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index.html.erb
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<%
require 'uv'
require 'json'
@counter = 0
def code_for(file, executable=false, show_load=true)
@counter += 1
return '' unless File.exists?("documentation/js/#{file}.js")
cs = File.read("documentation/coffee/#{file}.coffee")
js = File.read("documentation/js/#{file}.js")
cshtml = Uv.parse(cs, 'xhtml', 'coffeescript', false, 'idle', false)
jshtml = Uv.parse(js, 'xhtml', 'javascript', false, 'idle', false)
append = executable == true ? '' : "alert(#{executable});"
if executable and executable != true
cs.sub!(/(\S)\s*\Z/m, "\\1\n\nalert #{executable}")
end
run = executable == true ? 'run' : "run: #{executable}"
name = "example#{@counter}"
script = "<script>window.#{name} = #{cs.to_json}</script>"
import = show_load ? "<div class='minibutton load' onclick='javascript: loadConsole(#{name});'>load</div>" : ''
button = executable ? "<div class='minibutton ok' onclick='javascript: #{js};#{append}'>#{run}</div>" : ''
"<div class='code'>#{cshtml}#{jshtml}#{script}#{import}#{button}<br class='clear' /></div>"
end
%>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=UTF-8" />
<title>CoffeeScript</title>
<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" />
<script>
<%= File.read("documentation/js/tame_inc.js") %>
</script>
</head>
<body>
<div id="fadeout"></div>
<div id="flybar">
<a id="logo" href="#top"> </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="#language">Language Reference</a>
<a href="#tame">Tame Basic Features</a>
<a href="#tame_defer">Tame Defer Values</a>
<a href="#tame_expr">Tame Expressions</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, Heredocs, and Block Comments</a>
<a href="#regexes">Extended Regular Expressions</a>
<a href="#cake">Cake, and Cakefiles</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>
</div>
</div>
<div id="error" style="display:none;"></div>
</div>
<div class="container">
<span class="bookmark" id="top"></span>
<p>
CoffeeScript is a little language that compiles into JavaScript.
</p>
<p>
But mind you, this is a <b>fork</b> of CoffeeScript,
called <i>AdvancedCoffeeSubstitute (ACS)</i>.
<b>ACS is a drop-in replacement for CoffeeScript
designed to simplify and parallelize asynchronous control flow, both on the
browser, and in the server.</b>
</p>
<p>
The golden rule of CoffeeScript was: <i>"It's just JavaScript"</i>. ACS
is doing something deeper. It's selectively running code through a
<i>continuation-passing style conversion</i>. Original-style
CoffeeScript will compile as before, but code using the new ACS
features will be less recognizable. Still, the compiled output
is readable and pretty-printed, passes through
<a href="http://www.javascriptlint.com/">JavaScript Lint</a>
without warnings, will work in every JavaScript implementation, and tends
to run as fast or faster than the equivalent handwritten JavaScript.
</p>
<p>
<b>Latest Version:</b>
<a href="http://github.com/maxtaco/coffee-script/tarball/1.2.0">1.2.0</a>
</p>
<h2>
<span id="overview" class="bookmark"></span>
Overview
</h2>
<p><i>CoffeeScript on the left, compiled JavaScript output on the right.</i></p>
<%= code_for('overview', 'cubes', false) %>
<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 master 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 CoffeeScript compiler system-wide
under <tt>/usr/local</tt>, open the directory and run:
</p>
<pre>
sudo bin/cake install</pre>
<p>
If installing on Ubuntu or Debian,
<a href="http://opinionatedprogrammer.com/2010/12/installing-coffeescript-on-debian-or-ubuntu/">be
careful not to use the existing out-of-date package</a>.
</p>
<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 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>-l, --lint</code></td>
<td>
If the <tt>jsl</tt>
(<a href="http://www.javascriptlint.com/">JavaScript Lint</a>)
command is installed, use it
to check the compilation of a CoffeeScript file. (Handy in
conjunction with <br /> <tt>--watch</tt>)
</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>-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>-r, --require</code></td>
<td>
Load a library before compiling or executing your script. Can be used
to hook in to the compiler (to add Growl notifications, for example).
