (0 <= _c ? i < _c : i > _c); i += 12) {
        _b.push((function() {
          dozen = eggs.slice(i, i + 12);
          return deliver(new eggCarton(dozen));
        })());
      }
      return _b;
    };

Comprehensions can also be used to iterate over the keys and values in an object. Use `of` to signal comprehension over the properties of an object instead of the values in an array.

    yearsOld = max: 10, ida: 9, tim: 11

    ages = for child, age of yearsOld
      child + " is " + age

    var _a, _b, age, ages, child, yearsOld;
    var __hasProp = Object.prototype.hasOwnProperty;
    yearsOld = {
      max: 10,
      ida: 9,
      tim: 11
    };
    ages = (function() {
      _a = []; _b = yearsOld;
      for (child in _b) {
        if (!__hasProp.call(_b, child)) continue;
        age = _b[child];
        _a.push(child + " is " + age);
      }
      return _a;
    })();

By default, object comprehensions are safe, and use a `hasOwnProperty` check to make sure that you're dealing with properties on the current object. If you'd like the regular JavaScript
`for (key in obj) ...` loop, for speed or for another reason, you can use
`for all key, value of object` in CoffeeScript.

Array Slicing and Splicing 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]

    threeToSix = numbers[3..6]

    copy = numbers[0...numbers.length]

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

The same syntax can be used with assignment to replace a segment of an array with new values (to splice it).

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

    numbers[3..6] = [-3, -4, -5, -6]

    var numbers;
    numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
    numbers.splice.apply(numbers, [3, 6 - 3 + 1].concat([-3, -4, -5, -6]));

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.excellentWork
        "A+"
      else if student.okayStuff
        if student.triedHard then "B" else "B-"
      else
        "C"

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

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

Even though functions will always return their final value, it's both possible and encouraged to return early from a function body writing out the explicit return (`return value`), when you know that you're done.

Because variable declarations occur at the top of scope, assignment can be used within expressions, even for variables that haven't been seen before:

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

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

Things that would otherwise be statements in JavaScript, when used as part of an expression in CoffeeScript, are converted into expressions by wrapping them in a closure. This lets you do useful things, like assign the result of a comprehension to a variable:

    # The first ten global properties.

    globals = (name for name of window)[0...10]

    var _a, _b, _c, globals, name;
    var __hasProp = Object.prototype.hasOwnProperty;
    globals = (function() {
      _b = []; _c = window;
      for (name in _c) {
        if (!__hasProp.call(_c, name)) continue;
        _a = _c[name];
        _b.push(name);
      }
      return _b;
    })().slice(0, 10);

As well as silly things, like passing a try/catch statement directly into a function call:

    alert(
      try
        nonexistent / undefined
      catch error
        "And the error is ... " + error
    )

    alert((function() {
      try {
        return nonexistent / undefined;
      } catch (error) {
        return "And the error is ... " + error;
      }
    })());

There are a handful of statements in JavaScript that can't be meaningfully converted into expressions, namely `break`, `continue`, and `return`. If you make use of them within a block of code, CoffeeScript won't try to perform the conversion.

The Existential Operator It's a little difficult to check for the existence of a variable in JavaScript. `if (variable) ...` comes close, but fails for zero, the empty string, and false. CoffeeScript's existential operator `?` returns true unless a variable is null or undefined, which makes it analogous to Ruby's `nil?`

It can also be used for safer conditional assignment than `||=` provides, for cases where you may be handling numbers or strings.

    solipsism = true if mind? and not world?

    speed ?= 140

    var solipsism, speed;
    if ((typeof mind !== "undefined" && mind !== null) && !(typeof world !== "undefined" && world !== null)) {
      solipsism = true;
    }
    speed = (typeof speed !== "undefined" && speed !== null) ? speed : 140;

The accessor variant of the existential operator `?.` can be used to soak up null references in a chain of properties. Use it instead of the dot accessor `.` in cases where the base value may be null or undefined. If all of the properties exist then you'll get the expected result, if the chain is broken, undefined is returned instead of the TypeError that would be raised otherwise.

    lottery.drawWinner()?.address?.zipcode

    var _a, _b;
    (typeof (_b = ((_a = lottery.drawWinner()))) === "undefined" || _b === null) ? undefined : _b.address == null ? undefined : _b.address.zipcode;

Soaking up nulls is similar to Ruby's andand gem, and to the safe navigation operator in Groovy.

Classes, Inheritance, and Super 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.

