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<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=UTF-8" />
<title>CoffeeScript</title>
<link rel="canonical" href="http://coffeescript.org" />
<link rel="stylesheet" type="text/css" href="documentation/css/docs.css" />
<link rel="stylesheet" type="text/css" href="documentation/css/idle.css" />
<link rel="shortcut icon" href="documentation/images/favicon.ico" />
</head>
<body>
<div id="fadeout"></div>
<div id="flybar">
<a id="logo" href="#top"><img src="documentation/images/logo.png" width="225" height="39" alt="CoffeeScript" /></a>
<div class="navigation toc">
<div class="button">
Table of Contents
</div>
<div class="contents menu">
<a href="#overview">Overview</a>
<a href="#installation">Installation</a>
<a href="#usage">Usage</a>
<a href="#literate">Literate CoffeeScript</a>
<a href="#language">Language Reference</a>
<a href="#literals">Literals: Functions, Objects and Arrays</a>
<a href="#lexical-scope">Lexical Scoping and Variable Safety</a>
<a href="#conditionals">If, Else, Unless, and Conditional Assignment</a>
<a href="#splats">Splats...</a>
<a href="#loops">Loops and Comprehensions</a>
<a href="#slices">Array Slicing and Splicing</a>
<a href="#expressions">Everything is an Expression</a>
<a href="#operators">Operators and Aliases</a>
<a href="#classes">Classes, Inheritance, and Super</a>
<a href="#destructuring">Destructuring Assignment</a>
<a href="#fat-arrow">Function Binding</a>
<a href="#embedded">Embedded JavaScript</a>
<a href="#switch">Switch and Try/Catch</a>
<a href="#comparisons">Chained Comparisons</a>
<a href="#strings">String Interpolation, Block Strings, and Block Comments</a>
<a href="#regexes">Block Regular Expressions</a>
<a href="#cake">Cake, and Cakefiles</a>
<a href="#source-maps">Source Maps</a>
<a href="#scripts">"text/coffeescript" Script Tags</a>
<a href="#resources">Books, Screencasts, Examples and Resources</a>
<a href="#changelog">Change Log</a>
</div>
</div>
<div class="navigation try">
<div class="button">
Try CoffeeScript
<div class="repl_bridge"></div>
</div>
<div class="contents repl_wrapper">
<div class="code">
<div class="screenshadow tl"></div>
<div class="screenshadow tr"></div>
<div class="screenshadow bl"></div>
<div class="screenshadow br"></div>
<div id="repl_source_wrap">
<textarea id="repl_source" rows="100" spellcheck="false">alert "Hello CoffeeScript!"</textarea>
</div>
<div id="repl_results_wrap"><pre id="repl_results"></pre></div>
<div class="minibutton dark run" title="Ctrl-Enter">Run</div>
<a class="minibutton permalink" id="repl_permalink">Link</a>
<br class="clear" />
</div>
</div>
</div>
<div class="navigation annotated">
<div class="button">
Annotated Source
</div>
<div class="contents menu">
<a href="documentation/docs/grammar.html">Grammar Rules — src/grammar</a>
<a href="documentation/docs/lexer.html">Lexing Tokens — src/lexer</a>
<a href="documentation/docs/rewriter.html">The Rewriter — src/rewriter</a>
<a href="documentation/docs/nodes.html">The Syntax Tree — src/nodes</a>
<a href="documentation/docs/scope.html">Lexical Scope — src/scope</a>
<a href="documentation/docs/helpers.html">Helpers & Utility Functions — src/helpers</a>
<a href="documentation/docs/coffee-script.html">The CoffeeScript Module — src/coffee-script</a>
<a href="documentation/docs/cake.html">Cake & Cakefiles — src/cake</a>
<a href="documentation/docs/command.html">"coffee" Command-Line Utility — src/command</a>
<a href="documentation/docs/optparse.html">Option Parsing — src/optparse</a>
<a href="documentation/docs/repl.html">Interactive REPL — src/repl</a>
<a href="documentation/docs/sourcemap.html">Source Maps — src/sourcemap</a>
</div>
</div>
<div id="error" style="display:none;"></div>
</div>
<div class="container">
<span class="bookmark" id="top"></span>
<p>
<b>CoffeeScript is a little language that compiles into JavaScript.</b>
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.
</p>
<p>
The golden rule of CoffeeScript is: <i>"It's just JavaScript"</i>. 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.