</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> and <tt>--max-stack-size</tt>. Use this
flag to forward options directly to Node.js.
</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:<br />
<tt>coffee</tt>
</li>
</ul>
<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. <b>Early sections focus on new/forked ACS features</b>.
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="#functions">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 ="tame" class = "bookmark"></span>
<b class="header">Tame Basics</b>
Here is simple example that waits for 1 second, and then calls <tt>alert</tt>:
</p>
<p>
<%= code_for('tame_basics_1', true) %>
</p>
<p>
There is one new language addition here, the <tt>await { ... }</tt> block,
and also one new primitive
function, <tt>defer</tt>. The two of them work in concert. A
function must "wait" at the close of an await block until all
deferrals made in that await block are fulfilled. The
function <tt>defer</tt> returns a callback, and a callee in an
await block can fulfill a deferral by simply calling the callback
it was given. In the code above, there is only one deferral
produced, so after it's fulfilled by `setTimer` in 1000ms, control
continues past the `await` block, onto the alert. The code looks
and feels like threaded code, but is still in the asynchronous
idiom (if you look at the rewritten code output by
the <tt>coffee</tt> compiler).
</p>
<p>
The next example shows some of the power of the tame primitives.
In the following examples, there are two operations that fire in
parallel, and a third that fires after the first two complete:
</p>
<p>
<%= code_for('tame_basics_2', true) %>
</p>
<p>
As one might expect, the tame constructs interoperate with standard
coffee control structures. The following program will call
<tt>slowAlert</tt> three times sequentially:
</p>
<p>
<%= code_for('tame_loops_1', true) %>
</p>
<p>
To fire those asynchronous calls in parallel, simply exchange
the <tt>for</tt> and <tt>await</tt> blocks:
</p>
<p>
<%= code_for('tame_loops_2', true) %>
</p>
Things start to get interesting when you combine parallel and
serial control flow, but the syntax remains the same:
<p>
<%= code_for('tame_loops_3', true) %>
</p>
<p>
<span id ="tame_defer" class = "bookmark"></span>
<b class="header">Tame Defer Values</b>
Many JS and coffee libraries use callbacks passed as parameters
to signify when the given operation has completed, and which values
were produced. As such, <tt>defer</tt> can take one or more arguments
which are filled in after a deferral is fulfilled. Note that the
snippet below is <i>node.js</i> code that requires a DNS resolver:
<p>
<%= code_for('tame_defer') %>
</p>
<p>
<span id="tame_expr" class = "bookmark"></span>
<b class="header">Tame in an Expression Context</b>
It's possible
to use <tt>await</tt> blocks as expressions. And recursively
speaking, it's possible to use statements that contain `await`
blocks as expressions.
</p>
<p>
The simple rule is that an <tt>await</tt> block takes on the value of the
<tt>defer</tt> slot named <tt>_</tt> after its been fulfilled. If there
are multiple defer slots named <tt>_</tt> within the await block, then
the last writer wins. In practice, there's usually only one. Thus:
</p>
<p><%= code_for('tame_expr_1', true) %></p>
<p>
Of course, things can get arbitrarily compicated and nested, so it
gets hairy. Consider this:
</p>
<p><%= code_for('tame_expr_2', true) %></p>
<p>
The rule is that all nested <tt>await</tt> blocks (barf!) are evaluated
sequentially in DFS order. You will get <tt>10</tt> in the above example
after three sequential calls to <tt>add</tt>.
</p>
<p>
I really don't like this feature for two reasons: (1) it's tricky to
get the implementation right, and I'm sure I haven't tested all of the
corner cases yet; (2) it's difficult to read and understand what
happens in which order. I would suggest you save yourself the heartache,
and just write the above as this:
</p>
<p><%= code_for('tame_expr_3', true) %></p>
<p>
It's just so much clearer what happens in which order, and it's easier
to parallelize or serialize as you see fit.
</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>
<%= code_for('functions', 'cube(5)') %>
<p>
Functions may also have default values for arguments. Override the default
value by passing a non-null argument.