Instead of repetitively attaching functions to a prototype, CoffeeScript provides a basic `class` structure that allows you to name your class, set the superclass, assign prototypal properties, and define the constructor, in a single assignable expression.

    class Animal
      constructor: (@name) ->

      move: (meters) ->
        alert @name + " moved " + meters + "m."

    class Snake extends Animal
      move: ->
        alert "Slithering..."
        super 5

    class Horse extends Animal
      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;
    var __extends = function(child, parent) {
        var ctor = function(){};
        ctor.prototype = parent.prototype;
        child.prototype = new ctor();
        child.prototype.constructor = child;
        if (typeof parent.extended === "function") parent.extended(child);
        child.__super__ = parent.prototype;
      };
    Animal = function(_a) {
      this.name = _a;
      return this;
    };
    Animal.prototype.move = function(meters) {
      return alert(this.name + " moved " + meters + "m.");
    };
    Snake = function() {
      return Animal.apply(this, arguments);
    };
    __extends(Snake, Animal);
    Snake.prototype.move = function() {
      alert("Slithering...");
      return Snake.__super__.move.call(this, 5);
    };
    Horse = function() {
      return Animal.apply(this, arguments);
    };
    __extends(Horse, Animal);
    Horse.prototype.move = function() {
      alert("Galloping...");
      return Horse.__super__.move.call(this, 45);
    };
    sam = new Snake("Sammy the Python");
    tom = new Horse("Tommy the Palomino");
    sam.move();
    tom.move();

If structuring your prototypes classically isn't your cup of tea, CoffeeScript provides a couple of lower-level conveniences. The `extends` operator helps with proper prototype setup, `::` gives you quick access to an object's prototype, and `super()` is converted into a call against the immediate ancestor's method of the same name.

    String::dasherize = ->
      this.replace /_/g, "-"

    String.prototype.dasherize = function() {
      return this.replace(/_/g, "-");
    };

Finally, you may assign Class-level (static) properties within a class definition by using
`@property = value`

Pattern Matching (Destructuring Assignment) To make extracting values from complex arrays and objects more convenient, CoffeeScript implements ECMAScript Harmony's proposed destructuring assignment syntax. When you assign an array or object literal to a value, CoffeeScript breaks up and matches both sides against each other, assigning the values on the right to the variables on the left. In the simplest case, it can be used for parallel assignment:

    theBait   = 1000
    theSwitch = 0

    [theBait, theSwitch] = [theSwitch, theBait]

    var _a, theBait, theSwitch;
    theBait = 1000;
    theSwitch = 0;
    _a = [theSwitch, theBait];
    theBait = _a[0];
    theSwitch = _a[1];

But it's also helpful for dealing with functions that return multiple values.

    weatherReport = (location) ->
      # Make an Ajax request to fetch the weather...
      [location, 72, "Mostly Sunny"]

    [city, temp, forecast] = weatherReport "Berkeley, CA"

    var _a, city, forecast, temp, weatherReport;
    weatherReport = function(location) {
      return [location, 72, "Mostly Sunny"];
    };
    _a = weatherReport("Berkeley, CA");
    city = _a[0];
    temp = _a[1];
    forecast = _a[2];

Pattern matching can be used with any depth of array and object nesting, to help pull out deeply nested properties.

    futurists =
      sculptor: "Umberto Boccioni"
      painter:  "Vladimir Burliuk"
      poet:
        name:   "F.T. Marinetti"
        address: [
          "Via Roma 42R"
          "Bellagio, Italy 22021"
        ]

    {poet: {name, address: [street, city]}} = futurists

    var _a, _b, _c, city, futurists, name, street;
    futurists = {
      sculptor: "Umberto Boccioni",
      painter: "Vladimir Burliuk",
      poet: {
        name: "F.T. Marinetti",
        address: ["Via Roma 42R", "Bellagio, Italy 22021"]
      }
    };
    _a = futurists;
    _b = _a.poet;
    name = _b.name;
    _c = _b.address;
    street = _c[0];
    city = _c[1];

Pattern matching can even be combined with splats.

    tag = "<impossible>"

    [open, contents..., close] = tag.split("")

    var _a, close, contents, open, tag;
    var __slice = Array.prototype.slice;
    tag = "<impossible>";
    _a = tag.split("");
    open = _a[0];
    contents = __slice.call(_a, 1, _a.length - 1);
    close = _a[_a.length - 1];

Function binding In JavaScript, the `this` keyword is dynamically scoped to mean the object that the current function is attached to. If you pass a function as as callback, or attach it to a different object, the original value of `this` will be lost. If you're not familiar with this behavior, this Digital Web article gives a good overview of the quirks.

