Calling nested functions in Javascript OOP

Question

I am trying to become more familiar with Javascript OOP and so I wrote a little test script but I keep getting an exception when I test it:

Exception:

Uncaught TypeError: Cannot call method 'day' of undefined

Code:

(function () {
    function Time (date) {
        var self = this;
        var timeInWeek = 604800000;
        var timeInDay = 86400000;
        var dateInMilliSeconds = date.getTime();

        self.add = function (num) {
            self.day = function () {
                var newDate = new Date();
                newDate.setTime(dateInMilliSeconds + (timeInDay * num));
                return newDate;
            };
        };
    };
    var date = new Date();
    var time = new Time(date).add(1).day();
    console.log(time);
})();

And when I run the test script outside of the IIFE pattern I am getting the exception Time is undefined, I am new to Javascript OOP so when I tried reading other Javascript libraries a good chunk was over my head. Any help is appreciated.

user2864740 · Accepted Answer · 2013-12-04 19:40:28Z

4

The add method returns undefined as it has no return statement. The problem is unrelated to the use of the IIFE.

Instead, the add method simply added (possibly overwriting) the day method when it was executed so the following would have "worked":

var time = new Time(date)
t.add(1);    // returns undefined, but has a side-effect of adding "day" to t
t.day();

However, I suspect that the issue is two-fold:

add should return an object of a compatible type for Method Chaining; this can either be same object (for a mutable design) or it can be new object (for an immutable design).
day should be added directly to each Time object; this would make it so new Time(date).day() would work.

For instance:

function Time (date) {
    var self = this;
    var timeInWeek = 604800000;
    var timeInDay = 86400000;
    var dateInMilliSeconds = date.getTime();

    self.add = function (num) {
        // The idea here is to return an object of the same type for "chaining".
        // Here I returned a new Time object (you'll have to work out the details),
        // although for mutable objects, "return self" would be appropriate.
        return new Time(dateInMilliSeconds + (timeInDay * num));
    };
    self.day = function () {
        // actually return the "day", whatever that is.
        return ...;
    };
};

While this is can be a nice exercise, for production code I'd recommend moment.js unless there is as compelling reason otherwise. The source code for moment.js (which might make a good reference) is on github/moment. The "add" (with omissions and additional comments) looks like:

add : function (input, val) {
    // mutate this objects data (but not methods)
    addOrSubtractDurationFromMoment(this, dur, 1);
    // returns the same object for chaining
    return this;
},

edited Dec 4, 2013 at 19:40

answered Dec 4, 2013 at 19:11

user2864740

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2 Comments

Jonathan Jones Over a year ago

This is good information, thank you. I am aware of moment.js, I just wanted some JS OOP practice, I will take a closer look at its source code. I played around with a couple different returns, I noticed that returning day and any other function inside the add function as an object works as well. Is this not recommended? i.e. self.add = function () { return { day: function () {...} } };

user2864740 Over a year ago

Sure, it'll "work" here (as there is a day method-property of the returned object), but it might not be appropriate for the given task. What happens if the user does new Time(..).day()? Or what if they do new Time(..).add(..).add(..)? The former won't work because there is no day method on Time (only on the result of add) and the latter won't work unless the object returned from add also has the add method. For chaining it is usually best that the type of the result is equivalent (but usually the same) to the original object - such that all the same methods are allowed.

tewathia · Accepted Answer · 2013-12-04 19:50:47Z

You've missed a return this; (which must be there to allow daisy chaining)

function Time (date) {
        var self = this;
        var timeInWeek = 604800000;
        var timeInDay = 86400000;
        var dateInMilliSeconds = date.getTime();

        self.add = function (num) {
            self.day = function () {
                var newDate = new Date();
                newDate.setTime(dateInMilliSeconds + (timeInDay * num));
                return newDate;
            };
            return this; //this
        };
    };

Without the return statement, in new Time(date).add(1).day()) the day() is being treated as a method of the value returned by the add method of the new Time() instance. If the add method doesn't return an object that has a day method, you're sure to get an error.

The return this is necessary only for chaining. Your code would work fine with

var time = new Time(date);
time.add(1);
time.day();

OR

var time = new Time(date).add(1);
time.day()

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