CoffeeScript is a little language that compiles into JavaScript. Underneath that awkward Java-esque patina, JavaScript has always had a gorgeous heart. CoffeeScript is an attempt to expose the good parts of JavaScript in a simple way.

The golden rule of CoffeeScript is: "It's just JavaScript". The code compiles one-to-one into the equivalent JS, and there is no interpretation at runtime. You can use any existing JavaScript library seamlessly from CoffeeScript (and vice-versa). The compiled output is readable and pretty-printed, will work in every JavaScript runtime, and tends to run as fast or faster than the equivalent handwritten JavaScript.

Latest Version: 1.10.0

    npm install -g coffee-script

## 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 cubes, list, math, num, number, opposite, race, square,
      slice = [].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() {
      var runners, winner;
      winner = arguments[0], runners = 2 <= arguments.length ? slice.call(arguments, 1) : [];
      return print(winner, runners);
    };

    if (typeof elvis !== "undefined" && elvis !== null) {
      alert("I knew it!");
    }

    cubes = (function() {
      var i, len, results;
      results = [];
      for (i = 0, len = list.length; i < len; i++) {
        num = list[i];
        results.push(math.cube(num));
      }
      return results;
    })();

run: cubes

## Installation

The CoffeeScript compiler is itself written in CoffeeScript, using the Jison parser generator. The command-line version of `coffee` is available as a Node.js utility. The core compiler however, does not depend on Node, and can be run in any JavaScript environment, or in the browser (see "Try CoffeeScript", above).

To install, first make sure you have a working copy of the latest stable version of Node.js. You can then install CoffeeScript globally with npm:

    npm install -g coffee-script

When you need CoffeeScript as a dependency, install it locally:

    npm install --save coffee-script

If you'd prefer to install the latest master version of CoffeeScript, you can clone the CoffeeScript source repository from GitHub, or download the source directly. To install the latest master CoffeeScript compiler with npm:

    npm install -g jashkenas/coffeescript

Or, if you want to install to `/usr/local`, and don't want to use npm to manage it, open the `coffee-script` directory and run:

    sudo bin/cake install

## Usage

Once installed, you should have access to the `coffee` command, which can execute scripts, compile `.coffee` files into `.js`, and provide an interactive REPL. The `coffee` command takes the following options:

`-c, --compile` | Compile a `.coffee` script into a `.js` JavaScript file of the same name.
`-m, --map` | Generate source maps alongside the compiled JavaScript files. Adds `sourceMappingURL` directives to the JavaScript as well.
`-i, --interactive` | Launch an interactive CoffeeScript session to try short snippets. Identical to calling `coffee` with no arguments.
`-o, --output [DIR]` | Write out all compiled JavaScript files into the specified directory. Use in conjunction with `--compile` or `--watch`.
`-j, --join [FILE]` | Before compiling, concatenate all scripts together in the order they were passed, and write them into the specified file. Useful for building large projects.
`-w, --watch` | Watch files for changes, rerunning the specified command when any file is updated.
`-p, --print` | Instead of writing out the JavaScript as a file, print it directly to stdout.
`-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`
`-l, --literate` | Parses the code as Literate CoffeeScript. You only need to specify this when passing in code directly over stdio, or using some sort of extension-less file name.
`-e, --eval` | Compile and print a little snippet of CoffeeScript directly from the command line. For example: `coffee -e "console.log num for num in [10..1]"`
`-b, --bare` | Compile the JavaScript without the top-level function safety wrapper.
`-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"
`--nodejs` | The `node` executable has some useful options you can set, such as `--debug`, `--debug-brk`, `--max-stack-size`, and `--expose-gc`. Use this flag to forward options directly to Node.js. To pass multiple flags, use `--nodejs` multiple times.

Examples:

- Compile a directory tree of `.coffee` files in `src` into a parallel tree of `.js` files in `lib`:
  `coffee --compile --output lib/ src/`
- Watch a file for changes, and recompile it every time the file is saved:
  `coffee --watch --compile experimental.coffee`
- Concatenate a list of files into a single script:
  `coffee --join project.js --compile src/*.coffee`
- Print out the compiled JS from a one-liner:
  `coffee -bpe "alert i for i in [0..10]"`
- All together now, watch and recompile an entire project as you work on it:
  `coffee -o lib/ -cw src/`
- Start the CoffeeScript REPL (`Ctrl-D` to exit, `Ctrl-V`for multi-line):
  `coffee`

## Literate CoffeeScript

Besides being used as an ordinary programming language, CoffeeScript may also be written in "literate" mode. If you name your file with a `.litcoffee` extension, you can write it as a Markdown document — a document that also happens to be executable CoffeeScript code. The compiler will treat any indented blocks (Markdown's way of indicating source code) as code, and ignore the rest as comments.

Just for kicks, a little bit of the compiler is currently implemented in this fashion: See it as a document, raw, and properly highlighted in a text editor.

I'm fairly excited about this direction for the language, and am looking forward to writing (and more importantly, reading) more programs in this style. More information about Literate CoffeeScript, including an example program, are available in this blog post.

## 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 on the right, and can be loaded into the "Try CoffeeScript" console by pressing the load button on the left.