</p>
<p>
<b>Latest Version:</b>
<a href="http://github.com/jashkenas/coffee-script/tarball/1.6.3">1.6.3</a>
</p>
<pre>
sudo npm install -g coffee-script</pre>
<h2>
<span id="overview" class="bookmark"></span>
Overview
</h2>
<p><i>CoffeeScript on the left, compiled JavaScript output on the right.</i></p>
<div class='code'><pre class="idle"><span class="Comment"><span class="Comment">#</span> Assignment:</span>
number <span class="Keyword">=</span> <span class="Number">42</span>
opposite <span class="Keyword">=</span> <span class="BuiltInConstant">true</span>
<span class="Comment"><span class="Comment">#</span> Conditions:</span>
number <span class="Keyword">=</span> <span class="Keyword">-</span><span class="Number">42</span> <span class="Keyword">if</span> opposite
<span class="Comment"><span class="Comment">#</span> Functions:</span>
<span class="FunctionName">square </span><span class="Keyword">=</span> <span class="FunctionArgument">(x)</span> <span class="Storage">-></span> x <span class="Keyword">*</span> x
<span class="Comment"><span class="Comment">#</span> Arrays:</span>
list <span class="Keyword">=</span> [<span class="Number">1</span>, <span class="Number">2</span>, <span class="Number">3</span>, <span class="Number">4</span>, <span class="Number">5</span>]
<span class="Comment"><span class="Comment">#</span> Objects:</span>
math <span class="Keyword">=</span>
root: Math.sqrt
square: square
<span class="FunctionName">cube</span><span class="Keyword">:</span> <span class="FunctionArgument">(x)</span> <span class="Storage">-></span> x <span class="Keyword">*</span> square x
<span class="Comment"><span class="Comment">#</span> Splats:</span>
<span class="FunctionName">race </span><span class="Keyword">=</span> <span class="FunctionArgument">(winner, runners...)</span> <span class="Storage">-></span>
print winner, runners
<span class="Comment"><span class="Comment">#</span> Existence:</span>
alert <span class="String"><span class="String">"</span>I knew it!<span class="String">"</span></span> <span class="Keyword">if</span> elvis<span class="Keyword">?</span>
<span class="Comment"><span class="Comment">#</span> Array comprehensions:</span>
cubes <span class="Keyword">=</span> (math.cube num <span class="Keyword">for</span> num <span class="Keyword">in</span> list)
</pre><pre class="idle"><span class="Storage">var</span> cubes, list, math, num, number, opposite, race, square,
__slice <span class="Keyword">=</span> [].slice;
number <span class="Keyword">=</span> <span class="Number">42</span>;
opposite <span class="Keyword">=</span> <span class="BuiltInConstant">true</span>;
<span class="Keyword">if</span> (opposite) {
number <span class="Keyword">=</span> <span class="Keyword">-</span><span class="Number">42</span>;
}
<span class="FunctionName">square</span> = <span class="Storage">function</span>(<span class="FunctionArgument">x</span>) {
<span class="Keyword">return</span> x <span class="Keyword">*</span> x;
};
list <span class="Keyword">=</span> [<span class="Number">1</span>, <span class="Number">2</span>, <span class="Number">3</span>, <span class="Number">4</span>, <span class="Number">5</span>];
math <span class="Keyword">=</span> {
root: <span class="LibraryClassType">Math</span>.sqrt,
square: square,
<span class="FunctionName">cube</span>: <span class="Storage">function</span>(<span class="FunctionArgument">x</span>) {
<span class="Keyword">return</span> x <span class="Keyword">*</span> square(x);
}
};
<span class="FunctionName">race</span> = <span class="Storage">function</span>() {
<span class="Storage">var</span> runners, winner;
winner <span class="Keyword">=</span> arguments[<span class="Number">0</span>], runners <span class="Keyword">=</span> <span class="Number">2</span> <span class="Keyword"><=</span> arguments.<span class="LibraryConstant">length</span> ? __slice.<span class="LibraryFunction">call</span>(arguments, <span class="Number">1</span>) : [];
<span class="Keyword">return</span> <span class="LibraryFunction">print</span>(winner, runners);
};
<span class="Keyword">if</span> (<span class="Keyword">typeof</span> elvis <span class="Keyword">!</span><span class="Keyword">==</span> <span class="String"><span class="String">"</span>undefined<span class="String">"</span></span> <span class="Keyword">&</span><span class="Keyword">&</span> elvis <span class="Keyword">!</span><span class="Keyword">==</span> <span class="BuiltInConstant">null</span>) {
<span class="LibraryFunction">alert</span>(<span class="String"><span class="String">"</span>I knew it!<span class="String">"</span></span>);
}
cubes <span class="Keyword">=</span> (<span class="Storage">function</span>() {
<span class="Storage">var</span> _i, _len, _results;
_results <span class="Keyword">=</span> [];
<span class="Keyword">for</span> (_i <span class="Keyword">=</span> <span class="Number">0</span>, _len <span class="Keyword">=</span> list.<span class="LibraryConstant">length</span>; _i <span class="Keyword"><</span> _len; _i<span class="Keyword">++</span>) {
num <span class="Keyword">=</span> list[_i];
_results.<span class="LibraryFunction">push</span>(math.cube(num));
}
<span class="Keyword">return</span> _results;
})();
</pre><script>window.example1 = "# Assignment:\nnumber = 42\nopposite = true\n\n# Conditions:\nnumber = -42 if opposite\n\n# Functions:\nsquare = (x) -> x * x\n\n# Arrays:\nlist = [1, 2, 3, 4, 5]\n\n# Objects:\nmath =\n root: Math.sqrt\n square: square\n cube: (x) -> x * square x\n\n# Splats:\nrace = (winner, runners...) ->\n print winner, runners\n\n# Existence:\nalert \"I knew it!\" if elvis?\n\n# Array comprehensions:\ncubes = (math.cube num for num in list)\n\nalert cubes"</script><div class='minibutton ok' onclick='javascript: 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;
})();
;alert(cubes);'>run: cubes</div><br class='clear' /></div>
<h2>
<span id="installation" class="bookmark"></span>
Installation
</h2>
<p>
The CoffeeScript compiler is itself
<a href="documentation/docs/grammar.html">written in CoffeeScript</a>,
using the <a href="http://jison.org">Jison parser generator</a>. The
command-line version of <tt>coffee</tt> is available as a
<a href="http://nodejs.org/">Node.js</a> utility. The
<a href="extras/coffee-script.js">core compiler</a> however, does not
depend on Node, and can be run in any JavaScript environment, or in the
browser (see "Try CoffeeScript", above).
</p>
<p>
To install, first make sure you have a working copy of the latest stable version of
<a href="http://nodejs.org/">Node.js</a>, and <a href="http://npmjs.org">npm</a>
(the Node Package Manager). You can then install CoffeeScript with npm:
</p>
<pre>
npm install -g coffee-script</pre>
<p>
(Leave off the <tt>-g</tt> if you don't wish to install globally.)
