Now supported in all modern browsers without the need for any other libraries, but not IE9 (which is gradually being replaced by Edge).
There are two examples: main.js and missionControl.js. The first is a very simple example from Exploring JS, the second is from CodeCademy, Learn JavaScript: Modules.
Open the console (Win/Mac: F12 or cmd-opt-i) to see the result of both modules (lib.js and airplabne.js) being imported to and called from the main.js files above: