If I use foo.js, when the user reload the page, or goes to another page that also uses foo.js, the javascript file is in his cache and he won't need to re-downl

If I use foo.js, when the user reload the page, or goes to another page that also uses foo.js, the javascript file is in his cache and he won't need to re-download it. Given the size some of those js are, it makes a significant decrease. Especially in the mobile context this article talks about, when I visit my parents my phone's data connection is slow enough that 40kb of js make a huge difference. Of course, that also needs a properly configured server (a very large number of sites out there don't see proper client caching so the browser still need to ask the server if the file has changed every single time, for example).