</p>
<p>
If you'd prefer to install the latest <b>master</b> version of CoffeeScript, you
can clone the CoffeeScript
<a href="http://github.com/jashkenas/coffee-script">source repository</a>
from GitHub, or download
<a href="http://github.com/jashkenas/coffee-script/tarball/master">the source</a> directly.
To install the lastest master CoffeeScript compiler with npm:
</p>
<pre>
npm install -g http://github.com/jashkenas/coffee-script/tarball/master</pre>
<p>
Or, if you want to install to <tt>/usr/local</tt>, and don't want to use
npm to manage it, open the <tt>coffee-script</tt> directory and run:
</p>
<pre>
sudo bin/cake install</pre>
<h2>
<span id="usage" class="bookmark"></span>
Usage
</h2>
<p>
Once installed, you should have access to the <tt>coffee</tt> command,
which can execute scripts, compile <tt>.coffee</tt> files into <tt>.js</tt>,
and provide an interactive REPL. The <tt>coffee</tt> command takes the
following options:
</p>
<table>
<tr>
<td><code>-c, --compile</code></td>
<td>
Compile a <tt>.coffee</tt> script into a <tt>.js</tt> JavaScript file
of the same name.
</td>
</tr>
<tr>
<td><code>-m, --map</code></td>
<td>
Generate source maps alongside the compiled JavaScript files. Adds
<tt>sourceMappingURL</tt> directives to the JavaScript as well.
</td>
</tr>
<tr>
<td width="25%"><code>-i, --interactive</code></td>
<td>
Launch an interactive CoffeeScript session to try short snippets.
Identical to calling <tt>coffee</tt> with no arguments.
</td>
</tr>
<tr>
<td><code>-o, --output [DIR]</code></td>
<td>
Write out all compiled JavaScript files into the specified directory.
Use in conjunction with <tt>--compile</tt> or <tt>--watch</tt>.
</td>
</tr>
<tr>
<td><code>-j, --join [FILE]</code></td>
<td>
Before compiling, concatenate all scripts together in the order they
were passed, and write them into the specified file.
Useful for building large projects.
</td>
</tr>
<tr>
<td><code>-w, --watch</code></td>
<td>
Watch files for changes, rerunning the specified command when any
file is updated.
</td>
</tr>
<tr>
<td><code>-p, --print</code></td>
<td>
Instead of writing out the JavaScript as a file, print it
directly to <b>stdout</b>.
</td>
</tr>
<tr>
<td><code>-s, --stdio</code></td>
<td>
Pipe in CoffeeScript to STDIN and get back JavaScript over STDOUT.
Good for use with processes written in other languages. An example:<br />
<tt>cat src/cake.coffee | coffee -sc</tt>
</td>
</tr>
<tr>
<td><code>-l, --literate</code></td>
<td>
Parses the code as Literate CoffeeScript. You only need to specify
this when passing in code directly over <b>stdio</b>, or using some sort
of extension-less file name.
</td>
</tr>
<tr>
<td><code>-e, --eval</code></td>
<td>
Compile and print a little snippet of CoffeeScript directly from the
command line. For example:<br /><tt>coffee -e "console.log num for num in [10..1]"</tt>
</td>
</tr>
<tr>
<td><code>-b, --bare</code></td>
<td>
Compile the JavaScript without the
<a href="#lexical-scope">top-level function safety wrapper</a>.
</td>
</tr>
<tr>
<td><code>-t, --tokens</code></td>
<td>
Instead of parsing the CoffeeScript, just lex it, and print out the
token stream: <tt>[IDENTIFIER square] [ASSIGN =] [PARAM_START (]</tt> ...
</td>
</tr>
<tr>
<td><code>-n, --nodes</code></td>
<td>
Instead of compiling the CoffeeScript, just lex and parse it, and print
out the parse tree:
<pre class="no_bar">
Expressions
Assign
Value "square"
Code "x"
Op *
Value "x"
Value "x"</pre>
</td>
</tr>
<tr>
<td><code>--nodejs</code></td>
<td>
The <tt>node</tt> executable has some useful options you can set,
such as<br /> <tt>--debug</tt>, <tt>--debug-brk</tt>, <tt>--max-stack-size</tt>,
and <tt>--expose-gc</tt>. Use this flag to forward options directly to Node.js.
To pass multiple flags, use <tt>--nodejs</tt> multiple times.
</td>
</tr>
</table>
<p>
<b>Examples:</b>
</p>
<ul>
<li>
Compile a directory tree of <tt>.coffee</tt> files in <tt>src</tt> into a parallel
tree of <tt>.js</tt> files in <tt>lib</tt>:<br />
<tt>coffee --compile --output lib/ src/</tt>
</li>
<li>
Watch a file for changes, and recompile it every time the file is saved:<br />
<tt>coffee --watch --compile experimental.coffee</tt>
</li>
<li>
Concatenate a list of files into a single script:<br />
<tt>coffee --join project.js --compile src/*.coffee</tt>
</li>
<li>
Print out the compiled JS from a one-liner:<br />
<tt>coffee -bpe "alert i for i in [0..10]"</tt>
</li>
<li>
All together now, watch and recompile an entire project as you work on it:<br />
<tt>coffee -o lib/ -cw src/</tt>
</li>
<li>
Start the CoffeeScript REPL (<tt>Ctrl-D</tt> to exit, <tt>Ctrl-V</tt>for multi-line):<br />
<tt>coffee</tt>
</li>
</ul>
<h2>
<span id="literate" class="bookmark"></span>
Literate CoffeeScript
</h2>
<p>
Besides being used as an ordinary programming language, CoffeeScript may
also be written in "literate" mode. If you name your file with a
<tt>.litcoffee</tt> 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.
