Protractor: TypeError: fn is not a function

Question

I'm encountering the following error when I run my UI tests. I'm new to Javascript so I've been mostly following example code and I'm not sure where I've gone wrong. Could someone explain why I'm getting this error and what an appropriate fix would be?

Error:

[13:00:19] E/launcher - fn is not a function
[13:00:19] E/launcher - TypeError: fn is not a function

Homepage object:

Homepage.prototype = Object.create({}, {
    checkInBtn: {
        get: function () {
            return element(by.css('div.checkin.booking-date input'));
        }
    }, datePickerDay: {
        value: function (day) {
            return element(by.cssContainingText('.ui-datepicker-calendar a', day));
        }
    }, selectCheckInDate: {
        value: function (day) {
            return this.checkInBtn.click().then(this.datePickerDay(day).click());
        }
    },
});
module.exports = Homepage;

Cucumber-Protractor Stepfile

this.When(/^I enter the trip information and search$/, function (table) {
var page = new homepage();
        var checkOutDay = new Date(data["DepartureDate"]).getDate();
        page.selectCheckInDate(checkInDay);
        expect(page.checkInBtn.getText()).to.eventually.have.string(checkInDay);
        });

Answer 1

The problem is with the selectedCheckInDate

Homepage.prototype = Object.create({}, {
    checkInBtn: {
        get: function() {
            return element(by.css('div.checkin.booking-date input'));
        }
    },
    datePickerDay: {
        value: function(day) {
            return element(by.cssContainingText('.ui-datepicker-calendar a', day));
        }
    },
    selectCheckInDate: {
        value: function(day) {
            var self = this;
            this.checkInBtn.click().then(function() {
                self.datePickerDay(day).click();
            });
        }
    }
});
module.exports = Homepage;