How to extend the Javascript Date object?

Question

I'm trying to subclass/extend the native Date object, without modifying the native object itself.

I've tried this:

    var util = require('util');      function MyDate() {         Date.call(this);     }     util.inherits(MyDate, Date);      MyDate.prototype.doSomething = function() {         console.log('Doing something...');     };              var date = new MyDate();     date.doSomething();      console.log(date);     console.log(date.getHours()); 

and this:

function MyDate() {      }      MyDate.prototype = new Date();      MyDate.prototype.doSomething = function() {         console.log("DO");     }      var date = new MyDate();     date.doSomething();     console.log(date); 

In both cases, the date.doSomething() works, but when I call any of the native methods such as date.getHours() or even console.log(date), I get 'TypeError: this is not a Date object.'

Any ideas? Or am I stuck to extending the top-level Date object?

Answer 1

Looking at the v8 code, in date.js:

function DateGetHours() {   var t = DATE_VALUE(this);   if (NUMBER_IS_NAN(t)) return t;   return HOUR_FROM_TIME(LocalTimeNoCheck(t)); } 

And looks like DATE_VALUE is a macro that does this:

DATE_VALUE(arg) = (%_ClassOf(arg) === 'Date' ? %_ValueOf(arg) : ThrowDateTypeError()); 

So, seems like v8 won't let you subclass Date.

Answer 2

This can be done in ES5. It requires modifying the prototype chain directly. This is done using __proto__ or Object.setPrototypeOf(). I'm using __proto__ in the sample code since that's most widely supported (although the standard is Object.setPrototypeOf).

function XDate(a, b, c, d, e, f, g) {   var x;   switch (arguments.length) {     case 0:       x = new Date();       break;     case 1:       x = new Date(a);       break;     case 2:       x = new Date(a, b);       break;     case 3:       x = new Date(a, b, c);       break;     case 4:       x = new Date(a, b, c, d);       break;     case 5:       x = new Date(a, b, c, d, e);       break;     case 6:       x = new Date(a, b, c, d, e, f);       break;     default:       x = new Date(a, b, c, d, e, f, g);   }   x.__proto__ = XDate.prototype;   return x; }  XDate.prototype.__proto__ = Date.prototype;  XDate.prototype.foo = function() {   return 'bar'; }; 

The trick is that we actually instantiate a Date object (with the correct number of arguments) which gives us an object with it's internal [[Class]] set correctly. Then we modify it's prototype chain to make it an instance of XDate.

So, we can verify all this by doing:

var date = new XDate(2015, 5, 18) console.log(date instanceof Date) //true console.log(date instanceof XDate) //true console.log(Object.prototype.toString.call(date)) //[object Date] console.log(date.foo()) //bar console.log('' + date) //Thu Jun 18 2015 00:00:00 GMT-0700 (PDT) 

This is the only way I know of to subclass date because the Date() constructor does some magic to set the internal [[Class]] and most date methods require that to be set. This will work in Node, IE 9+ and almost all other JS engines.

Similar approach can be used for subclassing Array.