</p>
<p>
Just for kicks, a little bit of the compiler is currently implemented in this fashion:
See it
<a href="https://gist.github.com/jashkenas/3fc3c1a8b1009c00d9df">as a document</a>,
<a href="https://raw.github.com/jashkenas/coffee-script/master/src/scope.litcoffee">raw</a>,
and <a href="http://cl.ly/LxEu">properly highlighted in a text editor</a>.
</p>
<p>
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
<a href="https://github.com/jashkenas/journo">example program</a>,
are <a href="http://ashkenas.com/literate-coffeescript">available in this blog post</a>.
</p>
<h2>
<span id="language" class="bookmark"></span>
Language Reference
</h2>
<p>
<i>
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.
</i>
</p>
<p>
<i>
Many of the examples can be run (where it makes sense) by pressing the <b>run</b>
button on the right, and can be loaded into the "Try CoffeeScript"
console by pressing the <b>load</b> button on the left.
</i>
<p>
First, the basics: CoffeeScript uses significant whitespace to delimit blocks of code.
You don't need to use semicolons <tt>;</tt> 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
<tt>{ }</tt> to surround blocks of code in <a href="#literals">functions</a>,
<a href="#conditionals">if-statements</a>,
<a href="#switch">switch</a>, and <a href="#try">try/catch</a>,
use indentation.
</p>
<p>
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.<br />
<tt>console.log sys.inspect object</tt> → <tt>console.log(sys.inspect(object));</tt>
</p>
<p>
<span id="literals" class="bookmark"></span>
<b class="header">Functions</b>
Functions are defined by an optional list of parameters in parentheses,
an arrow, and the function body. The empty function looks like this:
<tt>-></tt>
</p>
<div class='code'><pre class="idle"><span class="FunctionName">square </span><span class="Keyword">=</span> <span class="FunctionArgument">(x)</span> <span class="Storage">-></span> x <span class="Keyword">*</span> x
<span class="FunctionName">cube </span><span class="Keyword">=</span> <span class="FunctionArgument">(x)</span> <span class="Storage">-></span> square(x) <span class="Keyword">*</span> x
</pre><pre class="idle"><span class="Storage">var</span> cube, square;
<span class="FunctionName">square</span> = <span class="Storage">function</span>(<span class="FunctionArgument">x</span>) {
<span class="Keyword">return</span> x <span class="Keyword">*</span> x;
};
<span class="FunctionName">cube</span> = <span class="Storage">function</span>(<span class="FunctionArgument">x</span>) {
<span class="Keyword">return</span> square(x) <span class="Keyword">*</span> x;
};
</pre><script>window.example2 = "square = (x) -> x * x\ncube = (x) -> square(x) * x\n\nalert cube(5)"</script><div class='minibutton load' onclick='javascript: loadConsole(example2);'>load</div><div class='minibutton ok' onclick='javascript: var cube, square;
square = function(x) {
return x * x;
};
cube = function(x) {
return square(x) * x;
};
;alert(cube(5));'>run: cube(5)</div><br class='clear' /></div>
<p>
Functions may also have default values for arguments. Override the default
value by passing a non-null argument.
</p>
<div class='code'><pre class="idle"><span class="FunctionName">fill </span><span class="Keyword">=</span> <span class="FunctionArgument">(container, liquid = "coffee")</span> <span class="Storage">-></span>
<span class="String"><span class="String">"</span>Filling the <span class="String"><span class="String">#{</span>container<span class="String">}</span></span> with <span class="String"><span class="String">#{</span>liquid<span class="String">}</span></span>...<span class="String">"</span></span>
</pre><pre class="idle"><span class="Storage">var</span> fill;
<span class="FunctionName">fill</span> = <span class="Storage">function</span>(<span class="FunctionArgument">container, liquid</span>) {
<span class="Keyword">if</span> (liquid <span class="Keyword">==</span> <span class="BuiltInConstant">null</span>) {
liquid <span class="Keyword">=</span> <span class="String"><span class="String">"</span>coffee<span class="String">"</span></span>;
}
<span class="Keyword">return</span> <span class="String"><span class="String">"</span>Filling the <span class="String">"</span></span> <span class="Keyword">+</span> container <span class="Keyword">+</span> <span class="String"><span class="String">"</span> with <span class="String">"</span></span> <span class="Keyword">+</span> liquid <span class="Keyword">+</span> <span class="String"><span class="String">"</span>...<span class="String">"</span></span>;
};
</pre><script>window.example3 = "fill = (container, liquid = \"coffee\") ->\n \"Filling the #{container} with #{liquid}...\"\n\nalert fill(\"cup\")"</script><div class='minibutton load' onclick='javascript: loadConsole(example3);'>load</div><div class='minibutton ok' onclick='javascript: var fill;
fill = function(container, liquid) {
if (liquid == null) {
liquid = "coffee";
}
return "Filling the " + container + " with " + liquid + "...";
};
;alert(fill("cup"));'>run: fill("cup")</div><br class='clear' /></div>
<p>
<span id="objects_and_arrays" class="bookmark"></span>
<b class="header">Objects and Arrays</b>
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 <a href="http://yaml.org">YAML</a>.
