According to this question How do I include a JavaScript file in another JavaScript file?. It seems a lots of people are interested in breaking big Javascript projects into small modules and export/import modules for code reuse.
After some research, import/export are designed for this feature. According to the references, they're initially defined in ES6.
Update
Latest version of main browsers shipped with this feature implemented. To have the latest status, please always refer to the References.
(If you're using nodejs, Modules (https://nodejs.org/dist/latest-v5.x/docs/api/modules.html) is the best aproach)
Refrerences:
https://developer.mozilla.org/en/docs/web/javascript/reference/statements/import https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export