First, the basics: CoffeeScript uses significant whitespace to delimit blocks of code. You don't need to use semicolons `;` to terminate expressions, ending the line will do just as well (although semicolons can still be used to fit multiple expressions onto a single line). Instead of using curly braces `{ }` to surround blocks of code in 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. The implicit call wraps forward to the end of the line or block expression.
`console.log sys.inspect object` → `console.log(sys.inspect(object));`

Functions Functions are defined by an optional list of parameters in parentheses, 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;
    };

load
run: cube(5)

Functions may also have default values for arguments, which will be used if the incoming argument is missing (`null` or `undefined`).

    fill = (container, liquid = "coffee") ->
      "Filling the #{container} with #{liquid}..."

    var fill;

    fill = function(container, liquid) {
      if (liquid == null) {
        liquid = "coffee";
      }
      return "Filling the " + container + " with " + liquid + "...";
    };

load
run: fill("cup")

Objects and Arrays The CoffeeScript literals for objects and arrays look very similar to their JavaScript cousins. When each property is listed on its own line, the commas are optional. Objects may be created using indentation instead of explicit braces, similar to YAML.

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

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

    bitlist = [
      1, 0, 1
      0, 0, 1
      1, 1, 0
    ]

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

    var bitlist, kids, singers, song;

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

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

    bitlist = [1, 0, 1, 0, 0, 1, 1, 1, 0];

    kids = {
      brother: {
        name: "Max",
        age: 11
      },
      sister: {
        name: "Ida",
        age: 9
      }
    };

load
run: song.join(" ... ")

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

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

    log object.class

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

    log(object["class"]);

load

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();

load
run: inner

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: `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 on a single line.

    mood = greatlyImproved if singing

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

    date = if friday then sue else jill

    var date, mood;

    if (singing) {
      mood = greatlyImproved;
    }

    if (happy && knowsIt) {
      clapsHands();
      chaChaCha();
    } else {
      showIt();
    }

    date = friday ? sue : jill;

load

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,
      slice = [].slice;

    gold = silver = rest = "unknown";

    awardMedals = function() {
      var first, others, second;
      first = arguments[0], second = arguments[1], others = 3 <= arguments.length ? 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(null, contenders);

    alert("Gold: " + gold);

    alert("Silver: " + silver);

    alert("The Field: " + rest);

load
run

Loops and Comprehensions Most of the loops you'll write in CoffeeScript will be comprehensions over arrays, objects, and ranges. Comprehensions 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.

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

    # Fine five course dining.
    courses = ['greens', 'caviar', 'truffles', 'roast', 'cake']
    menu i + 1, dish for dish, i in courses

    # Health conscious meal.
    foods = ['broccoli', 'spinach', 'chocolate']
    eat food for food in foods when food isnt 'chocolate'

    var courses, dish, food, foods, i, j, k, l, len, len1, len2, ref;

    ref = ['toast', 'cheese', 'wine'];
    for (j = 0, len = ref.length; j < len; j++) {
      food = ref[j];
      eat(food);
    }

    courses = ['greens', 'caviar', 'truffles', 'roast', 'cake'];

    for (i = k = 0, len1 = courses.length; k < len1; i = ++k) {
      dish = courses[i];
      menu(i + 1, dish);
    }

    foods = ['broccoli', 'spinach', 'chocolate'];

    for (l = 0, len2 = foods.length; l < len2; l++) {
      food = foods[l];
      if (food !== 'chocolate') {
        eat(food);
      }
    }

load

Comprehensions should be able to handle most places where you otherwise would use a loop, each/forEach, map, or select/filter, for example: `shortNames = (name for name in list when name.length < 5)`
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])

    var countdown, num;

    countdown = (function() {
      var i, results;
      results = [];
      for (num = i = 10; i >= 1; num = --i) {
        results.push(num);
      }
      return results;
    })();

load
run: countdown

Note how because we are assigning the value of the comprehensions to a variable in the example above, CoffeeScript is collecting the result of each iteration into an array. Sometimes functions end with loops that are intended to run only for their side-effects. Be careful that you're not accidentally returning the results of the comprehension in these cases, by adding a meaningful return value — like `true` — or `null`, to the bottom of your function.

To step through a range comprehension in fixed-size chunks, use `by`, for example:
`evens = (x for x in [0..10] by 2)`

If you don't need the current iteration value you may omit it:
`browser.closeCurrentTab() for [0...count]`

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 age, ages, child, yearsOld;

    yearsOld = {
      max: 10,
      ida: 9,
      tim: 11
    };

    ages = (function() {
      var results;
      results = [];
      for (child in yearsOld) {
        age = yearsOld[child];
        results.push(child + " is " + age);
      }
      return results;
    })();

load
run: ages.join(", ")

If you would like to iterate over just the keys that are defined on the object itself, by adding a `hasOwnProperty` check to avoid properties that may be inherited from the prototype, use
`for own key, value of object`

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 lyrics, num;

    if (this.studyingEconomics) {
      while (supply > demand) {
        buy();
      }
      while (!(supply > demand)) {
        sell();
      }
    }

    num = 6;

    lyrics = (function() {
      var results;
      results = [];
      while (num -= 1) {
        results.push(num + " little monkeys, jumping on the bed. One fell out and bumped his head.");
      }
      return results;
    })();

load
run: lyrics.join(" ")

For readability, the until keyword is equivalent to `while not`, and the loop keyword is equivalent to `while true`.

