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The Little Book on CoffeeScript by Alex MacCaw

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What Is CoffeeScript?

CoffeeScript is a little language that compiles down to JavaScript. The syntax is inspired by Ruby and Python, and implements many features from those two languages. This book is designed to help you learn CoffeeScript, understand best practices, and start building awesome client-side applications. The book is little, only six chapters, but that’s rather apt as CoffeeScript is a little language too.

This book is completely open source, and was written by Alex MacCaw (@maccman) with great contributions from David Griffiths, Satoshi Murakami, Chris Smith, Katsuya Noguchi, and Jeremy Ashkenas.

If you have any errata or suggestions, please don’t hesitate to open a ticket on the book’s GitHub page. Readers may also be interested in JavaScript Web Applications (O’Reilly), a book I authored that explores rich JavaScript applications and moving state to the client side.

So let’s dive right into it: why is CoffeeScript better than writing pure JavaScript? Well, for a start, there’s less code to write; CoffeeScript is very succinct, and takes white space into account. In my experience, this reduces code by a third to a half of the original pure JavaScript. In addition, CoffeeScript has some neat features, such as array comprehensions, prototype aliases, and classes that further reduce the amount of typing you need to do.

More importantly though, JavaScript has a lot of skeletons in its closet which can often trip up inexperienced developers. CoffeeScript neatly sidesteps these by only exposing a curated selection of JavaScript features, fixing many of the language’s oddities.

CoffeeScript is not a superset of JavaScript, so although you can use external JavaScript libraries from inside CoffeeScript, you’ll get syntax errors if you compile JavaScript as is, without converting it. The compiler converts CoffeeScript code into its counterpart JavaScript, there’s no interpretation at runtime.

So let’s get some common fallacies out of the way. You will need to know JavaScript in order to write CoffeeScript, as runtime errors require JavaScript knowledge. However, having said that, runtime errors are usually pretty obvious, and so far I haven’t found mapping JavaScript back to CoffeeScript to be an issue. The second problem I’ve often heard associated with CoffeeScript is speed (i.e., the code produced by the CoffeeScript compiler would run slower than its equivalent written in pure JavaScript). In practice though, it turns out this isn’t a problem either. CoffeeScript tends to run as fast or faster than handwritten JavaScript.

What are the disadvantages of using CoffeeScript? Well, it introduces another compile step between you and your JavaScript. CoffeeScript tries to mitigate the issue as best it can by producing clean and readable JavaScript, and with its server integrations which automate compilation. The other disadvantage, as with any new language, is the fact that the community is still small at this point, and you’ll have a hard time finding fellow collaborators who already know the language. CoffeeScript is quickly gaining momentum though, and its IRC list is well staffed; any questions you have are usually answered promptly.

CoffeeScript is not limited to the browser, and can be used to great effect in server-side JavaScript implementations, such as Node.js. Additionally, CoffeeScript is getting much wider use and integration, such as being a default in Rails 3.1. Now is definitely the time to jump on the CoffeeScript train. The time you invest in learning about the language now will be repaid by major time savings later.

Initial Setup

One of the easiest ways to initially play around with the library is to use it right inside the browser. Navigate to http://coffeescript.org and click on the Try CoffeeScript tab. The site uses a browser version of the CoffeeScript compiler, converting any CoffeeScript typed inside the left panel to JavaScript in the right panel.

You can also convert JavaScript back to CoffeeScript using the js2coffee project, especially useful when migrating JavaScript projects to CoffeeScript.

In fact, you can use the browser-based CoffeeScript compiler yourself, by including this script in a page, marking up any CoffeeScript script tags with the correct type:

<script src="http://jashkenas.github.com/coffee-script/extras/coffee-script.js" type="text/javascript" charset="utf-8"></script>
<script type="text/coffeescript">
  # Some CoffeeScript

Obviously, in production, you don’t want to be interpreting CoffeeScript at runtime, as it’ll slow things up for your clients. Instead, CoffeeScript offers a Node.js compiler to pre-process CoffeeScript files.

To install it, first make sure you have a working copy of the latest stable version of Node.js and npm (the Node Package Manager). You can then install CoffeeScript with npm:

npm install -g coffee-script

The -g flag is important, as it tells npm to install the coffee-script package globally, rather than locally. Without it, you won’t get the coffee executable.

If you execute the coffee executable without any command line options, it’ll give you the CoffeeScript console, which you can use to quickly execute CoffeeScript statements. To pre-process files, pass the --compile option:

coffee --compile my-script.coffee

If --output is not specified, CoffeeScript will write to a JavaScript file with the same name, in this case my-script.js. This will overwrite any existing files, so be careful you’re not overwriting any JavaScript files unintentionally. For a full list of the command line options available, pass --help.

You can also pass the --compile option a directory, and CoffeeScript will recursively compile every file with a .coffee extension:

coffee --output lib --compile src

If all this compilation seems like a bit of an inconvenience and bother, that’s because it is. We’ll be getting onto ways to solve this by automatically compiling CoffeeScript files, but first let’s take a look at the language’s syntax.

Conventions Used in This Book

The following typographical conventions are used in this book:


Indicates new terms, URLs, email addresses, filenames, and file extensions.

Constant width

Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.


This icon signifies a tip, suggestion, or general note.


This icon indicates a warning or caution.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “The Little Book on CoffeeScript by Alex MacCaw (O’Reilly). Copyright 2012 Alex MacCaw, 978-1-449-32105-5.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at .

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