main-module.js

"use strict";

// Require some stuff
const { myMethod } = require("my-method");

// Init code
myMethod("initializing");
...

// Cleaning up
exports.onUnload = () => {
  // Do cleanup
};

not-main-module.js

"use strict";

// Require some stuff
const { when: unload } = require("sdk/system/unload");

// Init code
const MY_CONSTANT = "foo";

// Exports (there are multiple way to organize these, some like to define then export, some prefer to directly export)
exports.aMethod = () => MY_CONSTANT;

// Unload
unload(() => {
  // Clean up the mess this module made.
});