The fat arrow `=>` can be used to both define a function, and to bind it to the current value of `this`, right on the spot. This is helpful when using callback-based libraries like Prototype or jQuery, for creating iterator functions to pass to `each`, or event-handler functions to use with `bind`. Functions created with the fat arrow are able to access properties of the `this` where they're defined.

    Account = (customer, cart) ->
      @customer = customer
      @cart = cart

      $('.shopping_cart').bind 'click', (event) =>
        @customer.purchase @cart

    var Account;
    var __bind = function(func, context) {
        return function(){ return func.apply(context, arguments); };
      };
    Account = function(customer, cart) {
      this.customer = customer;
      this.cart = cart;
      return $('.shopping_cart').bind('click', __bind(function(event) {
        return this.customer.purchase(this.cart);
      }, this));
    };

If we had used `->` in the callback above, `@customer` would have referred to the undefined "customer" property of the DOM element, and trying to call `purchase()` on it would have raised an exception.

Embedded JavaScript Hopefully, you'll never need to use it, but if you ever need to intersperse snippets of JavaScript within your CoffeeScript, you can use backticks to pass it 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 a bit awkward. You need to remember to break at the end of every case statement to avoid accidentally falling through to the default case. CoffeeScript prevents accidental fall-through, and can convert the `switch` into a returnable, assignable expression. The format is: `switch` condition, `when` clauses, `else` the default case.

As in Ruby, switch statements in CoffeeScript can take multiple values for each when clause. If any of the values match, the clause runs.

    switch day
      when "Mon" then go work
      when "Tue" then go relax
      when "Thu" then go iceFishing
      when "Fri", "Sat"
        if day is bingoDay
          go bingo
          go dancing
      when "Sun" then go church
      else go work

    switch (day) {
    case "Mon":
      go(work);
      break;
    case "Tue":
      go(relax);
      break;
    case "Thu":
      go(iceFishing);
      break;
    case "Fri":
    case "Sat":
      if (day === bingoDay) {
        go(bingo);
        go(dancing);
      }
      break;
    case "Sun":
      go(church);
      break;
    default:
      go(work);
    }

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

    try
      allHellBreaksLoose()
      catsAndDogsLivingTogether()
    catch error
      print error
    finally
      cleanUp()

    try {
      allHellBreaksLoose();
      catsAndDogsLivingTogether();
    } catch (error) {
      print(error);
    } finally {
      cleanUp();
    }

Chained Comparisons CoffeeScript borrows chained comparisons from Python — making it easy to test if a value falls within a certain range.

    cholesterol = 127

    healthy = 200 > cholesterol > 60

    var cholesterol, healthy;
    cholesterol = 127;
    healthy = (200 > cholesterol) && (cholesterol > 60);

String and RegExp Interpolation Ruby-style string interpolation is included in CoffeeScript. Double-quoted strings allow for interpolated values, while single-quoted strings are literal.

    author = "Wittgenstein"
    quote  = "A picture is a fact. -- #{author}"

    var author, quote;
    author = "Wittgenstein";
    quote = ("A picture is a fact. -- " + (author));

And arbitrary expressions can be interpolated by using brackets `#{ ... }`
Interpolation works the same way within regular expressions.

    sentence = "#{ 22 / 7 } is a decent approximation of π"

    sep   = "[.\\/\\- ]"
    dates = /\d+#{sep}\d+#{sep}\d+/g

    var dates, sentence, sep;
    sentence = ("" + (22 / 7) + " is a decent approximation of π");
    sep = "[.\\/\\- ]";
    dates = (new RegExp("\\d+" + (sep) + "\\d+" + (sep) + "\\d+", "g"));

Multiline Strings, Heredocs, and Block Comments Multiline strings are allowed in CoffeeScript.

    mobyDick = "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 mobyDick;
    mobyDick = "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...";

Heredocs can be used to hold formatted or indentation-sensitive text (or, if you just don't feel like escaping quotes and apostrophes). The indentation level that begins the heredoc is maintained throughout, so you can keep it all aligned with the body of your code.

    html = '''
           <strong>
             cup of coffeescript
           </strong>
           '''

    var html;
    html = '<strong>\n  cup of coffeescript\n</strong>';

Double-quoted heredocs, like double-quoted strings, allow interpolation.