When using a JavaScript loop to generate functions, it's common to insert a closure wrapper in order to ensure that loop variables are closed over, and all the generated functions don't just share the final values. CoffeeScript provides the `do` keyword, which immediately invokes a passed function, forwarding any arguments.

    for filename in list
      do (filename) ->
        fs.readFile filename, (err, contents) ->
          compile filename, contents.toString()

    var filename, fn, i, len;

    fn = function(filename) {
      return fs.readFile(filename, function(err, contents) {
        return compile(filename, contents.toString());
      });
    };
    for (i = 0, len = list.length; i < len; i++) {
      filename = list[i];
      fn(filename);
    }

load

Array Slicing and Splicing with Ranges Ranges can also be used to extract slices of arrays. With two dots (`3..6`), the range is inclusive (`3, 4, 5, 6`); with three dots (`3...6`), the range excludes the end (`3, 4, 5`). Slices indices have useful defaults. An omitted first index defaults to zero and an omitted second index defaults to the size of the array.

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

    start   = numbers[0..2]

    middle  = numbers[3...-2]

    end     = numbers[-2..]

    copy    = numbers[..]

    var copy, end, middle, numbers, start;

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

    start = numbers.slice(0, 3);

    middle = numbers.slice(3, -2);

    end = numbers.slice(-2);

    copy = numbers.slice(0);

load
run: middle

The same syntax can be used with assignment to replace a segment of an array with new values, splicing it.

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

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

    var numbers, ref;

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

    [].splice.apply(numbers, [3, 4].concat(ref = [-3, -4, -5, -6])), ref;

load
run: numbers

Note that JavaScript strings are immutable, and can't be spliced.

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) {
        if (student.triedHard) {
          return "B";
        } else {
          return "B-";
        }
      } else {
        return "C";
      }
    };

    eldest = 24 > 21 ? "Liz" : "Ike";

load
run: eldest

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);

load
run: six

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 globals, name;

    globals = ((function() {
      var results;
      results = [];
      for (name in window) {
        results.push(name);
      }
      return results;
    })()).slice(0, 10);

load
run: globals

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}"
    )

    var error;

    alert((function() {
      var error1;
      try {
        return nonexistent / void 0;
      } catch (error1) {
        error = error1;
        return "And the error is ... " + error;
      }
    })());

load
run

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.

Operators and 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`.

`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.

To simplify math expressions, `**` can be used for exponentiation and `//` performs integer division. `%` works just like in JavaScript, while `%%` provides “dividend dependent modulo”:

    -7 % 5 == -2 # The remainder of 7 / 5
    -7 %% 5 == 3 # n %% 5 is always between 0 and 4

    tabs.selectTabAtIndex((tabs.currentIndex - count) %% tabs.length)

    var modulo = function(a, b) { return (+a % (b = +b) + b) % b; };

    -7 % 5 === -2;

    modulo(-7, 5) === 3;

    tabs.selectTabAtIndex(modulo(tabs.currentIndex - count, tabs.length));

load

All together now:

CoffeeScript | JavaScript
--- | ---
`is` | `===`
`isnt` | `!==`
`not` | `!`
`and` | `&&`
`or` | `||`
`true`, `yes`, `on` | `true`
`false`, `no`, `off` | `false`
`@`, `this` | `this`
`of` | `in`
`in` | no JS equivalent
`a ** b` | `Math.pow(a, b)`
`a // b` | `Math.floor(a / b)`
`a %% b` | `(a % b + b) % b`

    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 (pick === 47 || pick === 92 || pick === 13) {
      winner = true;
    }

    print(inspect("My name is " + this.name));

load

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 = 0
    speed ?= 15

    footprints = yeti ? "bear"

    var footprints, solipsism, speed;

    if ((typeof mind !== "undefined" && mind !== null) && (typeof world === "undefined" || world === null)) {
      solipsism = true;
    }

    speed = 0;

    if (speed == null) {
      speed = 15;
    }

    footprints = typeof yeti !== "undefined" && yeti !== null ? yeti : "bear";

load
run: footprints

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.

    zip = lottery.drawWinner?().address?.zipcode

    var ref, zip;

    zip = typeof lottery.drawWinner === "function" ? (ref = lottery.drawWinner().address) != null ? ref.zipcode : void 0 : void 0;

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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.

Constructor functions are named, to better support helpful stack traces. In the first class in the example below, `this.constructor.name is "Animal"`.

    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,
      extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },
      hasProp = {}.hasOwnProperty;

    Animal = (function() {
      function Animal(name) {
        this.name = name;
      }

      Animal.prototype.move = function(meters) {
        return alert(this.name + (" moved " + meters + "m."));
      };

      return Animal;

    })();

    Snake = (function(superClass) {
      extend(Snake, superClass);

      function Snake() {
        return Snake.__super__.constructor.apply(this, arguments);
      }

      Snake.prototype.move = function() {
        alert("Slithering...");
        return Snake.__super__.move.call(this, 5);
      };

      return Snake;

    })(Animal);

    Horse = (function(superClass) {
      extend(Horse, superClass);

      function Horse() {
        return Horse.__super__.constructor.apply(this, arguments);
      }

      Horse.prototype.move = function() {
        alert("Galloping...");
        return Horse.__super__.move.call(this, 45);
      };

      return Horse;

    })(Animal);

    sam = new Snake("Sammy the Python");

    tom = new Horse("Tommy the Palomino");

    sam.move();

    tom.move();

