checkStorage.js

module.exports.checkStorage = function () {
  try {
    global.mozSimpleStorage = require('sdk/simple-storage');
  } catch() {
    global.mozSimpleStorage = false;
  }

  if (chrome.storage) { // We're running inside the Chrome/Web Extension context
    return 'chromeOrWebExt';
  } else if (window.localStorage) { // We're running inside a normal webpage
    return 'localStorage';
  } else if (mozSimpleStorage) { //We're running in a Firefox Add-On context
    return 'firefoxAddOn';
  }
};

testStorage.js

var storage = require('checkStorage.js');
var test = require('tape');

test ('checkStorage tests with chrome', function (t) {
  global.chrome = {'storage': true}; // Mock a chrome/Web extension environment
  t.equal(storage.checkStorage(), 'chromeOrWebExt', 'if we find the chrome API, return chromeOrWebExt');
  global.chrome = {'storage': false};
  t.end();
});

test ('checkStorage tests with localStorage', function (t) {
  global.window = {'localStorage': true}; // Add localStorage API mocking.
  t.equal(storage.checkStorage(), 'localStorage', 'if we find the localStorage API, return localStorage');
  global.window = {'localStorage': false};
  t.end();
});

test ('checkStorage tests with simple-storage', function (t) {
  global.mozSimpleStorage = true;
  t,equal(...)
  t.end();
});