Sometimes you'd like to pass a block comment through to the generated JavaScript. For example, when you need to embed a licensing header at the top of a file. Block comments, which mirror the synax for heredocs, are preserved in the generated code.

    ###
    CoffeeScript Compiler v0.9.3
    Released under the MIT License
    ###

    /*CoffeeScript Compiler v0.9.3
    Released under the MIT License
    */

## Cake, and Cakefiles

CoffeeScript includes a simple build system similar to Make and Rake. Naturally, it's called Cake, and is used for the build and test tasks for the CoffeeScript language itself. Tasks are defined in a file named `Cakefile`, and can be invoked by running `cake taskname` from within the directory. To print a list of all the tasks and options, just run `cake`.

Task definitions are written in CoffeeScript, so you can put arbitrary code in your Cakefile. Define a task with a name, a long description, and the function to invoke when the task is run. If your task takes a command-line option, you can define the option with short and long flags, and it will be made available in the `options` object. Here's a task that uses the Node.js API to rebuild CoffeeScript's parser:

    fs = require 'fs'

    option '-o', '--output [DIR]', 'directory for compiled code'

    task 'build:parser', 'rebuild the Jison parser', (options) ->
      require 'jison'
      code = require('./lib/grammar').parser.generate()
      dir  = options.output or 'lib'
      fs.writeFile "#{dir}/parser.js", code

    var fs;
    fs = require('fs');
    option('-o', '--output [DIR]', 'directory for compiled code');
    task('build:parser', 'rebuild the Jison parser', function(options) {
      var code, dir;
      require('jison');
      code = require('./lib/grammar').parser.generate();
      dir = options.output || 'lib';
      return fs.writeFile("" + (dir) + "/parser.js", code);
    });

## "text/coffeescript" Script Tags

While it's not recommended for serious use, CoffeeScripts may be included directly within the browser using `<script type="text/coffeescript">` tags. The source includes a compressed and minified version of the compiler (Download current version here, 43k when gzipped) as `extras/coffee-script.js`. Include this file on a page with inline CoffeeScript tags, and it will compile and evaluate them in order.

In fact, the little bit of glue script that runs "Try CoffeeScript" above, as well as jQuery for the menu, is implemented in just this way. View source and look at the bottom of the page to see the example. Including the script also gives you access to `CoffeeScript.compile()` so you can pop open Firebug and try compiling some strings.

The usual caveats about CoffeeScript apply — your inline scripts will run within a closure wrapper, so if you want to expose global variables or functions, attach them to the `window` object.

## Resources

- Source Code
  Use `bin/coffee` to test your changes,
  `bin/cake test` to run the test suite,
  `bin/cake build` to rebuild the CoffeeScript compiler, and
  `bin/cake build:parser` to regenerate the Jison parser if you're working on the grammar.
  `git checkout lib && bin/cake build:full` is a good command to run when you're working on the core language. It'll refresh the lib directory (in case you broke something), build your altered compiler, use that to rebuild itself (a good sanity test) and then run all of the tests. If they pass, there's a good chance you've made a successful change.
- CoffeeScript Issues
  Bugs reports, feature requests, and general discussion all belong here.
- If you'd like to chat, stop by `#coffeescript` on Freenode in the IRC client of your choice, or on webchat.freenode.net.
- yeungda's JCoffeeScript — A Java Library that uses Rhino to compile CoffeeScript, allowing compilation within Java projects or on systems that Node.js doesn't support.
- defunkt's CoffeeScript Major Mode — a Emacs major mode that provides syntax highlighting, indentation support, and some bonus commands.
- jashkenas's CoffeeScript TextMate Bundle — which provides syntax highlighting, snippet expansion, and the ability to run bits of CoffeeScript from within TextMate itself.
- kchmck's Vim CoffeeScript — which adds Vim syntax highlighting and indentation support.
- wavded's gedit-coffeescript — a CoffeeScript syntax highlighter for the gedit text editor.
- yeungda's coffeescript-idea — a plugin for IntelliJ IDEA and RubyMine providing syntax highlighting.
- mattly's rack-coffee — a small Rack middleware for serving CoffeeScript files as compiled JavaScript on the fly.
- jnicklas's BistroCar — a plugin that serves and bundles CoffeeScript from within your Rails application.
- dsc's CoffeeCup — a Python WSGI middleware that compiles CoffeeScript to JavaScript on-demand during development.
- sutto's Barista — a BistroCar alternative that integrates well with Jammit and Rails 3.
- inem and gerad's coffee-haml-filter — a custom filter for rendering CoffeeScript inline within HAML templates.
- chrislloyd's Roast — a CoffeeScript compiler plug-in that allows you to include external source files.
- jashkenas's Docco — a quick-and-dirty literate-programming-style documentation generator for CoffeeScript. Used to produce the annotated source.