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run

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, and can be used to create an inheritance chain between any pair of constructor functions; `::` 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, "-");
    };

load
run: "one_two".dasherize()

Finally, class definitions are blocks of executable code, which make for interesting metaprogramming possibilities. Because in the context of a class definition, `this` is the class object itself (the constructor function), you can assign static properties by using
`@property: value`, and call functions defined in parent classes: `@attr 'title', type: 'text'`

Destructuring Assignment Just like JavaScript (since ES2015), CoffeeScript has 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 ref, theBait, theSwitch;

    theBait = 1000;

    theSwitch = 0;

    ref = [theSwitch, theBait], theBait = ref[0], theSwitch = ref[1];

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run: theBait

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 city, forecast, ref, temp, weatherReport;

    weatherReport = function(location) {
      return [location, 72, "Mostly Sunny"];
    };

    ref = weatherReport("Berkeley, CA"), city = ref[0], temp = ref[1], forecast = ref[2];

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run: forecast

Destructuring assignment 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 city, futurists, name, ref, ref1, street;

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

    ref = futurists.poet, name = ref.name, (ref1 = ref.address, street = ref1[0], city = ref1[1]);

load
run: name + "-" + street

Destructuring assignment can even be combined with splats.

    tag = "<impossible>"

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

    var close, contents, i, open, ref, tag,
      slice = [].slice;

    tag = "<impossible>";

    ref = tag.split(""), open = ref[0], contents = 3 <= ref.length ? slice.call(ref, 1, i = ref.length - 1) : (i = 1, []), close = ref[i++];

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run: contents.join("")

Expansion can be used to retrieve elements from the end of an array without having to assign the rest of its values. It works in function parameter lists as well.

    text = "Every literary critic believes he will
            outwit history and have the last word"

    [first, ..., last] = text.split " "

    var first, last, ref, text;

    text = "Every literary critic believes he will outwit history and have the last word";

    ref = text.split(" "), first = ref[0], last = ref[ref.length - 1];

load
run: first + " " + last

Destructuring assignment is also useful when combined with class constructors to assign properties to your instance from an options object passed to the constructor.

    class Person
      constructor: (options) ->
        {@name, @age, @height = 'average'} = options

    tim = new Person name: 'Tim', age: 4

    var Person, tim;

    Person = (function() {
      function Person(options) {
        var ref;
        this.name = options.name, this.age = options.age, this.height = (ref = options.height) != null ? ref : 'average';
      }

      return Person;

    })();

    tim = new Person({
      name: 'Tim',
      age: 4
    });

load
run: tim.age + " " + tim.height

The above example also demonstrates that if properties are missing in the destructured object or array, you can, just like in JavaScript, provide defaults. The difference with JavaScript is that CoffeeScript, as always, treats both null and undefined the same.

Bound Functions, Generator Functions 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 a 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 `on`. 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').on 'click', (event) =>
        @customer.purchase @cart

    var Account;

    Account = function(customer, cart) {
      this.customer = customer;
      this.cart = cart;
      return $('.shopping_cart').on('click', (function(_this) {
        return function(event) {
          return _this.customer.purchase(_this.cart);
        };
      })(this));
    };

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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.

When used in a class definition, methods declared with the fat arrow will be automatically bound to each instance of the class when the instance is constructed.

CoffeeScript functions also support ES6 generator functions through the `yield` keyword. There's no `function*(){}` nonsense — a generator in CoffeeScript is simply a function that yields.

    perfectSquares = ->
      num = 0
      loop
        num += 1
        yield num * num
      return

    window.ps or= perfectSquares()

    var perfectSquares;

    perfectSquares = function*() {
      var num;
      num = 0;
      while (true) {
        num += 1;
        (yield num * num);
      }
    };

    window.ps || (window.ps = perfectSquares());

load
run: ps.next().value

`yield*` is called `yield from`, and `yield return` may be used if you need to force a generator that doesn't yield.

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(": ");
    };

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run: hi()

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);
    }

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Switch statements can also be used without a control expression, turning them in to a cleaner alternative to if/else chains.

    score = 76
    grade = switch
      when score < 60 then 'F'
      when score < 70 then 'D'
      when score < 80 then 'C'
      when score < 90 then 'B'
      else 'A'
    # grade == 'C'

    var grade, score;

    score = 76;

    grade = (function() {
      switch (false) {
        case !(score < 60):
          return 'F';
        case !(score < 70):
          return 'D';
        case !(score < 80):
          return 'C';
        case !(score < 90):
          return 'B';
        default:
          return 'A';
      }
    })();

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Try/Catch/Finally Try-expressions have the same semantics as try-statements in JavaScript, though in CoffeeScript, you may omit both the catch and finally parts. The catch part may also omit the error parameter if it is not needed.

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

    var error, error1;

    try {
      allHellBreaksLoose();
      catsAndDogsLivingTogether();
    } catch (error1) {
      error = error1;
      print(error);
    } finally {
      cleanUp();
    }

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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);

load
run: healthy

String Interpolation, Block Strings, and Block Comments Ruby-style string interpolation is included in CoffeeScript. Double-quoted strings allow for interpolated values, using `#{ ... }`, and single-quoted strings are literal. You may even use interpolation in object keys.

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

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

    var author, quote, sentence;

    author = "Wittgenstein";

    quote = "A picture is a fact. -- " + author;

    sentence = (22 / 7) + " is a decent approximation of π";

load
run: sentence

Multiline strings are allowed in CoffeeScript. Lines are joined by a single space unless they end with a backslash. Indentation is ignored.

