metamask-extension/app/scripts/lib/notifications.js

const extension = require('./extension')

const notifications = {
  show,
  getPopup,
  closePopup,
}
module.exports = notifications
window.METAMASK_NOTIFIER = notifications

function show () {
  getPopup((err, popup) => {
    if (err) throw err

    if (popup) {

      // bring focus to existing popup
      extension.windows.update(popup.id, { focused: true })

    } else {

      // create new popup
      extension.windows.create({
        url: 'notification.html',
        type: 'popup',
        focused: true,
        width: 360,
        height: 500,
      })

    }
  })
}

function getWindows(cb) {
  // Ignore in test environment
  if (!extension.windows) {
    return cb()
  }

  extension.windows.getAll({}, (windows) => {
    cb(null, windows)
  })
}

function getPopup(cb) {
  getWindows((err, windows) => {
    if (err) throw err
    cb(null, getPopupIn(windows))
  })
}

function getPopupIn(windows) {
  return windows ? windows.find((win) => win.type === 'popup') : null
}

function closePopup() {
  getPopup((err, popup) => {
    if (err) throw err
    if (!popup) return
    extension.windows.remove(popup.id, console.error)
  })
}