nightwatch-setup.js

// package.json scripts
"test:e2e-setup": "mkdir -p bin && node ./test/testSupport/selenium-download.js",
"test:e2e": "nightwatch --config ./test/e2e/conf/nightwatch.conf.js -e default",

// test/testSupport/selenium-download.js
var selenium = require('selenium-download')
selenium.ensure('./bin', function (error) {
  if (error) {
    console.error(error ? error.stack : 'Could not download selenium, most likely a network issue and not with `selenium-download`.')
    process.exit(0)
  }
})