DateInputHelper

// Automatically convert date inputs to a required format
// Works, but needs a lot of refactoring

var dateInputHelper =  new DateInputHelper();

function DateInputHelper() {
    var self = this;
    self.cursorPosition = null;

    self.buildDateInput = function($input, separator, minYear, maxYearLimit) {
        var maxYear = self.buildMaxYear(maxYearLimit);

        var altSeparator = '/';
        if (separator == '/') {
            altSeparator = '.';
        }

        $input.on("keypress", function (e) {
            return self.isKeyAllowed($input.val(), e.key);
        }).on('keydown', function (e) {
            if(e.key === altSeparator) {
                setTimeout(function() {
                    e.target.value += separator;
                }, 4);
                e.preventDefault();
            }
        }).on('change', function (e) {
            var value = $input.val();
            value = value.replace(/\//g, '.');
            $input.val(value);
        }).on('keyup', function (e) {
            var val = $input.val();

            if (isNaN(e.key)) {
                // not a number
                return false;
            }

            // auto add slash
            if (
                (
                    (this.selectionEnd === 3 && val.length === 3) ||
                    (this.selectionEnd === 6 && val.length === 6)
                ) && val.substring(0, val.length - 1).slice(-1) != separator && val.split(separator).length < 3
            ) {
                $input.val(val.substring(0, val.length - 1)+separator+e.key);
            }

            self.saveCursorPosition(this);

            var clearedValue = self.autoCorrectDate($input.val(), separator, minYear, maxYear, 'date', false);
            $input.val(clearedValue);

            self.revertCursorPosition(this);
        }).on("focusout", function () {
            if ($input.val().length > 0) {
                var clearedValue = self.autoCorrectDate($input.val(), separator, minYear, maxYear, 'date', true);
                $input.val(clearedValue);
            }
        });

    };

    self.buildMonthInput = function($input, separator, minYear, maxYearLimit) {
        var maxYear = self.buildMaxYear(maxYearLimit);

        var altSeparator = '/';
        if (separator == '/') {
            altSeparator = '.';
        }

        $input.on("keypress", function (e) {
            return self.isKeyAllowed($input.val(), e.key);
        }).on('keydown', function (e) {
            if(e.key === altSeparator) {
                setTimeout(function() {
                    e.target.value += separator;
                }, 4);
                e.preventDefault();
            }
        }).on('change', function (e) {
            var value = $input.val();
            value = value.replace(/\//g, '.');
            $input.val(value);
        }).on('keyup', function (e) {
            var val = $input.val();

            if (isNaN(e.key)) {
                // not a number
                return false;
            }

            // auto add slash
            if ((this.selectionEnd === 3 && val.length === 3) && val.substring(0, val.length - 1).slice(-1) != separator) {
                $input.val(val.substring(0, val.length - 1)+separator+e.key);
            }

            self.saveCursorPosition(this);

            var clearedValue = self.autoCorrectDate($input.val(), separator, minYear, maxYear, 'month', false);
            $input.val(clearedValue);

            self.revertCursorPosition(this);
        }).on("focusout", function () {
            if ($input.val().length > 0) {
                var clearedValue = self.autoCorrectDate($input.val(), separator, minYear, maxYear, 'month', true);
                $input.val(clearedValue);
            }
        });

    };

    self.autoCorrectDate = function (value, separator, minYear, maxYear, mode, padZero) {
        if (separator == '/') {
            value = value.replace(/\./g, '/');
        } else {
            value = value.replace(/\//g, '.');
        }

        var dateParts = value.split(separator);
        var clearedValue = '';

        switch(mode) {
            case 'date':
                var maxLength = 3;
                var dayIndex = 0;
                var monthIndex = 1;
                var yearIndex = 2;
                break;
            case 'month':
                var maxLength = 2;
                var dayIndex = null;
                var monthIndex = 0;
                var yearIndex = 1;
                break;
            default:
                throw new Error('Invalid autocorrect mode');
        }

        if (dateParts.length > maxLength) {
            dateParts.length = maxLength;
        }

        for (var i in dateParts) {
            var numberString = dateParts[i];
            var originalNumber = parseInt(numberString);
            var number = parseInt(numberString);

            // day
            if (i == dayIndex) {
                if (isNaN(number)) {
                    number = 1;
                }

                number = self.padBetween(number, 0, self.getMaxDays(dateParts));

                if (number == 0 && dateParts.length > 1) {
                    number = '01';
                }

                if (padZero && (number < 10 && number > 0)) {
                    number = '0' + number.toString();
                }
            }

            // month
            if (i == monthIndex) {
                if (isNaN(number)) {
                    number = 1;
                }

                number = self.padBetween(number, 0, 12);

                if (number == 0 && ((mode == 'month' && dateParts.length > 1) || dateParts.length > 2)) {
                    number = '01';
                }

                if (padZero && (number < 10 && dateParts.length > 2)) {
                    number = '0' + number.toString();
                }
            }

            // year
            if (i == yearIndex && numberString.length > 3) {
                if (isNaN(number)) {
                    number = 2000;
                }

                number = self.padBetween(number, minYear, maxYear);
            }

            if (number === originalNumber) {
                number = numberString;
            }

            if (i > 0) {
                clearedValue += separator;
            }

            if (!isNaN(number)) {
                clearedValue += number;
            }
        }

        return clearedValue;
    };

    self.buildMaxYear = function (maxYearLimit) {
        var currentYear = new Date().getFullYear();
        return currentYear+maxYearLimit;
    };

    self.isKeyAllowed = function (value, key) {
        if (parseInt(key) !== parseInt(key) && key !== '/' && key !== '.') {
            // not a number and not a slash
            return false;
        }
        if (value === '' && (key === '/' || key === '.')) {
            // slash not allowed as first char
            return false;
        }
        return true;
    };

    self.saveCursorPosition = function (element) {
        self.cursorPosition = element.selectionEnd;
    };

    self.revertCursorPosition = function (element) {
        element.selectionEnd = self.cursorPosition;
    };

    self.padBetween = function(number, min, max) {
        if (number < min) {
            number = min;
        }
        if (number > max) {
            number = max;
        }
        return number;
    };

    self.getMaxDays = function (dateParts) {
        var maxDays = 31;
        if (dateParts.length > 1) {
            var month = parseInt(dateParts[1]);
            if (month > 0) {
                if ([1, 3, 5, 7, 8, 10, 12].indexOf(month) !== -1) {
                    maxDays = 31;
                } else if ([4, 6, 9, 11].indexOf(month) !== -1) {
                    maxDays = 30;
                } else if (month == 2) {
                    maxDays = 29;
                }
            }
        }
        return maxDays;
    };
}