    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...";

load
run: mobyDick

Block strings 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 block 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>";

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run: html

Double-quoted block strings, like other 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 syntax for block strings, are preserved in the generated code.

    ###
    SkinnyMochaHalfCaffScript Compiler v1.0
    Released under the MIT License
    ###

    /*
    SkinnyMochaHalfCaffScript Compiler v1.0
    Released under the MIT License
     */

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Block Regular Expressions Similar to block strings and comments, CoffeeScript supports block regexes — extended regular expressions that ignore internal whitespace and can contain comments and interpolation. Modeled after Perl's `/x` modifier, CoffeeScript's block regexes are delimited by `///` and go a long way towards making complex regular expressions readable. To quote from the CoffeeScript source:

    OPERATOR = /// ^ (
      ?: [-=]>             # function
       | [-+*/%<>&|^!?=]=  # compound assign / compare
       | >>>=?             # zero-fill right shift
       | ([-+:])\1         # doubles
       | ([&|<>])\2=?      # logic / shift
       | \?\.              # soak access
       | \.{2,3}           # range or splat
    ) ///

    var OPERATOR;

    OPERATOR = /^(?:[-=]>|[-+*\/%<>&|^!?=]=|>>>=?|([-+:])\1|([&|<>])\2=?|\?\.|\.{2,3})/;

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## Cake, and Cakefiles

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

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If you need to invoke one task before another — for example, running `build` before `test`, you can use the `invoke` function: `invoke 'build'`. Cake tasks are a minimal way to expose your CoffeeScript functions to the command line, so don't expect any fanciness built-in. If you need dependencies, or async callbacks, it's best to put them in your code itself — not the cake task.

## Source Maps

CoffeeScript 1.6.1 and above include support for generating source maps, a way to tell your JavaScript engine what part of your CoffeeScript program matches up with the code being evaluated. Browsers that support it can automatically use source maps to show your original source code in the debugger. To generate source maps alongside your JavaScript files, pass the `--map` or `-m` flag to the compiler.

For a full introduction to source maps, how they work, and how to hook them up in your browser, read the HTML5 Tutorial.

## "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, 39k 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 the 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.

## Books

There are a number of excellent resources to help you get started with CoffeeScript, some of which are freely available online.

- The Little Book on CoffeeScript is a brief 5-chapter introduction to CoffeeScript, written with great clarity and precision by Alex MacCaw.
- Smooth CoffeeScript is a reimagination of the excellent book Eloquent JavaScript, as if it had been written in CoffeeScript instead. Covers language features as well as the functional and object oriented programming styles. By E. Hoigaard.
- CoffeeScript: Accelerated JavaScript Development is Trevor Burnham's thorough introduction to the language. By the end of the book, you'll have built a fast-paced multiplayer word game, writing both the client-side and Node.js portions in CoffeeScript.
- CoffeeScript Programming with jQuery, Rails, and Node.js is a new book by Michael Erasmus that covers CoffeeScript with an eye towards real-world usage both in the browser (jQuery) and on the server size (Rails, Node).
- CoffeeScript Ristretto is a deep dive into CoffeeScript's semantics from simple functions up through closures, higher-order functions, objects, classes, combinators, and decorators. By Reg Braithwaite.
- Testing with CoffeeScript is a succinct and freely downloadable guide to building testable applications with CoffeeScript and Jasmine.
- CoffeeScript Application Development from Packt, introduces CoffeeScript while walking through the process of building a demonstration web application. A CoffeeScript Application Development Coookbook with over 90 "recipes" is also available.
- CoffeeScript in Action from Manning Publications, covers CoffeeScript syntax, composition techniques and application development.
- CoffeeScript: Die Alternative zu JavaScript from dpunkt.verlag, is the first CoffeeScript book in Deutsch.

## Screencasts

- A Sip of CoffeeScript is a Code School Course which combines 6 screencasts with in-browser coding to make learning fun. The first level is free to try out.
- Meet CoffeeScript is a 75-minute long screencast by PeepCode. Highly memorable for its animations which demonstrate transforming CoffeeScript into the equivalent JS.
- If you're looking for less of a time commitment, RailsCasts' CoffeeScript Basics should have you covered, hitting all of the important notes about CoffeeScript in 11 minutes.

## Examples

The best list of open-source CoffeeScript examples can be found on GitHub. But just to throw out few more:

- github's Hubot, a friendly IRC robot that can perform any number of useful and useless tasks.
- sstephenson's Pow, a zero-configuration Rack server, with comprehensive annotated source.
- technoweenie's Coffee-Resque, a port of Resque for Node.js.
- assaf's Zombie.js, a headless, full-stack, faux-browser testing library for Node.js.
- jashkenas' Underscore.coffee, a port of the Underscore.js library of helper functions.
- stephank's Orona, a remake of the Bolo tank game for modern browsers.
- github's Atom, a hackable text editor built on web technologies.