</p>
<div class='code'><pre class="idle">song <span class="Keyword">=</span> [<span class="String"><span class="String">"</span>do<span class="String">"</span></span>, <span class="String"><span class="String">"</span>re<span class="String">"</span></span>, <span class="String"><span class="String">"</span>mi<span class="String">"</span></span>, <span class="String"><span class="String">"</span>fa<span class="String">"</span></span>, <span class="String"><span class="String">"</span>so<span class="String">"</span></span>]
singers <span class="Keyword">=</span> {Jagger: <span class="String"><span class="String">"</span>Rock<span class="String">"</span></span>, Elvis: <span class="String"><span class="String">"</span>Roll<span class="String">"</span></span>}
bitlist <span class="Keyword">=</span> [
<span class="Number">1</span>, <span class="Number">0</span>, <span class="Number">1</span>
<span class="Number">0</span>, <span class="Number">0</span>, <span class="Number">1</span>
<span class="Number">1</span>, <span class="Number">1</span>, <span class="Number">0</span>
]
kids <span class="Keyword">=</span>
brother:
name: <span class="String"><span class="String">"</span>Max<span class="String">"</span></span>
age: <span class="Number">11</span>
sister:
name: <span class="String"><span class="String">"</span>Ida<span class="String">"</span></span>
age: <span class="Number">9</span>
</pre><pre class="idle"><span class="Storage">var</span> bitlist, kids, singers, song;
song <span class="Keyword">=</span> [<span class="String"><span class="String">"</span>do<span class="String">"</span></span>, <span class="String"><span class="String">"</span>re<span class="String">"</span></span>, <span class="String"><span class="String">"</span>mi<span class="String">"</span></span>, <span class="String"><span class="String">"</span>fa<span class="String">"</span></span>, <span class="String"><span class="String">"</span>so<span class="String">"</span></span>];
singers <span class="Keyword">=</span> {
Jagger: <span class="String"><span class="String">"</span>Rock<span class="String">"</span></span>,
Elvis: <span class="String"><span class="String">"</span>Roll<span class="String">"</span></span>
};
bitlist <span class="Keyword">=</span> [<span class="Number">1</span>, <span class="Number">0</span>, <span class="Number">1</span>, <span class="Number">0</span>, <span class="Number">0</span>, <span class="Number">1</span>, <span class="Number">1</span>, <span class="Number">1</span>, <span class="Number">0</span>];
kids <span class="Keyword">=</span> {
brother: {
name: <span class="String"><span class="String">"</span>Max<span class="String">"</span></span>,
age: <span class="Number">11</span>
},
sister: {
name: <span class="String"><span class="String">"</span>Ida<span class="String">"</span></span>,
age: <span class="Number">9</span>
}
};
</pre><script>window.example4 = "song = [\"do\", \"re\", \"mi\", \"fa\", \"so\"]\n\nsingers = {Jagger: \"Rock\", Elvis: \"Roll\"}\n\nbitlist = [\n 1, 0, 1\n 0, 0, 1\n 1, 1, 0\n]\n\nkids =\n brother:\n name: \"Max\"\n age: 11\n sister:\n name: \"Ida\"\n age: 9\n\nalert song.join(\" ... \")"</script><div class='minibutton load' onclick='javascript: loadConsole(example4);'>load</div><div class='minibutton ok' onclick='javascript: 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
}
};
;alert(song.join(" ... "));'>run: song.join(" ... ")</div><br class='clear' /></div>
<p>
In JavaScript, you can't use reserved words, like <tt>class</tt>, 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).
</p>
<div class='code'><pre class="idle">$(<span class="String"><span class="String">'</span>.account<span class="String">'</span></span>).attr class: <span class="String"><span class="String">'</span>active<span class="String">'</span></span>
log object.<span class="Storage">class</span>
</pre><pre class="idle"><span class="Keyword">$</span>(<span class="String"><span class="String">'</span>.account<span class="String">'</span></span>).attr({
<span class="String"><span class="String">"</span>class<span class="String">"</span></span>: <span class="String"><span class="String">'</span>active<span class="String">'</span></span>
});
<span class="LibraryFunction">log</span>(object[<span class="String"><span class="String">"</span>class<span class="String">"</span></span>]);
</pre><script>window.example5 = "$('.account').attr class: 'active'\n\nlog object.class\n\n\n"</script><div class='minibutton load' onclick='javascript: loadConsole(example5);'>load</div><br class='clear' /></div>
<p>
<span id="lexical-scope" class="bookmark"></span>
<b class="header">Lexical Scoping and Variable Safety</b>
The CoffeeScript compiler takes care to make sure that all of your variables
are properly declared within lexical scope — you never need to write
<tt>var</tt> yourself.
</p>
<div class='code'><pre class="idle">outer <span class="Keyword">=</span> <span class="Number">1</span>
<span class="FunctionName">changeNumbers </span><span class="Keyword">=</span> <span class="Keyword">-</span><span class="Keyword">></span>
inner <span class="Keyword">=</span> <span class="Keyword">-</span><span class="Number">1</span>
outer <span class="Keyword">=</span> <span class="Number">10</span>
inner <span class="Keyword">=</span> changeNumbers()
</pre><pre class="idle"><span class="Storage">var</span> changeNumbers, inner, outer;
outer <span class="Keyword">=</span> <span class="Number">1</span>;
<span class="FunctionName">changeNumbers</span> = <span class="Storage">function</span>() {
<span class="Storage">var</span> inner;
inner <span class="Keyword">=</span> <span class="Keyword">-</span><span class="Number">1</span>;
<span class="Keyword">return</span> outer <span class="Keyword">=</span> <span class="Number">10</span>;
};
inner <span class="Keyword">=</span> changeNumbers();
</pre><script>window.example6 = "outer = 1\nchangeNumbers = ->\n inner = -1\n outer = 10\ninner = changeNumbers()\n\nalert inner"</script><div class='minibutton load' onclick='javascript: loadConsole(example6);'>load</div><div class='minibutton ok' onclick='javascript: var changeNumbers, inner, outer;
outer = 1;
changeNumbers = function() {
var inner;
inner = -1;
return outer = 10;
};
inner = changeNumbers();
;alert(inner);'>run: inner</div><br class='clear' /></div>
<p>
Notice how all of the variable declarations have been pushed up to
the top of the closest scope, the first time they appear.
