How Can Some Javascript Libraries Export a PDF?

How can some JavaScript libraries export a PDF, such as "jsPDF", as I understand that JS cannot write a file due to the security aspect?

There's a big difference between knowing javascript, and knowing how to use Angular, Phantom, Backbne, Ember, and the incredibly increasing number of MV* frameworks and libraries... If you want to learn javascript, I would start reading tutorials and doing some exercises to understand the javascript principles, like closures, public properties, local variables, function invocations, context binding, IOC, etc. Javascript has a really different way of handling all this stuff compared to a more traditional OO language. On the other hand if you wanna learn the nowadays more used frameworks and libraries, just point to their home pages or youtube and take the tutorials t offer, which in most cases are pretty good. Try (and get used to!) jsfiddle cause you will use it a LOT in the future, as well as browsers js debuggers. Search for exercises using their built in api's and functions, and maybe even start building a client only app (Or tie it up to a server backend if you wish...) like a traditional calndar, or todo list, something simple which will let you focus on client dom & data manipulation. However if I were you, I would start with the javascript basics, cause although you won't need it to use a framework, it will be really helpfull to understand how the framework itself works, how to extend it in case you need to, and what are the best practices for using it.