## 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
  Bug reports, feature proposals, and ideas for changes to the language belong here.
- CoffeeScript Google Group
  If you'd like to ask a question, the mailing list is a good place to get help.
- The CoffeeScript Wiki
  If you've ever learned a neat CoffeeScript tip or trick, or ran into a gotcha — share it on the wiki. The wiki also serves as a directory of handy text editor extensions, web framework plugins, and general CoffeeScript build tools.
- The FAQ
  Perhaps your CoffeeScript-related question has been asked before. Check the FAQ first.
- JS2Coffee
  Is a very well done reverse JavaScript-to-CoffeeScript compiler. It's not going to be perfect (infer what your JavaScript classes are, when you need bound functions, and so on...) — but it's a great starting point for converting simple scripts.
- High-Rez Logo
  The CoffeeScript logo is available in Illustrator, EPS and PSD formats, for use in presentations.

## 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

1.10.0 — September 4, 2015

- CoffeeScript now supports ES6-style destructuring defaults.
- `(offsetHeight: height) ->` no longer compiles. That syntax was accidental and partly broken. Use `({offsetHeight: height}) ->` instead. Object destructuring always requires braces.

- Several minor bug fixes, including:

  - A bug where the REPL would sometimes report valid code as invalid, based on what you had typed earlier.
  - A problem with multiple JS contexts in the jest test framework.
  - An error in io.js where strict mode is set on internal modules.
  - A variable name clash for the caught error in `catch` blocks.

1.9.3 — May 27, 2015

- Bugfix for interpolation in the first key of an object literal in an implicit call.
- Fixed broken error messages in the REPL, as well as a few minor bugs with the REPL.
- Fixed source mappings for tokens at the beginning of lines when compiling with the `--bare` option. This has the nice side effect of generating smaller source maps.
- Slight formatting improvement of compiled block comments.
- Better error messages for `on`, `off`, `yes` and `no`.

1.9.2 — April 15, 2015

- Fixed a watch mode error introduced in 1.9.1 when compiling multiple files with the same filename.
- Bugfix for `yield` around expressions containing `this`.
- Added a Ruby-style `-r` option to the REPL, which allows requiring a module before execution with `--eval` or `--interactive`.
- In `<script type="text/coffeescript">` tags, to avoid possible duplicate browser requests for .coffee files, you can now use the `data-src` attribute instead of `src`.
- Minor bug fixes for IE8, strict ES5 regular expressions and Browserify.

1.9.1 — February 18, 2015

- Interpolation now works in object literal keys (again). You can use this to dynamically name properties.
- Internal compiler variable names no longer start with underscores. This makes the generated JavaScript a bit prettier, and also fixes an issue with the completely broken and ungodly way that AngularJS "parses" function arguments.
- Fixed a few `yield`-related edge cases with `yield return` and `yield throw`.
- Minor bug fixes and various improvements to compiler error messages.

1.9.0 — January 29, 2015

- CoffeeScript now supports ES6 generators. A generator is simply a function that `yield`s.
- More robust parsing and improved error messages for strings and regexes — especially with respect to interpolation.
- Changed strategy for the generation of internal compiler variable names. Note that this means that `@example` function parameters are no longer available as naked `example` variables within the function body.
- Fixed REPL compatibility with latest versions of Node and Io.js.
- Various minor bug fixes.

1.8.0 — August 26, 2014

- The `--join` option of the CLI is now deprecated.
- Source maps now use `.js.map` as file extension, instead of just `.map`.
- The CLI now exits with the exit code 1 when it fails to write a file to disk.
- The compiler no longer crashes on unterminated, single-quoted strings.
- Fixed location data for string interpolations, which made source maps out of sync.
- The error marker in error messages is now correctly positioned if the code is indented with tabs.
- Fixed a slight formatting error in CoffeeScript’s source map-patched stack traces.
- The `%%` operator now coerces its right operand only once.
- It is now possible to require CoffeeScript files from Cakefiles without having to register the compiler first.
- The CoffeeScript REPL is now exported and can be required using `require 'coffee-script/repl'`.
- Fixes for the REPL in Node 0.11.

1.7.1 — January 29, 2014

- Fixed a typo that broke node module lookup when running a script directly with the `coffee` binary.

1.7.0 — January 28, 2014

- When requiring CoffeeScript files in Node you must now explicitly register the compiler. This can be done with `require 'coffee-script/register'` or `CoffeeScript.register()`. Also for configuration such as Mocha's, use coffee-script/register.
- Improved error messages, source maps and stack traces. Source maps now use the updated `//#` syntax.
- Leading `.` now closes all open calls, allowing for simpler chaining syntax.

    $ 'body'
    .click (e) ->
      $ '.box'
      .fadeIn 'fast'
      .addClass '.active'
    .css 'background', 'white'

    $('body').click(function(e) {
      return $('.box').fadeIn('fast').addClass('.active');
    }).css('background', 'white');

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- Added `**`, `//` and `%%` operators and `...` expansion in parameter lists and destructuring expressions.
- Multiline strings are now joined by a single space and ignore all indentation. A backslash at the end of a line can denote the amount of whitespace between lines, in both strings and heredocs. Backslashes correctly escape whitespace in block regexes.
- Closing brackets can now be indented and therefore no longer cause unexpected error.
- Several breaking compilation fixes. Non-callable literals (strings, numbers etc.) don't compile in a call now and multiple postfix conditionals compile properly. Postfix conditionals and loops always bind object literals. Conditional assignment compiles properly in subexpressions. `super` is disallowed outside of methods and works correctly inside `for` loops.
- Formatting of compiled block comments has been improved.
- No more `-p` folders on Windows.
- The `options` object passed to CoffeeScript is no longer mutated.

