How to add a default return statement for objects instantiated by constructor functions?

Question

Consider how JavaScript's Date constructor creates an object that returns a default string when referenced directly:

var date = new Date();
document.write(date); // Outputs string: Mon May 23 2016 08:48:14 GMT-0400 (EDT)
                      // Expected output: [object Object]

document.write('<br/>', date.getFullYear()); // Can call methods as expected.
document.write('<br/>', date.toString());

According to MDN documentation:

If no arguments are provided, the constructor creates a JavaScript Date object for the current date and time according to system settings.

(emphasis mine)

The Date constructor returns an object, yet when that object is referenced directly, it returns a string representing the current date and time instead of a representation of the methods and properties as expected.

How can I achieve the same behavior with an object constructor of my own creation?

For example:

// My CoolObj constructor function
var CoolObj = function () {
  var self = this;
  var i = 0;
  this.coolMethod = function () {
    i += 1;
    return self.coolProperty + ' and increment ' + i;
  };
  this.coolProperty = 'My cool string';
}

var myCoolObj = new CoolObj(); // Instantiate new object from constructor;
                               // Just like `var date = new Date();` above.

document.write(myCoolObj); // Outputs [object Object];
                           // I want to output a string, like Date does.
                           // For example: 'My cool direct-reference string.'

document.write('<br/>', myCoolObj.coolProperty); // Can call properties...
document.write('<br/>', myCoolObj.coolMethod()); // ...and methods as expected.

I'd like it to return a string of some kind when it is referenced directly, while providing the ability to call its methods and properties as normal.

---

UPDATE:

The linked question does provide the answer:
Is it possible to override JavaScript's toString() function to provide meaningful output for debugging?

tl;dr: Define CoolObj.prototype.toString which will get called by functions like console.log and document.write that attempt to cast objects to a string before output.

// My CoolObj constructor function
var CoolObj = function () {
  var self = this;
  var i = 0;
  this.coolMethod = function () {
    i += 1;
    return self.coolProperty + ' and increment ' + i;
  };
  this.coolProperty = 'My cool string';
}

// Define a custom `toString` method on the constructor prototype
CoolObj.prototype.toString = function () {
  return 'My cool direct-reference string.';
}

var myCoolObj = new CoolObj(); // Instantiate new object from constructor;
                               // Just like `var date = new Date();` above.

document.write(myCoolObj); // Outputs [object Object];
                           // I want to output a string, like Date does.
                           // For example: 'My cool direct-reference string.'

document.write('<br/>', myCoolObj.coolProperty); // Can call properties...
document.write('<br/>', myCoolObj.coolMethod()); // ...and methods as expected.

Answer 1

I am not seeking help for specifying string format for objects in console.

But that's the behaviour you are seeing. There is no such thing as a "default return statement for objects".

Consider how JavaScript's Date constructor creates an object that returns a default string when referenced directly

It doesn't. It just creates an object. A special native Date object maybe, but there's nothing about "default strings".
It really is only the console (console.log) that formats these native Date instances special.

There is nothing you can do to achieve that yourself for arbitrary objects, what you are asking for is impossible.