Any idea why this does not work in Chrome?
var foo = (new Date).getDate;
foo();
I get a TypeError: this is not a Date object. However (new Date).getDate() works

Any idea why this does not work in Chrome?
var foo = (new Date).getDate;
foo();
I get a TypeError: this is not a Date object. However (new Date).getDate() works

• javascript

• 2 If you've stumbled across this question and you're using a Proxy, read this: 2ality./2016/11/… – mpen Commented May 15, 2017 at 16:50

5 Answers 5

The function is not properly bound in your example. The "this" object for that foo call isn't the original date object.

One way to make the logic work is to bind the function:

var x = new Date();
var foo = x.getDate.bind(x);
foo();

In JavaScript, the this context is not bound to each method of an object. Rather, it is determined at run-time by the way you call that method Check this answer for more about the binding behaviour..

In your code, foo receives the getDate property of new Date, which it receives from the Date.prototype through the prototype chain. Thus, your code is really equivalent to:

var foo = Date.prototype.getDate;
foo();

(Test this yourself: verify in your console that (new Date).getDate === Date.prototype.getDate is indeed true.)

Now, it should be clear that there's no actual this context for that call. You could set it up beforehand by manually binding the function to an object. (Note: older browsers need a shiv for Function.prototype.bind.)

var foo = Date.prototype.getDate.bind(new Date);
foo();

Alternatively, set up the proper this context when you call/apply the function.

var foo = Date.prototype.getDate;
foo.call(new Date);

The problem is that this, when you call the function, isn't a date but the global context (window).

You may do this :

foo.call(new Date());

Or, if you want to be able to use the function from anywhere and still use the original date, you may use

var date = new Date();
var foo = function() { return date.getDate() }; // returns always the same date

or

var foo = function() { return (new Date()).getDate() }; // returns the current date

If there weren't IE8, you could also use bind :

var foo = date.bind(date);

What you want to do is either

var date = new Date;
var foo = date.getDate.bind(Date)
foo()

or

var date = new Date;
var foo = date.getDate;
foo.call(date);

When you call foo as you did, it won't have a reference to a date object, which is why it throws an error.

Because foo in your code is just a function which is not bind with any object. It requires some other info from Date object in order to be called. You can do this:

var date = new Date()
var foo = date.getDate
foo.call(date)