1.6.3 — June 2, 2013

- The CoffeeScript REPL now remembers your history between sessions. Just like a proper REPL should.
- You can now use `require` in Node to load `.coffee.md` Literate CoffeeScript files. In the browser, `text/literate-coffeescript` script tags.
- The old `coffee --lint` command has been removed. It was useful while originally working on the compiler, but has been surpassed by JSHint. You may now use `-l` to pass literate files in over stdio.
- Bugfixes for Windows path separators, `catch` without naming the error, and executable-class-bodies-with- prototypal-property-attachment.

1.6.2 — March 18, 2013

- Source maps have been used to provide automatic line-mapping when running CoffeeScript directly via the `coffee` command, and for automatic line-mapping when running CoffeeScript directly in the browser. Also, to provide better error messages for semantic errors thrown by the compiler — with colors, even.
- Improved support for mixed literate/vanilla-style CoffeeScript projects, and generating source maps for both at the same time.
- Fixes for 1.6.x regressions with overriding inherited bound functions, and for Windows file path management.
- The `coffee` command can now correctly `fork()` both `.coffee` and `.js` files. (Requires Node.js 0.9+)

1.6.1 — March 5, 2013

- First release of source maps. Pass the `--map` flag to the compiler, and off you go. Direct all your thanks over to Jason Walton.
- Fixed a 1.5.0 regression with multiple implicit calls against an indented implicit object. Combinations of implicit function calls and implicit objects should generally be parsed better now — but it still isn't good style to nest them too heavily.
- `.coffee.md` is now also supported as a Literate CoffeeScript file extension, for existing tooling. `.litcoffee` remains the canonical one.
- Several minor fixes surrounding member properties, bound methods and `super` in class declarations.

1.5.0 — February 25, 2013

- First release of Literate CoffeeScript.
- The CoffeeScript REPL is now based on the Node.js REPL, and should work better and more familiarly.
- Returning explicit values from constructors is now forbidden. If you want to return an arbitrary value, use a function, not a constructor.
- You can now loop over an array backwards, without having to manually deal with the indexes: `for item in list by -1`
- Source locations are now preserved in the CoffeeScript AST, although source maps are not yet being emitted.

1.4.0 — October 23, 2012

- The CoffeeScript compiler now strips Microsoft's UTF-8 BOM if it exists, allowing you to compile BOM-borked source files.
- Fix Node/compiler deprecation warnings by removing `registerExtension`, and moving from `path.exists` to `fs.exists`.
- Small tweaks to splat compilation, backticks, slicing, and the error for duplicate keys in object literals.

1.3.3 — May 15, 2012

- Due to the new semantics of JavaScript's strict mode, CoffeeScript no longer guarantees that constructor functions have names in all runtimes. See #2052 for discussion.
- Inside of a nested function inside of an instance method, it's now possible to call `super` more reliably (walks recursively up).
- Named loop variables no longer have different scoping heuristics than other local variables. (Reverts #643)
- Fix for splats nested within the LHS of destructuring assignment.
- Corrections to our compile time strict mode forbidding of octal literals.

1.3.1 — April 10, 2012

- CoffeeScript now enforces all of JavaScript's Strict Mode early syntax errors at compile time. This includes old-style octal literals, duplicate property names in object literals, duplicate parameters in a function definition, deleting naked variables, setting the value of `eval` or `arguments`, and more. See a full discussion at #1547.
- The REPL now has a handy new multi-line mode for entering large blocks of code. It's useful when copy-and-pasting examples into the REPL. Enter multi-line mode with `Ctrl-V`. You may also now pipe input directly into the REPL.
- CoffeeScript now prints a `Generated by CoffeeScript VERSION` header at the top of each compiled file.
- Conditional assignment of previously undefined variables `a or= b` is now considered a syntax error.
- A tweak to the semantics of `do`, which can now be used to more easily simulate a namespace: `do (x = 1, y = 2) -> ...`
- Loop indices are now mutable within a loop iteration, and immutable between them.
- Both endpoints of a slice are now allowed to be omitted for consistency, effectively creating a shallow copy of the list.
- Additional tweaks and improvements to `coffee --watch` under Node's "new" file watching API. Watch will now beep by default if you introduce a syntax error into a watched script. We also now ignore hidden directories by default when watching recursively.

1.2.0 — December 18, 2011

- Multiple improvements to `coffee --watch` and `--join`. You may now use both together, as well as add and remove files and directories within a `--watch`'d folder.
- The `throw` statement can now be used as part of an expression.
- Block comments at the top of the file will now appear outside of the safety closure wrapper.
- Fixed a number of minor 1.1.3 regressions having to do with trailing operators and unfinished lines, and a more major 1.1.3 regression that caused bound functions within bound class functions to have the incorrect `this`.

1.1.3 — November 8, 2011

- Ahh, whitespace. CoffeeScript's compiled JS now tries to space things out and keep it readable, as you can see in the examples on this page.
- You can now call `super` in class level methods in class bodies, and bound class methods now preserve their correct context.
- JavaScript has always supported octal numbers `010 is 8`, and hexadecimal numbers `0xf is 15`, but CoffeeScript now also supports binary numbers: `0b10 is 2`.
- The CoffeeScript module has been nested under a subdirectory to make it easier to `require` individual components separately, without having to use npm. For example, after adding the CoffeeScript folder to your path: `require('coffee-script/lexer')`
- There's a new "link" feature in Try CoffeeScript on this webpage. Use it to get a shareable permalink for your example script.
- The `coffee --watch` feature now only works on Node.js 0.6.0 and higher, but now also works properly on Windows.
- Lots of small bug fixes from @michaelficarra, @geraldalewis, @satyr, and @trevorburnham.