</p>
<%= code_for('default_args', 'fill("cup")') %>
<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>
<%= code_for('objects_and_arrays', 'song.join(" ... ")') %>
<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>
<%= code_for('objects_reserved') %>
<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>
<%= code_for('scope', 'inner') %>
<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>
<%= code_for('conditionals') %>
<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>
<%= code_for('splats', true) %>
<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>
<%= code_for('array_comprehensions') %>
<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>
<%= code_for('range_comprehensions', 'countdown') %>
<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>
<%= code_for('object_comprehensions', 'ages.join(", ")') %>
<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 interited 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.
</p>
<%= code_for('while', 'lyrics.join("\n")') %>
<p>
For readability, the <b>until</b> keyword is equivalent to <tt>while not</tt>,
and the <b>loop</b> keyword is equivalent to <tt>while true</tt>.
</p>
<p>
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 <tt>do</tt> keyword, which immediately invokes a passed function,
forwarding any arguments.
</p>
<%= code_for('do') %>
<p>
<span id="slices" class="bookmark"></span>
<b class="header">Array Slicing and Splicing with Ranges</b>
Ranges can also be used to extract slices of arrays.
With two dots (<tt>3..6</tt>), the range is inclusive (<tt>3, 4, 5, 6</tt>);
with three dots (<tt>3...6</tt>), the range excludes the end (<tt>3, 4, 5</tt>).
</p>
<%= code_for('slices', 'middle') %>
<p>
The same syntax can be used with assignment to replace a segment of an array
with new values, splicing it.
</p>
<%= code_for('splices', 'numbers') %>
<p>
Note that JavaScript strings are immutable, and can't be spliced.
</p>
<p>
<span id="expressions" class="bookmark"></span>
<b class="header">Everything is an Expression (at least, as much as possible)</b>
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 <tt>return</tt> gets
pushed down into each possible branch of execution in the function
below.
</p>
<%= code_for('expressions', 'eldest') %>
<p>
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 (<tt>return value</tt>), when you know that you're done.
</p>
<p>
Because variable declarations occur at the top of scope, assignment can
be used within expressions, even for variables that haven't been seen before:
</p>
<%= code_for('expressions_assignment', 'six') %>
<p>
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:
</p>
<%= code_for('expressions_comprehension', 'globals') %>
<p>
As well as silly things, like passing a <b>try/catch</b> statement directly
into a function call:
</p>
<%= code_for('expressions_try', true) %>
<p>
There are a handful of statements in JavaScript that can't be meaningfully
converted into expressions, namely <tt>break</tt>, <tt>continue</tt>,
and <tt>return</tt>. If you make use of them within a block of code,
CoffeeScript won't try to perform the conversion.
</p>
<p>
<span id="operators" class="bookmark"></span>
<b class="header">Operators and Aliases</b>
Because the <tt>==</tt> operator frequently causes undesirable coercion,
is intransitive, and has a different meaning than in other languages,
CoffeeScript compiles <tt>==</tt> into <tt>===</tt>, and <tt>!=</tt> into
<tt>!==</tt>.
In addition, <tt>is</tt> compiles into <tt>===</tt>,
and <tt>isnt</tt> into <tt>!==</tt>.
</p>
<p>
You can use <tt>not</tt> as an alias for <tt>!</tt>.
</p>
<p>
For logic, <tt>and</tt> compiles to <tt>&&</tt>, and <tt>or</tt>
into <tt>||</tt>.
</p>
<p>
Instead of a newline or semicolon, <tt>then</tt> can be used to separate
conditions from expressions, in <b>while</b>,
<b>if</b>/<b>else</b>, and <b>switch</b>/<b>when</b> statements.
</p>
<p>
As in <a href="http://yaml.org/">YAML</a>, <tt>on</tt> and <tt>yes</tt>
are the same as boolean <tt>true</tt>, while <tt>off</tt> and <tt>no</tt> are boolean <tt>false</tt>.
</p>
<p>
<tt>unless</tt> can be used as the inverse of <tt>if</tt>.
</p>
<p>
As a shortcut for <tt>this.property</tt>, you can use <tt>@property</tt>.
</p>
<p>
You can use <tt>in</tt> to test for array presence, and <tt>of</tt> to
test for JavaScript object-key presence.