<b>outer</b> is not redeclared within the inner function, because it's
already in scope; <b>inner</b> 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.
</p>
<p>
This behavior is effectively identical to Ruby's scope for local variables.
Because you don't have direct access to the <tt>var</tt> 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.
</p>
<p>
Although suppressed within this documentation for clarity, all
CoffeeScript output is wrapped in an anonymous function:
<tt>(function(){ ... })();</tt> This safety wrapper, combined with the
automatic generation of the <tt>var</tt> keyword, make it exceedingly difficult
to pollute the global namespace by accident.
</p>
<p>
If you'd like to create top-level variables for other scripts to use,
attach them as properties on <b>window</b>, or on the <b>exports</b>
object in CommonJS. The <b>existential operator</b> (covered below), gives you a
reliable way to figure out where to add them; if you're targeting both
CommonJS and the browser: <tt>exports ? this</tt>
</p>
<p>
<span id="conditionals" class="bookmark"></span>
<b class="header">If, Else, Unless, and Conditional Assignment</b>
<b>If/else</b> 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 <tt>if</tt> or <tt>unless</tt> at the end.
</p>
<p>
CoffeeScript can compile <b>if</b> 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 <b>if</b> statement on a single line.
</p>
<div class='code'><pre class="idle">mood <span class="Keyword">=</span> greatlyImproved <span class="Keyword">if</span> singing
<span class="Keyword">if</span> happy <span class="Keyword">and</span> knowsIt
clapsHands()
chaChaCha()
<span class="Keyword">else</span>
showIt()
date <span class="Keyword">=</span> <span class="Keyword">if</span> friday <span class="Keyword">then</span> sue <span class="Keyword">else</span> jill
</pre><pre class="idle"><span class="Storage">var</span> date, mood;
<span class="Keyword">if</span> (singing) {
mood <span class="Keyword">=</span> greatlyImproved;
}
<span class="Keyword">if</span> (happy <span class="Keyword">&</span><span class="Keyword">&</span> knowsIt) {
clapsHands();
chaChaCha();
} <span class="Keyword">else</span> {
showIt();
}
date <span class="Keyword">=</span> friday ? sue : jill;
</pre><script>window.example7 = "mood = greatlyImproved if singing\n\nif happy and knowsIt\n clapsHands()\n chaChaCha()\nelse\n showIt()\n\ndate = if friday then sue else jill\n\n\n\n"</script><div class='minibutton load' onclick='javascript: loadConsole(example7);'>load</div><br class='clear' /></div>
<p>
<span id="splats" class="bookmark"></span>
<b class="header">Splats...</b>
The JavaScript <b>arguments object</b> is a useful way to work with
functions that accept variable numbers of arguments. CoffeeScript provides
splats <tt>...</tt>, both for function definition as well as invocation,
making variable numbers of arguments a little bit more palatable.
</p>
<div class='code'><pre class="idle">gold <span class="Keyword">=</span> silver <span class="Keyword">=</span> rest <span class="Keyword">=</span> <span class="String"><span class="String">"</span>unknown<span class="String">"</span></span>
<span class="FunctionName">awardMedals </span><span class="Keyword">=</span> <span class="FunctionArgument">(first, second, others...)</span> <span class="Storage">-></span>
gold <span class="Keyword">=</span> first
silver <span class="Keyword">=</span> second
rest <span class="Keyword">=</span> others
contenders <span class="Keyword">=</span> [
<span class="String"><span class="String">"</span>Michael Phelps<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Liu Xiang<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Yao Ming<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Allyson Felix<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Shawn Johnson<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Roman Sebrle<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Guo Jingjing<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Tyson Gay<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Asafa Powell<span class="String">"</span></span>
<span class="String"><span class="String">"</span>Usain Bolt<span class="String">"</span></span>
]
awardMedals contenders...