## Web Chat (IRC)

Quick help and advice can usually be found in the CoffeeScript IRC room. Join `#coffeescript` on `irc.freenode.net`, or click the button below to open a webchat session on this page.

## Change Log

0.9.3 CoffeeScript `switch` statements now compile into JS `switch` statements — they previously compiled into `if/else` chains for JavaScript 1.3 compatibility. Soaking a function invocation is now supported. Users of the RubyMine editor should now be able to use `--watch` mode.

0.9.2 Specifying the start and end of a range literal is now optional, eg. `array[3..]`. You can now say `a not instanceof b`. Fixed important bugs with nested significant and non-significant indentation (Issue #637). Added a `--require` flag that allows you to hook into the `coffee` command. Added a custom `jsl.conf` file for our preferred JavaScriptLint setup. Sped up Jison grammar compilation time by flattening rules for operations. Block comments can now be used with JavaScript-minifier-friendly syntax. Added JavaScript's compound assignment bitwise operators. Bugfixes to implicit object literals with leading number and string keys, as the subject of implicit calls, and as part of compound assignment.

0.9.1 Bugfix release for 0.9.1. Greatly improves the handling of mixed implicit objects, implicit function calls, and implicit indentation. String and regex interpolation is now strictly `#{ ... }` (Ruby style). The compiler now takes a `--require` flag, which specifies scripts to run before compilation.

0.9.0 The CoffeeScript 0.9 series is considered to be a release candidate for 1.0; let's give her a shakedown cruise. 0.9.0 introduces a massive backwards-incompatible change: Assignment now uses `=`, and object literals use `:`, as in JavaScript. This allows us to have implicit object literals, and YAML-style object definitions. Half assignments are removed, in favor of `+=`, `or=`, and friends. Interpolation now uses a hash mark `#` instead of the dollar sign `$` — because dollar signs may be part of a valid JS identifier. Downwards range comprehensions are now safe again, and are optimized to straight for loops when created with integer endpoints. A fast, unguarded form of object comprehension was added: `for all key, value of object`. Mentioning the `super` keyword with no arguments now forwards all arguments passed to the function, as in Ruby. If you extend class `B` from parent class `A`, if `A` has an `extended` method defined, it will be called, passing in `B` — this enables static inheritance, among other things. Cleaner output for functions bound with the fat arrow. `@variables` can now be used in parameter lists, with the parameter being automatically set as a property on the object — useful in constructors and setter functions. Constructor functions can now take splats.

0.7.2 Quick bugfix (right after 0.7.1) for a problem that prevented `coffee` command-line options from being parsed in some circumstances.

0.7.1 Block-style comments are now passed through and printed as JavaScript block comments -- making them useful for licenses and copyright headers. Better support for running coffee scripts standalone via hashbangs. Improved syntax errors for tokens that are not in the grammar.

0.7.0 Official CoffeeScript variable style is now camelCase, as in JavaScript. Reserved words are now allowed as object keys, and will be quoted for you. Range comprehensions now generate cleaner code, but you have to specify `by -1` if you'd like to iterate downward. Reporting of syntax errors is greatly improved from the previous release. Running `coffee` with no arguments now launches the REPL, with Readline support. The `<-` bind operator has been removed from CoffeeScript. The `loop` keyword was added, which is equivalent to a `while true` loop. Comprehensions that contain closures will now close over their variables, like the semantics of a `forEach`. You can now use bound function in class definitions (bound to the instance). For consistency, `a in b` is now an array presence check, and `a of b` is an object-key check. Comments are no longer passed through to the generated JavaScript.

0.6.2 The `coffee` command will now preserve directory structure when compiling a directory full of scripts. Fixed two omissions that were preventing the CoffeeScript compiler from running live within Internet Explorer. There's now a syntax for block comments, similar in spirit to CoffeeScript's heredocs. ECMA Harmony DRY-style pattern matching is now supported, where the name of the property is the same as the name of the value: `{name, length}: func`. Pattern matching is now allowed within comprehension variables. `unless` is now allowed in block form. `until` loops were added, as the inverse of `while` loops. `switch` statements are now allowed without switch object clauses. Compatible with Node.js v0.1.95.

