Why this JS code is not valid?

Question

Below code snipped throws an error TypeError: myObj.prototype is undefined. Could someone explain me why?

Why there is no prototype for new Object() & object literals as specified below?

var myObj = {
    a : "This is a",
    b : "This is b"
}

myObj.prototype.c= "This is c";  // TypeError: myObj.prototype is undefined

If this is not valid approach then how can I achieve this?

Answer 1

In earlier versions of EcmaScript you could not directly access the prototype of objects; the prototype property existed only on functions, and it comes into play when they are used as constructors. So you could do this:

// This is the myObj constuctor
function myObj() {
    this.a = "This is a";
    this.b = "This is b";
}

// Setting a property on the constructor prototype
// All instances will share this
myObj.prototype.c= "This is c";

// Creating a new object and testing its "c" property
var obj = new myObj();
alert(obj.c); // "This is c"

Modern browsers implement Object.getPrototypeOf, which means you can do this:

var myObj = {
    a : "This is a",
    b : "This is b"
}

Object.getPrototypeOf(myObj).c= "This is c";

However, you have to be careful! If you do this, then all objects that exist now and all objects that get created in the future will inherit the property c through their prototype chain!

This is because myObj is of type Object, and the prototype of Object is inherited by everything that is an object of any type. This leads to:

var myObj = {
    a : "This is a",
    b : "This is b"
}

Object.getPrototypeOf(myObj).c= "This is c";

var anotherObject = {};

alert(anotherObject.c); // "This is c" -- was it expected?

See it in action.