alert <span class="String"><span class="String">"</span>Gold: <span class="String">"</span></span> <span class="Keyword">+</span> gold
alert <span class="String"><span class="String">"</span>Silver: <span class="String">"</span></span> <span class="Keyword">+</span> silver
alert <span class="String"><span class="String">"</span>The Field: <span class="String">"</span></span> <span class="Keyword">+</span> rest
</pre><pre class="idle"><span class="Storage">var</span> awardMedals, contenders, gold, rest, silver,
__slice <span class="Keyword">=</span> [].slice;
gold <span class="Keyword">=</span> silver <span class="Keyword">=</span> rest <span class="Keyword">=</span> <span class="String"><span class="String">"</span>unknown<span class="String">"</span></span>;
<span class="FunctionName">awardMedals</span> = <span class="Storage">function</span>() {
<span class="Storage">var</span> first, others, second;
first <span class="Keyword">=</span> arguments[<span class="Number">0</span>], second <span class="Keyword">=</span> arguments[<span class="Number">1</span>], others <span class="Keyword">=</span> <span class="Number">3</span> <span class="Keyword"><=</span> arguments.<span class="LibraryConstant">length</span> ? __slice.<span class="LibraryFunction">call</span>(arguments, <span class="Number">2</span>) : [];
gold <span class="Keyword">=</span> first;
silver <span class="Keyword">=</span> second;
<span class="Keyword">return</span> rest <span class="Keyword">=</span> others;
};
contenders <span class="Keyword">=</span> [<span class="String"><span class="String">"</span>Michael Phelps<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Liu Xiang<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Yao Ming<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Allyson Felix<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Shawn Johnson<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Roman Sebrle<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Guo Jingjing<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Tyson Gay<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Asafa Powell<span class="String">"</span></span>, <span class="String"><span class="String">"</span>Usain Bolt<span class="String">"</span></span>];
awardMedals.<span class="LibraryFunction">apply</span>(<span class="BuiltInConstant">null</span>, contenders);
<span class="LibraryFunction">alert</span>(<span class="String"><span class="String">"</span>Gold: <span class="String">"</span></span> <span class="Keyword">+</span> gold);
<span class="LibraryFunction">alert</span>(<span class="String"><span class="String">"</span>Silver: <span class="String">"</span></span> <span class="Keyword">+</span> silver);
<span class="LibraryFunction">alert</span>(<span class="String"><span class="String">"</span>The Field: <span class="String">"</span></span> <span class="Keyword">+</span> rest);
</pre><script>window.example8 = "gold = silver = rest = \"unknown\"\n\nawardMedals = (first, second, others...) ->\n gold = first\n silver = second\n rest = others\n\ncontenders = [\n \"Michael Phelps\"\n \"Liu Xiang\"\n \"Yao Ming\"\n \"Allyson Felix\"\n \"Shawn Johnson\"\n \"Roman Sebrle\"\n \"Guo Jingjing\"\n \"Tyson Gay\"\n \"Asafa Powell\"\n \"Usain Bolt\"\n]\n\nawardMedals contenders...\n\nalert \"Gold: \" + gold\nalert \"Silver: \" + silver\nalert \"The Field: \" + rest\n\n\n"</script><div class='minibutton load' onclick='javascript: loadConsole(example8);'>load</div><div class='minibutton ok' onclick='javascript: 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);
;'>run</div><br class='clear' /></div>
<p>
<span id="loops" class="bookmark"></span>
<b class="header">Loops and Comprehensions</b>
Most of the loops you'll write in CoffeeScript will be <b>comprehensions</b>
over arrays, objects, and ranges. Comprehensions replace (and compile into)
<b>for</b> 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.
</p>
<div class='code'><pre class="idle"><span class="Comment"><span class="Comment">#</span> Eat lunch.</span>
eat food <span class="Keyword">for</span> food <span class="Keyword">in</span> [<span class="String"><span class="String">'</span>toast<span class="String">'</span></span>, <span class="String"><span class="String">'</span>cheese<span class="String">'</span></span>, <span class="String"><span class="String">'</span>wine<span class="String">'</span></span>]
<span class="Comment"><span class="Comment">#</span> Fine five course dining.</span>
courses <span class="Keyword">=</span> [<span class="String"><span class="String">'</span>greens<span class="String">'</span></span>, <span class="String"><span class="String">'</span>caviar<span class="String">'</span></span>, <span class="String"><span class="String">'</span>truffles<span class="String">'</span></span>, <span class="String"><span class="String">'</span>roast<span class="String">'</span></span>, <span class="String"><span class="String">'</span>cake<span class="String">'</span></span>]
menu i <span class="Keyword">+</span> <span class="Number">1</span>, dish <span class="Keyword">for</span> dish, i <span class="Keyword">in</span> courses
<span class="Comment"><span class="Comment">#</span> Health conscious meal.</span>
foods <span class="Keyword">=</span> [<span class="String"><span class="String">'</span>broccoli<span class="String">'</span></span>, <span class="String"><span class="String">'</span>spinach<span class="String">'</span></span>, <span class="String"><span class="String">'</span>chocolate<span class="String">'</span></span>]
eat food <span class="Keyword">for</span> food <span class="Keyword">in</span> foods <span class="Keyword">when</span> food <span class="Keyword">isnt</span> <span class="String"><span class="String">'</span>chocolate<span class="String">'</span></span>
</pre><pre class="idle"><span class="Storage">var</span> courses, dish, food, foods, i, _i, _j, _k, _len, _len1, _len2, _ref;
_ref <span class="Keyword">=</span> [<span class="String"><span class="String">'</span>toast<span class="String">'</span></span>, <span class="String"><span class="String">'</span>cheese<span class="String">'</span></span>, <span class="String"><span class="String">'</span>wine<span class="String">'</span></span>];
<span class="Keyword">for</span> (_i <span class="Keyword">=</span> <span class="Number">0</span>, _len <span class="Keyword">=</span> _ref.<span class="LibraryConstant">length</span>; _i <span class="Keyword"><</span> _len; _i<span class="Keyword">++</span>) {
food <span class="Keyword">=</span> _ref[_i];
eat(food);
}
courses <span class="Keyword">=</span> [<span class="String"><span class="String">'</span>greens<span class="String">'</span></span>, <span class="String"><span class="String">'</span>caviar<span class="String">'</span></span>, <span class="String"><span class="String">'</span>truffles<span class="String">'</span></span>, <span class="String"><span class="String">'</span>roast<span class="String">'</span></span>, <span class="String"><span class="String">'</span>cake<span class="String">'</span></span>];
<span class="Keyword">for</span> (i <span class="Keyword">=</span> _j <span class="Keyword">=</span> <span class="Number">0</span>, _len1 <span class="Keyword">=</span> courses.<span class="LibraryConstant">length</span>; _j <span class="Keyword"><</span> _len1; i <span class="Keyword">=</span> <span class="Keyword">++</span>_j) {
dish <span class="Keyword">=</span> courses[i];
menu(i <span class="Keyword">+</span> <span class="Number">1</span>, dish);
}
foods <span class="Keyword">=</span> [<span class="String"><span class="String">'</span>broccoli<span class="String">'</span></span>, <span class="String"><span class="String">'</span>spinach<span class="String">'</span></span>, <span class="String"><span class="String">'</span>chocolate<span class="String">'</span></span>];
<span class="Keyword">for</span> (_k <span class="Keyword">=</span> <span class="Number">0</span>, _len2 <span class="Keyword">=</span> foods.<span class="LibraryConstant">length</span>; _k <span class="Keyword"><</span> _len2; _k<span class="Keyword">++</span>) {
food <span class="Keyword">=</span> foods[_k];
<span class="Keyword">if</span> (food <span class="Keyword">!</span><span class="Keyword">==</span> <span class="String"><span class="String">'</span>chocolate<span class="String">'</span></span>) {
eat(food);
}
}
</pre><script>window.example9 = "# Eat lunch.\neat food for food in ['toast', 'cheese', 'wine']\n\n# Fine five course dining.\ncourses = ['greens', 'caviar', 'truffles', 'roast', 'cake']\nmenu i + 1, dish for dish, i in courses\n\n# Health conscious meal.\nfoods = ['broccoli', 'spinach', 'chocolate']\neat food for food in foods when food isnt 'chocolate'\n"</script><div class='minibutton load' onclick='javascript: loadConsole(example9);'>load</div><br class='clear' /></div>
<p>
Comprehensions should be able to handle most places where you otherwise
would use a loop, <b>each</b>/<b>forEach</b>, <b>map</b>, or <b>select</b>/<b>filter</b>, for example:
<tt>shortNames = (name for name in list when name.length < 5)</tt><br />
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.