0.6.1 Upgraded CoffeeScript for compatibility with the new Node.js v0.1.90 series.

0.6.0 Trailing commas are now allowed, a-la Python. Static properties may be assigned directly within class definitions, using `@property` notation.

0.5.6 Interpolation can now be used within regular expressions and heredocs, as well as strings. Added the `<-` bind operator. Allowing assignment to half-expressions instead of special `||=`-style operators. The arguments object is no longer automatically converted into an array. After requiring `coffee-script`, Node.js can now directly load `.coffee` files, thanks to registerExtension. Multiple splats can now be used in function calls, arrays, and pattern matching.

0.5.5 String interpolation, contributed by Stan Angeloff. Since `--run` has been the default since 0.5.3, updating `--stdio` and `--eval` to run by default, pass `--compile` as well if you'd like to print the result.

0.5.4 Bugfix that corrects the Node.js global constants `__filename` and `__dirname`. Tweaks for more flexible parsing of nested function literals and improperly-indented comments. Updates for the latest Node.js API.

0.5.3 CoffeeScript now has a syntax for defining classes. Many of the core components (Nodes, Lexer, Rewriter, Scope, Optparse) are using them. Cakefiles can use `optparse.coffee` to define options for tasks. `--run` is now the default flag for the `coffee` command, use `--compile` to save JavaScripts. Bugfix for an ambiguity between RegExp literals and chained divisions.

0.5.2 Added a compressed version of the compiler for inclusion in web pages as
`extras/coffee-script.js`. It'll automatically run any script tags with type `text/coffeescript` for you. Added a `--stdio` option to the `coffee` command, for piped-in compiles.

0.5.1 Improvements to null soaking with the existential operator, including soaks on indexed properties. Added conditions to `while` loops, so you can use them as filters with `when`, in the same manner as comprehensions.

0.5.0 CoffeeScript 0.5.0 is a major release, While there are no language changes, the Ruby compiler has been removed in favor of a self-hosting compiler written in pure CoffeeScript.

0.3.2 `@property` is now a shorthand for `this.property`.
Switched the default JavaScript engine from Narwhal to Node.js. Pass the `--narwhal` flag if you'd like to continue using it.

0.3.0 CoffeeScript 0.3 includes major syntax changes:
The function symbol was changed to `->`, and the bound function symbol is now `=>`.
Parameter lists in function definitions must now be wrapped in parentheses.
Added property soaking, with the `?.` operator.
Made parentheses optional, when invoking functions with arguments.
Removed the obsolete block literal syntax.

0.2.6 Added Python-style chained comparisons, the conditional existence operator `?=`, and some examples from Beautiful Code. Bugfixes relating to statement-to-expression conversion, arguments-to-array conversion, and the TextMate syntax highlighter.

0.2.5 The conditions in switch statements can now take multiple values at once — If any of them are true, the case will run. Added the long arrow `==>`, which defines and immediately binds a function to `this`. While loops can now be used as expressions, in the same way that comprehensions can. Splats can be used within pattern matches to soak up the rest of an array.

0.2.4 Added ECMAScript Harmony style destructuring assignment, for dealing with extracting values from nested arrays and objects. Added indentation-sensitive heredocs for nicely formatted strings or chunks of code.

0.2.3 Axed the unsatisfactory `ino` keyword, replacing it with `of` for object comprehensions. They now look like: `for prop, value of object`.

0.2.2 When performing a comprehension over an object, use `ino`, instead of `in`, which helps us generate smaller, more efficient code at compile time.
Added `::` as a shorthand for saying `.prototype.`
The "splat" symbol has been changed from a prefix asterisk `*`, to a postfix ellipsis `...`
Added JavaScript's `in` operator, empty `return` statements, and empty `while` loops.
Constructor functions that start with capital letters now include a safety check to make sure that the new instance of the object is returned.
The `extends` keyword now functions identically to `goog.inherits` in Google's Closure Library.

0.2.1 Arguments objects are now converted into real arrays when referenced.

0.2.0 Major release. Significant whitespace. Better statement-to-expression conversion. Splats. Splice literals. Object comprehensions. Blocks. The existential operator. Many thanks to all the folks who posted issues, with special thanks to Liam O'Connor-Davis for whitespace and expression help.

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.