The latest version of the ECMAScript Language - ES6 - introduces a lot of new features. One of the most interesting of these is the introduction of a module system that is built into the language. The way this works is, unfortunately, very different to how any of the pre-existing ES5 module systems work.
Wait, what? Most of the time when people discuss the new ES6 module system it is talked about as being one of the brand new features of ES6, and not an existing feature that has been fit into the language - such as Promises. However, there are already a number of module systems that are used and very well supported in ES5. The fact that there are different module systems, and that they aren’t trivially compatible, is a big problem. There are ways of making the three major module systems work together, but it’s not great to have to do that just to work around this fact. As such, the fact that there is a language-level module system in ES6 is a good step forwards. It’s just that the new module system isn’t a perfect solution.