Uncaught TypeError: this is not a Date object when adding a function to Date prototype

Question

I have a problem with this code in google chrome and firefox. this.setUTCDate(1); throws Uncaught TypeError: this is not a Date object. How can I fix it?

I've looked at this and this, but I didn't find anything to solve my problem.

Date.prototype.setUTCDateOnly = function(date) {
    var tmp = new Date(date);
    this.setUTCDate(1);
};

I am calling it in this way:

var date = Date.prototype.setUTCDateOnly(item.date);

@RocketHazmat in this way : var date = Date.prototype.setUTCDateOnly(item.date); — Pooya, Dec 31 '15 at 15:49
Possible duplicate of [TypeError: this is not a Date object](http://stackoverflow.com/questions/17899598/typeerror-this-is-not-a-date-object) — Hacketo, Dec 31 '15 at 15:49
Well there you go. It indeed is not a Date object. Why are you calling it that way? — , Dec 31 '15 at 15:50
I used a library for calendar. it's developer does not answer any questions. so I started to edit the code. the above codes works already before I upgraded my browser — Pooya, Dec 31 '15 at 16:00
@Пуя You have not explained what it is that you want the function to actually do. — Pointy, Dec 31 '15 at 16:01
It has another codes after that line. I want to set only the year, month, date parts (keep existing time) — Pooya, Dec 31 '15 at 16:03

score 3 · Answer 1 · answered Dec 31 '15 at 15:59

Methods added to the .prototype of a constructor are meant to operate on instances created from that constructor. Instead, you're operating on the .prototype object itself.

Why? Because the value of this in your case is set by looking to the left hand side of the . operator. In other words, the object on which the method is called becomes the value of this in the method.

That's not the only way this can be set, but it is the default, implicit behavior when invoking methods.

So what you need to do is create a Date object like you normally would, then call your method on that object to have it operate on that object.

var mydate = new Date();
mydate.setUTCDateOnly(item.date);

score 2 · Accepted Answer · answered Dec 31 '15 at 16:07

Since you call your method like this

Date.prototype.setUTCDateOnly(item.date);

then the this value will be Date.prototype.

In ES5, Date.prototype was a Date object:

The Date prototype object is itself a Date object (its [[Class]] is "Date") whose [[PrimitiveValue]] is NaN.

Therefore, you could call setUTCDate on it. The NaN was just transformed into a +0 by ToInteger inside MakeDay.

However, this changed in ES6, now Date.prototype is defined as

The Date prototype object is itself an ordinary object. It is not a Date instance and does not have a [[DateValue]] internal slot.

Therefore, attempting to call setUTCDate on it throws.

Thanks, Because I didn't know that the new google chrome changed the ECMA script engine to 6 — Pooya, Dec 31 '15 at 16:14
@Пуя It's more like a gradual transition. New browser versions are becoming closer to the ES6 spec. — Oriol, Dec 31 '15 at 16:17

Uncaught TypeError: this is not a Date object when adding a function to Date prototype

2 Answers2