moment-test.js

var moment = require('moment-timezone')
  , prevAbbr = moment.fn.zoneAbbr
  , timezoneTranslations = {
      'Europe/Istanbul': 'TSİ'
    }

moment.fn.zoneAbbr = function () {
  var customTimezoneTranslation = timezoneTranslations[this._z.name]
  return customTimezoneTranslation || prevAbbr.call(this)
}

var str = moment().tz('Europe/Istanbul').format('HH:mm:ss z')

console.log(str)

// 15:11:39 TSİ