Nicholas Zakas:
The differences between scripts and modules are subtle enough that it’s hard for developers to understand the restriction of declaring what a JavaScript file represents ahead of time. My hope is that this post clarifies some of the reasons why it’s not possible to autodetect modules from inspecting the source code and why tools such as ESLint ask you to specify the file type before executing. There will be a point in the future where ES6 modules are the dominant JavaScript file type and script files are left only on legacy applications, and at that point, it’s likely that tools will default to assuming that files are modules. In the meantime, we’re going through a difficult adolescence between scripts and modules where mixing the two is going to be a bit painful.