CoffeeScript in a Nutshell, Part 1: An Introduction to CoffeeScript
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 candownload the code from this article here。
What Is CoffeeScript?
CoffeeScriptis 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 2011readwrite.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 thewithstatement).
- Compiled CoffeeScript code often runs as fast as JavaScript code—or even faster. As Ashkenas reported, "[You] can avoid slowforEachstatements, and get the speed of nativeforloops 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 ofsource 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: thecore compiler, which runs in any JavaScript environment (including a browser), and thecommand-line compiler。不管你想用哪个版本,visit theofficial CoffeeScript websiteto obtain the compiler.
Core Compiler
At the time of writing, the core compiler wasVersion 1.6.3。You can download this file and access your local copy or embed its link into your HTML file. Listing 1 presents anhw.htmlfile 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.
Hello, World
Theelement in Listing 1 references the core compiler via the previously mentioned link. It also presents aelement 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 inelements whosetypeattributes are set totext/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 arange。Here,itakes on successive whole numbers that range from 0 through one less thanmsg.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:
- Visitnodejs.organd install the latest version ofNode.js, a platform built on Chrome's JavaScript runtime for easily building fast, scalable network applications. I clicked the Install button, downloadednode-v0.10.5-x64.msifor my Windows 7 platform, double-clicked this Microsoft Installer file, and followed subsequent instructions to installNode.js。
- From the command line, execute this command to install CoffeeScript:
npm install -g coffee-script
- While you're still at the command line, type the wordcoffee。If all goes well, you should be prompted withcoffee>。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]
- 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
- You must specify at least a single space beforeconsole(you'll learn why later in this series). Note thatconsole.loglets you send output to the console window. You can't usealert()because the concept of a dialog box doesn't make sense in a console-oriented context.
Executingcoffeewith no arguments demonstratesinteractive mode, in which you enter and execute short snippets of CoffeeScript code interactively. You can achieve the same effect by specifyingcoffee -iorcoffee --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 namedhw.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。coffeeextension isn't mandatory: I could just as easily have named this filehw.c, buthw.coffeeis more descriptive. Execute the following command line to compilehw.coffeeintohw.js:
coffee -c hw.coffee
You could specify--compileinstead of-cto perform the compilation.
If all goes well, you should observe a newhw.jssource 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 aclosure。Thistop-level function safety wrapperavoids name conflicts with the global namespace and is a CoffeeScript feature.
Notice that variablesiandmsgare 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 aforloop that successively assigns range values from0through one less thanmsg.lengthtoi。It then invokesalert()with the string at this array location.
Listing 4 presents a revisedhw.htmlfile that accesseshw.js。
Listing 4 - Where performance matters, access JavaScript source files from your HTML files, rather than embedding CoffeeScript in those files.
Hello, World
Thecoffeecommand offers additional options for specifying an output directory, executingJavaScript Lintto warn against questionable coding practices, watching files for changes, and more. You can find a complete list of options at themain 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 seriesintroduces you to CoffeeScript's basic language features.