I'm developing in CoffeeScript and want to start to use Underscore.js. I know that any JS library will work in CoffeeScript.
Online there is the regular UnderscoreJS and also a CoffeScript version. Are there any difference in implication of the two? Is it perfectly ok to use the underscore JS version for my CoffeeScript needs?
You'll want to use the JavaScript version. The CoffeeScript version was likely just the author playing around with CoffeeScript, which makes sense since he is the author for both CoffeeScript and Underscore. Also, the CoffeeScript version introduces a compile step (assuming you are using this in the browser rather than on the server with node.js).
As another option, check out Lodash. It is a drop-in replacement for Underscore and for many reasons is the better option. It just released v1.0 in the past few days.
Usually when you are developing in Coffeescript, you are going to need something to compile your various Coffeescript files together to Javascript so that a browser can run it. How you want to use the library determines which version you will use.
Option 1: Manually add the Underscore library (in JS form) as a <script> tag in your page and also add your compiled coffeescript as a <script> tag. Quick and easy dirty way to get things to work, but results in a buildup of <script> and <meta> tags as you add more libraries/styles to your page, and spaghetti code.
Option 2: Use a tool to compile all your Coffeescript and CSS into a single JS/CSS file, which you then reference in your HTML. Then you would use the Coffeescript form of Underscore and compile that with the rest of your code. This is the approach I use, with the additional advantage of being able to use tools like npm to manage dependencies. Additionally, it allows you to have a test web server that compiles your code in real time as you edit the Coffeescript. Check out my post on using hem, npm, (and Spine).
For option 2, something else you can check out is requireJS.