</p>
<div class='code'><pre class="idle">countdown <span class="Keyword">=</span> (num <span class="Keyword">for</span> num <span class="Keyword">in</span> [<span class="Number">10</span>..<span class="Number">1</span>])
</pre><pre class="idle"><span class="Storage">var</span> countdown, num;
countdown <span class="Keyword">=</span> (<span class="Storage">function</span>() {
<span class="Storage">var</span> _i, _results;
_results <span class="Keyword">=</span> [];
<span class="Keyword">for</span> (num <span class="Keyword">=</span> _i <span class="Keyword">=</span> <span class="Number">10</span>; _i <span class="Keyword">>=</span> <span class="Number">1</span>; num <span class="Keyword">=</span> <span class="Keyword">--</span>_i) {
_results.<span class="LibraryFunction">push</span>(num);
}
<span class="Keyword">return</span> _results;
})();
</pre><script>window.example10 = "countdown = (num for num in [10..1])\n\nalert countdown"</script><div class='minibutton load' onclick='javascript: loadConsole(example10);'>load</div><div class='minibutton ok' onclick='javascript: var countdown, num;
countdown = (function() {
var _i, _results;
_results = [];
for (num = _i = 10; _i >= 1; num = --_i) {
_results.push(num);
}
return _results;
})();
;alert(countdown);'>run: countdown</div><br class='clear' /></div>
<p>
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 <tt>true</tt> — or <tt>null</tt>,
to the bottom of your function.
</p>
<p>
To step through a range comprehension in fixed-size chunks,
use <tt>by</tt>, for example:<br />
<tt>evens = (x for x in [0..10] by 2)</tt>
</p>
<p>
Comprehensions can also be used to iterate over the keys and values in
an object. Use <tt>of</tt> to signal comprehension over the properties of
an object instead of the values in an array.
</p>
<div class='code'><pre class="idle">yearsOld <span class="Keyword">=</span> max: <span class="Number">10</span>, ida: <span class="Number">9</span>, tim: <span class="Number">11</span>
ages <span class="Keyword">=</span> <span class="Keyword">for</span> child, age <span class="Keyword">of</span> yearsOld
<span class="String"><span class="String">"</span><span class="String"><span class="String">#{</span>child<span class="String">}</span></span> is <span class="String"><span class="String">#{</span>age<span class="String">}</span></span><span class="String">"</span></span>
</pre><pre class="idle"><span class="Storage">var</span> age, ages, child, yearsOld;
yearsOld <span class="Keyword">=</span> {
max: <span class="Number">10</span>,
ida: <span class="Number">9</span>,
tim: <span class="Number">11</span>
};
ages <span class="Keyword">=</span> (<span class="Storage">function</span>() {
<span class="Storage">var</span> _results;
_results <span class="Keyword">=</span> [];
<span class="Keyword">for</span> (child <span class="Keyword">in</span> yearsOld) {
age <span class="Keyword">=</span> yearsOld[child];
_results.<span class="LibraryFunction">push</span>(<span class="String"><span class="String">"</span><span class="String">"</span></span> <span class="Keyword">+</span> child <span class="Keyword">+</span> <span class="String"><span class="String">"</span> is <span class="String">"</span></span> <span class="Keyword">+</span> age);
}
<span class="Keyword">return</span> _results;
})();
</pre><script>window.example11 = "yearsOld = max: 10, ida: 9, tim: 11\n\nages = for child, age of yearsOld\n \"#{child} is #{age}\"\n\nalert ages.join(\", \")"</script><div class='minibutton load' onclick='javascript: loadConsole(example11);'>load</div><div class='minibutton ok' onclick='javascript: 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;
})();
;alert(ages.join(", "));'>run: ages.join(", ")</div><br class='clear' /></div>
<p>
If you would like to iterate over just the keys that are defined on the
object itself, by adding a <tt>hasOwnProperty</tt>
check to avoid properties that may be inherited from the prototype, use<br />
<tt>for own key, value of object</tt>
</p>
<p>
The only low-level loop that CoffeeScript provides is the <b>while</b> loop. The
main difference from JavaScript is that the <b>while</b> loop can be used
as an expression, returning an array containing the result of each iteration
through the loop.