Third-party JavaScript

There is also a lot of JavaScript code available that is not built in. JavaScript from third parties is usually referred to as a "library" or "plugin". One of my favourites is called Underscore.js. Let's go and grab it and load it into our page! First go to the Underscore site, http://underscorejs.org/ ⇗, click on the download link (I usually use development versions because they are easier to read but both will give you the same basic functionality), and then copy all the code onto your clipboard (you can use Select All from the Edit menu to select everything). Then paste it into your console and hit enter. Now your browser has a new variable in it: _. Underscore gives you a ton of helpful functions to play with. We'll learn more about how to use them later.