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CoffeeScript in a Nutshell, Part 1: An Introduction to CoffeeScript

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A productive alternative to JavaScript, the CoffeeScript language can speed up development of your HTML5-based games and other browser apps. In Part 1 of a four-part series on CoffeeScript, Jeff Friesen introduces you to CoffeeScript and shows you how to obtain and use the CoffeeScript compiler.

In December 2012, I wrote an article for SitePoint titled "Happy Holidays from CoffeeScript." Although I had planned to write the article's HHFCS application source code in JavaScript, I found it much easier and faster to write the code (especially the class-oriented code) in CoffeeScript. I think you'll find that CoffeeScript improves your productivity as well.

In this four-part series, I'll introduce you to the CoffeeScript technology. Part 1 (this article) begins by defining CoffeeScript and listing its advantages (and one disadvantage) versus JavaScript. Then I'll show you how to obtain and use the CoffeeScript compiler, contrasting the sample CoffeeScript code with its JavaScript equivalent. You can download the code from this article here.

What Is CoffeeScript?

CoffeeScript is a small programming language that compiles into JavaScript. Inspired by languages such as Ruby and Python, its syntactic sugar enhances source code readability, promotes brevity, and offers new features such as list comprehensions. CoffeeScript source code is often much shorter than its JavaScript equivalent—without sacrificing runtime performance.

In December 2009, developer Jeremy Ashkenas announced the release of CoffeeScript and its compiler, which was initially written in Ruby and subsequently rewritten in CoffeeScript. In a 2011 readwrite.com interview, Ashkenas pointed out several advantages of CoffeeScript over its JavaScript counterpart:

  • CoffeeScript offers a more succinct and consistent syntax. Ashkenas pointed out that his CoffeeScript ports of existing JavaScript libraries often resulted in one-third less code.
  • CoffeeScript embraces JavaScript's good parts (for example, the beautiful object model that underlies JavaScript) while solving some of its bad parts (such as eliminating the with statement).
  • Compiled CoffeeScript code often runs as fast as JavaScript code—or even faster. As Ashkenas reported, "[You] can avoid slow forEach statements, and get the speed of native for loops for many operations."
  • CoffeeScript offers many helpful features, such as "correct prototype-based–classes, comprehensions over arrays and objects, bound function literals, safe lexical variables, destructuring assignments," and more.

    CoffeeScript also works well with frameworks such as jQuery, but we won't deal much with jQuery in this series. Check out blog posts such as Stefan Buhrmester's "How CoffeeScript Makes jQuery More Fun Than Ever" to learn about using CoffeeScript with jQuery.

Of course, CoffeeScript has some disadvantages. Ashkenas points out that "the main disadvantage lies in introducing yet another compile step in between you and your JavaScript.[… We] try to mitigate that disadvantage as much as possible by compiling into clean, readable JavaScript. And by providing conveniences like --watch, which can keep an entire directory of CoffeeScript files always up-to-date."

In the past, CoffeeScript has been criticized for its lack of a proper debugging capability. However, CoffeeScript 1.6.1 and later support the generation of source maps, which tell a JavaScript engine what part of a CoffeeScript program matches up with the code being evaluated. Browsers that support source maps can automatically use them to show the original source code in a debugger.

CoffeeScript Compilers

The CoffeeScript compiler translates CoffeeScript source code into its JavaScript equivalent. Two versions of the compiler are available: the core compiler, which runs in any JavaScript environment (including a browser), and the command-line compiler. Regardless of which version you want to use, visit the official CoffeeScript website to obtain the compiler.

Core Compiler

At the time of writing, the core compiler was Version 1.6.3. You can download this file and access your local copy or embed its link into your HTML file. Listing 1 presents an hw.html file that accesses the compiler via the link and embeds some CoffeeScript code.

Listing 1 - The core compiler lets you embed CoffeeScript source code in an HTML file.

<html>
  <head>
    <title>
      Hello, World
    </title>

    <script type="text/javascript"
            src="http://coffeescript.org/extras/coffee-script.js">
    </script>
  </head>

  <body>
    <script type="text/coffeescript">
      msg = ["Hello", "World"]
      for i in [0...msg.length]
         alert msg[i]
    </script>
  </body>
</html>

The <script> element in Listing 1 references the core compiler via the previously mentioned link. It also presents a <script type="text/coffeescript"> element that embeds some CoffeeScript code. This code creates a two-element string array and then iterates over the array, outputting each element via an alert dialog box.

The core compiler automatically detects, compiles, and executes all scripts present in <script> elements whose type attributes are set to text/coffeescript.