</p>
<p>
All together now:
</p>
<table class="definitions">
<tr><th>CoffeeScript</th><th>JavaScript</th></tr>
<tr><td><tt>is</tt></td><td><tt>===</tt></td></tr>
<tr><td><tt>isnt</tt></td><td><tt>!==</tt></td></tr>
<tr><td><tt>not</tt></td><td><tt>!</tt></td></tr>
<tr><td><tt>and</tt></td><td><tt>&&</tt></td></tr>
<tr><td><tt>or</tt></td><td><tt>||</tt></td></tr>
<tr><td><tt>true, yes, on</tt></td><td><tt>true</tt></td></tr>
<tr><td><tt>false, no, off</tt></td><td><tt>false</tt></td></tr>
<tr><td><tt>@, this</tt></td><td><tt>this</tt></td></tr>
<tr><td><tt>of</tt></td><td><tt>in</tt></td></tr>
<tr><td><tt>in</tt></td><td><i><small>no JS equivalent</small></i></td></tr>
</table>
<%= code_for('aliases') %>
<p>
<b class="header">The Existential Operator</b>
It's a little difficult to check for the existence of a variable in
JavaScript. <tt>if (variable) ...</tt> comes close, but fails for zero,
the empty string, and false. CoffeeScript's existential operator <tt>?</tt> returns true unless
a variable is <b>null</b> or <b>undefined</b>, which makes it analogous
to Ruby's <tt>nil?</tt>
</p>
<p>
It can also be used for safer conditional assignment than <tt>||=</tt>
provides, for cases where you may be handling numbers or strings.
</p>
<%= code_for('existence', 'footprints') %>
<p>
The accessor variant of the existential operator <tt>?.</tt> can be used to soak
up null references in a chain of properties. Use it instead
of the dot accessor <tt>.</tt> in cases where the base value may be <b>null</b>
or <b>undefined</b>. If all of the properties exist then you'll get the expected
result, if the chain is broken, <b>undefined</b> is returned instead of
the <b>TypeError</b> that would be raised otherwise.
</p>
<%= code_for('soaks') %>
<p>
Soaking up nulls is similar to Ruby's
<a href="http://andand.rubyforge.org/">andand gem</a>, and to the
<a href="http://groovy.codehaus.org/Operators#Operators-SafeNavigationOperator%28%3F.%29">safe navigation operator</a>
in Groovy.
</p>
<p>
<span id="classes" class="bookmark"></span>
<b class="header">Classes, Inheritance, and Super</b>
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:
<a href="http://code.google.com/p/base2/">Base2</a>,
<a href="http://prototypejs.org/">Prototype.js</a>,
<a href="http://jsclass.jcoglan.com/">JS.Class</a>, 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 <b>super</b> (the prototype object's
implementation of the current function), and it's awkward to correctly
set the prototype chain.
</p>
<p>
Instead of repetitively attaching functions to a prototype, CoffeeScript
provides a basic <tt>class</tt> structure that allows you to name your class,
set the superclass, assign prototypal properties, and define the constructor,
in a single assignable expression.
</p>
<p>
Constructor functions are named, to better support helpful stack traces.
In the first class in the example below, <tt>this.constructor.name is "Animal"</tt>.
</p>
<%= code_for('classes', true) %>
<p>
If structuring your prototypes classically isn't your cup of tea, CoffeeScript
provides a couple of lower-level conveniences. The <tt>extends</tt> operator
helps with proper prototype setup, and can be used to create an inheritance
chain between any pair of constructor functions; <tt>::</tt> gives you
quick access to an object's prototype; and <tt>super()</tt>
is converted into a call against the immediate ancestor's method of the same name.
</p>
<%= code_for('prototypes', '"one_two".dasherize()') %>
<p>
Finally, class definitions are blocks of executable code, which make for interesting
metaprogramming possibilities. Because in the context of a class definition,
<tt>this</tt> is the class object itself (the constructor function), you
can assign static properties by using <br /><tt>@property: value</tt>, and call
functions defined in parent classes: <tt>@attr 'title', type: 'text'</tt>
</p>
<p>
<span id="destructuring" class="bookmark"></span>
<b class="header">Destructuring Assignment</b>
To make extracting values from complex arrays and objects more convenient,
CoffeeScript implements ECMAScript Harmony's proposed
<a href="http://wiki.ecmascript.org/doku.php?id=harmony:destructuring">destructuring assignment</a>
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:
</p>
<%= code_for('parallel_assignment', 'theBait') %>
<p>
But it's also helpful for dealing with functions that return multiple
values.
