String prototype extension fail!

string_prototype_extension_fail.js

/**
 * Comparing a String object and a simple string
 **/

var myStringSimple = 'abcd';
console.log(myStringSimple instanceof String); // false
console.log(typeof(myStringSimple)); // 'string'

var myStringObject = new String('abcd');
console.log(myStringSimple instanceof String); // true
console.log(typeof(mySimpleString)); // 'object'

// BUT: all the methods from String.prototype are available to myStringSimple
String.prototype.test = function () {
  console.log(1);
};
'xyz'.test(); // 1 is output to the console
// Why ? Because internally it's (new String('xyz')).test() that is executed

myStringSimple == myStringObject; //true (thanks to type coercion)
myStringSimple === myStringObject; //false 

/************************************************************************************************/

/**
 * Pitfalls of augmenting the prototype of String
 **/
String.prototype.test = function () {
  console.log(1);
};

var myString = new String('abcd');

//Looping over the elements of myString
for (var index in myString) {
  console.log(index, myString[index]);
}

//// => returns
// 0 a
// 1 b
// 2 c
// 3 d
// test function () { console.log(1); }

// => The enumerability of String happens on the string 'abcd' + the augmented prototype methods!!

/************************************************************************************************/

/**
 * Pitfalls of inheriting String.prototype
 **/

var ExtString = function () {
  String.apply(this, arguments); //arguments is the JS notation for argv
};

ExtString.prototype = String.prototype; // ExtString.prototype = new String(); is equivalent

ExtString.prototype.extTest = function () {
  console.log('my inheritance Test');
};

var myExtString = new ExtString('123456789');
console.log(myExtString.length); // 0
// => Fail...