formatea 3654 en 3.654,00 €

function format_number(number){
    const number_string = number.toString()
    const array = number_string.split('')
    const array_from_back = array.reverse()
    const array_with_points = array_from_back.splice(2, 0, '.')
    const array_from_back_with_comma = array_with_points.splice(6, 0, ',')
    const array_from_back_with_comma_reverse = array_from_back_with_comma.reverse()
    const array_from_back_with_comma_reverse_join = array_from_back_with_comma_reverse.join('')
    return array_from_back_with_comma_reverse_join + ' €'
}

format_number(3654);