How to initialise a global variable in unit test runs?

You just need to add a setup file in test/mocha.opts that will be loaded before to start any test, then you will be available to access those global variables, for example:

test/mocha.opts

--require should
--require ./test/setup
--ui bdd
--globals global
--timeout 200

test/setup.js

global.app = require('some-library')
global.window = {}
global.window.document = {}

docs:

http://unitjs.com/guide/mocha.html#mocha-opts