Listing 1 demonstrates two items that are unique to CoffeeScript (you'll learn more about them later in this series):

  • [0...msg.length] is an example of a range. Here, i takes on successive whole numbers that range from 0 through one less than msg.length.
  • alert msg[i] reveals that you don't have to surround a function/method call's arguments with parentheses (although you must provide them occasionally).

Command-Line Compiler

Because the core compiler compiles CoffeeScript code before each run, which can impact performance, you'll often want to use the command-line version of the compiler to precompile CoffeeScript source files into JavaScript files and then reference these files from your HTML. Complete the following steps to obtain and start using the command-line compiler:

  1. Visit nodejs.org and install the latest version of Node.js, a platform built on Chrome's JavaScript runtime for easily building fast, scalable network applications. I clicked the Install button, downloaded node-v0.10.5-x64.msi for my Windows 7 platform, double-clicked this Microsoft Installer file, and followed subsequent instructions to install Node.js.
  2. From the command line, execute this command to install CoffeeScript:
    npm install -g coffee-script
  3. While you're still at the command line, type the word coffee. If all goes well, you should be prompted with coffee>. Press Ctrl-V to change the prompt to ------> to signify input mode, and then enter the following lines:
    msg = ["hello", "world"]
    for i in [0...msg.length]
     console.log msg[i]
  4. The prompt changes to ....... for the second and third lines. After entering these lines, press Ctrl-V to exit input mode and execute these lines. You should observe the following output:
    hello
    world
  5. You must specify at least a single space before console (you'll learn why later in this series). Note that console.log lets you send output to the console window. You can't use alert() because the concept of a dialog box doesn't make sense in a console-oriented context.

Executing coffee with no arguments demonstrates interactive mode, in which you enter and execute short snippets of CoffeeScript code interactively. You can achieve the same effect by specifying coffee -i or coffee --interactive. However, you'll often want to compile a CoffeeScript source file into its JavaScript equivalent, as I'll now demonstrate.

Listing 2 presents the contents of a small text file named hw.coffee.

Listing 2 - This source file contains the same CoffeeScript source code as found in Listing 1.

msg = ["Hello", "World"]
for i in [0...msg.length]
   alert msg[i]

The .coffee extension isn't mandatory: I could just as easily have named this file hw.c, but hw.coffee is more descriptive. Execute the following command line to compile hw.coffee into hw.js:

coffee -c hw.coffee

You could specify --compile instead of -c to perform the compilation.

If all goes well, you should observe a new hw.js source file in the current directory. Listing 3 presents this file's generated JavaScript source code (slightly modified for readability).

Listing 3 - The equivalent JavaScript code is placed in a closure.

// Generated by CoffeeScript 1.6.2
(function() {
  var i, msg, _i, _ref;

  msg = ["Hello", "World"];

  for (i = _i = 0, _ref = msg.length; 0 <= _ref ? _i < _ref : _i > _ref;
       i = 0 <= _ref ? ++_i : --_i) {
    alert(msg[i]);
  }

}).call(this);

Listing 3 presents JavaScript code within the context of a closure. This top-level function safety wrapper avoids name conflicts with the global namespace and is a CoffeeScript feature.

Notice that variables i and msg are declared at the beginning of the closure. This technique makes them local variables, which cannot conflict with any same-named global variables. (Name conflicts are one of the JavaScript problems that CoffeeScript solves.)

After declaring the array in much the same manner as in JavaScript, Listing 3 presents a for loop that successively assigns range values from 0 through one less than msg.length to i. It then invokes alert() with the string at this array location.

Listing 4 presents a revised hw.html file that accesses hw.js.

Listing 4 - Where performance matters, access JavaScript source files from your HTML files, rather than embedding CoffeeScript in those files.

<html>
  <head>
    <title>
      Hello, World
    </title>

    <script type="text/javascript"
            src="http://coffeescript.org/extras/coffee-script.js">
    </script>
  </head>

  <body>
    <script type="text/javascript" src="hw.js">
    </script>
  </body>
</html>

The coffee command offers additional options for specifying an output directory, executing JavaScript Lint to warn against questionable coding practices, watching files for changes, and more. You can find a complete list of options at the main CoffeeScript website.

Conclusion

Now that you know what CoffeeScript is, how to embed CoffeeScript code in an HTML document for compilation via the core compiler, and how to use the command-line compiler to precompile CoffeeScript code into JavaScript, you're ready to explore the CoffeeScript language. Part 2 of this series introduces you to CoffeeScript's basic language features.

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