</p>
<%= code_for('multiple_return_values', 'forecast') %>
<p>
Destructuring assignment can be used with any depth of array and object nesting,
to help pull out deeply nested properties.
</p>
<%= code_for('object_extraction', 'name + " - " + street') %>
<p>
Destructuring assignment can even be combined with splats.
</p>
<%= code_for('patterns_and_splats', 'contents.join("")') %>
<p>
<span id="fat_arrow" class="bookmark"></span>
<b class="header">Function binding</b>
In JavaScript, the <tt>this</tt> 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 <tt>this</tt>
will be lost. If you're not familiar with this behavior,
<a href="http://www.digital-web.com/articles/scope_in_javascript/">this Digital Web article</a>
gives a good overview of the quirks.
</p>
<p>
The fat arrow <tt>=></tt> can be used to both define a function, and to bind
it to the current value of <tt>this</tt>, right on the spot. This is helpful
when using callback-based libraries like Prototype or jQuery, for creating
iterator functions to pass to <tt>each</tt>, or event-handler functions
to use with <tt>bind</tt>. Functions created with the fat arrow are able to access
properties of the <tt>this</tt> where they're defined.
</p>
<%= code_for('fat_arrow') %>
<p>
If we had used <tt>-></tt> in the callback above, <tt>@customer</tt> would
have referred to the undefined "customer" property of the DOM element,
and trying to call <tt>purchase()</tt> on it would have raised an exception.
</p>
<p>
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.
</p>
<p>
<span id="embedded" class="bookmark"></span>
<b class="header">Embedded JavaScript</b>
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.
</p>
<%= code_for('embedded', 'hi()') %>
<p>
<span id="switch" class="bookmark"></span>
<b class="header">Switch/When/Else</b>
<b>Switch</b> statements in JavaScript are a bit awkward. You need to
remember to <b>break</b> at the end of every <b>case</b> statement to
avoid accidentally falling through to the default case.
CoffeeScript prevents accidental fall-through, and can convert the <tt>switch</tt>
into a returnable, assignable expression. The format is: <tt>switch</tt> condition,
<tt>when</tt> clauses, <tt>else</tt> the default case.
</p>
<p>
As in Ruby, <b>switch</b> statements in CoffeeScript can take multiple
values for each <b>when</b> clause. If any of the values match, the clause
runs.
</p>
<%= code_for('switch') %>
<p>
<span id="try" class="bookmark"></span>
<b class="header">Try/Catch/Finally</b>
Try/catch statements are just about the same as JavaScript (although
they work as expressions).
</p>
<%= code_for('try') %>
<p>
<span id="comparisons" class="bookmark"></span>
<b class="header">Chained Comparisons</b>
CoffeeScript borrows
<a href="http://docs.python.org/reference/expressions.html#notin">chained comparisons</a>
from Python — making it easy to test if a value falls within a
certain range.
</p>
<%= code_for('comparisons', 'healthy') %>
<p>
<span id="strings" class="bookmark"></span>
<b class="header">String Interpolation, Heredocs, and Block Comments</b>
Ruby-style string interpolation is included in CoffeeScript. Double-quoted
strings allow for interpolated values, using <tt>#{ ... }</tt>,
and single-quoted strings are literal.
</p>
<%= code_for('interpolation', 'sentence') %>
<p>
Multiline strings are allowed in CoffeeScript.
</p>
<%= code_for('strings', 'mobyDick') %>
<p>
Heredocs can be used to hold formatted or indentation-sensitive text
(or, if you just don't feel like escaping quotes and apostrophes). The
indentation level that begins the heredoc is maintained throughout, so
you can keep it all aligned with the body of your code.
</p>
<%= code_for('heredocs', 'html') %>
<p>
Double-quoted heredocs, like double-quoted strings, allow interpolation.
</p>
<p>
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 heredocs,
are preserved in the generated code.
</p>
<%= code_for('block_comment') %>