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\')         


        

Answer 1

var SubDate = function() { 
    var dateInst = new Date(...arguments); // spread arguments object
    /* Object.getPrototypeOf(dateInst) === Date.prototype */
    Object.setPrototypeOf(dateInst, SubDate.prototype);   // redirectionA
    return dateInst; // now instanceof SubDate
};

Object.setPrototypeOf(SubDate.prototype, Date.prototype); // redirectionB

// do something useful
Object.defineProperty(SubDate.prototype, 'year', {
    get: function() {return this.getFullYear();},
    set: function(y) {this.setFullYear(y);}
});

var subDate = new SubDate(); 
subDate.year;                                 // now
subDate.year = 2050; subDate.getFullYear();   // 2050

The problem with the Date constructor function is already explained in the other answers. You can read about the Date.call(this, ...arguments) problem on Date | MDN (first Note).

This solution is a compact workaround which works as intended in all supporting browsers.