How to get all files like: js,css,images from a website to save it for offline use?

I am not talking about the softwares like surf online, HTtrack, or any other 'save page' feature of browsers but I need to know how actually it happens in the background. I am interested in making my own program to do that. Also is it possible to do in JavaScript. If yes what are the libraries I should look into or any other APIs that could be helpful. Please give me any kind of information about the topic I couldn't find any relevant thing to contribute my research.