1.1.2 — August 4, 2011 Fixes for block comment formatting, `?=` compilation, implicit calls against control structures, implicit invocation of a try/catch block, variadic arguments leaking from local scope, line numbers in syntax errors following heregexes, property access on parenthesized number literals, bound class methods and super with reserved names, a REPL overhaul, consecutive compiled semicolons, block comments in implicitly called objects, and a Chrome bug.

1.1.1 — May 10, 2011 Bugfix release for classes with external constructor functions, see issue #1182.

1.1.0 — May 1, 2011 When running via the `coffee` executable, `process.argv` and friends now report `coffee` instead of `node`. Better compatibility with Node.js 0.4.x module lookup changes. The output in the REPL is now colorized, like Node's is. Giving your concatenated CoffeeScripts a name when using `--join` is now mandatory. Fix for lexing compound division `/=` as a regex accidentally. All `text/coffeescript` tags should now execute in the order they're included. Fixed an issue with extended subclasses using external constructor functions. Fixed an edge-case infinite loop in `addImplicitParentheses`. Fixed exponential slowdown with long chains of function calls. Globals no longer leak into the CoffeeScript REPL. Splatted parameters are declared local to the function.

1.0.1 — January 31, 2011 Fixed a lexer bug with Unicode identifiers. Updated REPL for compatibility with Node.js 0.3.7. Fixed requiring relative paths in the REPL. Trailing `return` and `return undefined` are now optimized away. Stopped requiring the core Node.js `"util"` module for back-compatibility with Node.js 0.2.5. Fixed a case where a conditional `return` would cause fallthrough in a `switch` statement. Optimized empty objects in destructuring assignment.

1.0.0 — December 24, 2010 CoffeeScript loops no longer try to preserve block scope when functions are being generated within the loop body. Instead, you can use the `do` keyword to create a convenient closure wrapper. Added a `--nodejs` flag for passing through options directly to the `node` executable. Better behavior around the use of pure statements within expressions. Fixed inclusive slicing through `-1`, for all browsers, and splicing with arbitrary expressions as endpoints.

0.9.6 — December 6, 2010 The REPL now properly formats stacktraces, and stays alive through asynchronous exceptions. Using `--watch` now prints timestamps as files are compiled. Fixed some accidentally-leaking variables within plucked closure-loops. Constructors now maintain their declaration location within a class body. Dynamic object keys were removed. Nested classes are now supported. Fixes execution context for naked splatted functions. Bugfix for inversion of chained comparisons. Chained class instantiation now works properly with splats.

0.9.5 — November 21, 2010 0.9.5 should be considered the first release candidate for CoffeeScript 1.0. There have been a large number of internal changes since the previous release, many contributed from satyr's Coco dialect of CoffeeScript. Heregexes (extended regexes) were added. Functions can now have default arguments. Class bodies are now executable code. Improved syntax errors for invalid CoffeeScript. `undefined` now works like `null`, and cannot be assigned a new value. There was a precedence change with respect to single-line comprehensions: `result = i for i in list`
used to parse as `result = (i for i in list)` by default ... it now parses as
`(result = i) for i in list`.

0.9.4 — September 21, 2010 CoffeeScript now uses appropriately-named temporary variables, and recycles their references after use. Added `require.extensions` support for Node.js 0.3. Loading CoffeeScript in the browser now adds just a single `CoffeeScript` object to global scope. Fixes for implicit object and block comment edge cases.

0.9.3 — September 16, 2010 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 — August 23, 2010 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 — August 11, 2010 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 — August 4, 2010 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 — July 12, 2010 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 — July 11, 2010 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 — June 28, 2010 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 — May 15, 2010 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 — April 12, 2010 Upgraded CoffeeScript for compatibility with the new Node.js v0.1.90 series.

0.6.0 — April 3, 2010 Trailing commas are now allowed, a-la Python. Static properties may be assigned directly within class definitions, using `@property` notation.

0.5.6 — March 23, 2010 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 — March 8, 2010 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 — March 3, 2010 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 — February 27, 2010 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 — February 25, 2010 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 — February 24, 2010 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 — February 21, 2010 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 — February 8, 2010 `@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 — January 26, 2010 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 — January 17, 2010 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 — January 13, 2010 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 — January 12, 2010 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 — January 11, 2010 Axed the unsatisfactory `ino` keyword, replacing it with `of` for object comprehensions. They now look like: `for prop, value of object`.

0.2.2 — January 10, 2010 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 — January 5, 2010 Arguments objects are now converted into real arrays when referenced.

0.2.0 — January 5, 2010 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 — December 27, 2009 Bugfix for running `coffee --interactive` and `--run` from outside of the CoffeeScript directory. Bugfix for nested function/if-statements.

0.1.5 — December 26, 2009 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 — December 25, 2009 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 — December 25, 2009 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 — December 24, 2009 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 — December 24, 2009 Added `instanceof` and `typeof` as operators.

0.1.0 — December 24, 2009 Initial CoffeeScript release.