The Little Book on CoffeeScript

Preface

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.