Object.assign(String.prototype, {
    formatDateYMD (splitChar) {
        let matchedString = this.match('[0-9]{4}([-/])[0-9]{2}[-/][0-9]{2}')
        const dutchMonths = ['jan', 'feb', 'mrt', 'apr', 'mei', 'jun', 'jul', 'aug', 'sep', 'okt', 'nov', 'dec']
        matchedString = matchedString[0].split(splitChar)

        return `${matchedString[2]} ${dutchMonths[Number(matchedString[1] - 1)]}`
    }
})

const myString = '1992-09-21'
myString.formatDateYMD('-') // returns '21 sep'