Develop Reference

JS prototype inheritance confusing

Actual code.
function Animal(name) {
this.name = name;
}
function Rabbit(speed) {
Animal.call(this, 'Rabbit');
this.speed = speed;
}
Rabbit.prototype = Animal;
var rabbit = new Rabbit(50);
console.log(rabbit.name, rabbit.speed);
I expected to have console output:
Rabbit 50
But I have
Animal 50
Can anybody explain why Animal function doesn't rewrite its name property?

Animal just a function
'function' object has a property 'name'
for example
function app(){
console.log("hello")
}
console.log(app.name) // "app"
function Animal(name){
this.name = name;
}
console.log(Animal.name) // "Animal"
so Animal object has a property 'name', function object also has a property 'name'
you can try this
function Animal(name) {
this.notname = name; //do not use "name" as a property name
}
function Rabbit(speed) {
Animal.call(this, 'Rabbit');
this.speed = speed;
}
Rabbit.prototype = Animal;
var rabbit = new Rabbit(50);
console.log(rabbit.notname, rabbit.speed);
or
function Animal(name) {
this.name = name;
}
function Rabbit(speed) {
Animal.call(this, 'Rabbit');
this.speed = speed;
}
Rabbit.prototype = new Animal();
//you should point the Rabbit.prototype to a Animal object,not a function
var rabbit = new Rabbit(50);
console.log(rabbit.name, rabbit.speed);

Using Object.create, you should end up with something like:
function Animal(name) {
this.name = name
}
function Rabbit(speed) {
Animal.call(this, 'Rabbit')
this.speed = speed
}
Rabbit.prototype = Object.create(Animal.prototype)
// If you don't set this line, `new Rabbit` would call `Animal` constructor
Rabbit.prototype.constructor = Rabbit
var rabbit = new Rabbit(50)
console.log(rabbit.name, rabbit.speed)

Getting undefined value while inheritance in JavaScript OOPS

Getting undefined value while inheritance in JavaScript OOPS. Student object doesn't inheritance the Person Object
function person(name, age) {
this.name = name;
this.age = age;
this.say = function() {
return this.name + " says Hi..";
}
}
var p1 = new person("Mahesh", "33");
var p2 = new person("Girish", "30");
console.log(p1.say());
console.log(p2.say());
// Inheritance
function student() {};
student.prototype = new person();
var stud1 = new student("Nakktu", "32");
console.log(stud1.say());

You still have to call your super class from within the constructor of the sub class. See this MDN link for more information.
function person(name, age) {
// When no name is provided, throw an error.
if (name === undefined) {
throw 'Unable to create instance of person. Name is required.';
}
this.name = name;
this.age = age;
this.say = function() {
return this.name + " says Hi..";
}
}
var p1 = new person("Mahesh", "33");
var p2 = new person("Girish", "30");
console.log(p1.say());
console.log(p2.say());
// Inheritance
function student(name, age) {
// You need to call your super class.
person.call(this, name, age);
};
// Don't use "new person()", your code will stop working when person() throws
// an error when the 'name' param is required and missing.
student.prototype = Object.create(person.prototype);
var stud1 = new student("Nakktu", "32");
console.log(stud1.say());

JavaScript OOP: Uncaught TypeError: is not a function

Have trouble with object creation. Console says that something wrong in the last line. Please tell how it should be, I more familar with Java, so this is little bit confusing for me.
var dog = {
name:"Dog",
age:"11",
getName : function() {
alert(this.name);
}
}
function Dog(name, age) {
this.name = name;
this.age = age;
}
var d1 = new Dog("Rex", 8);
d1.getName();

Your dog is just a simple Object literal,
that means that your getName is bound to it, not to your Dog class.
You can make that function a method of Dog instead:
/*var dog = {
name:"Dog",
age:"11",
getName : function() {
alert(this.name);
}
}*/
function Dog(name, age) {
this.name = name;
this.age = age;
}
Dog.prototype.getName = function() {
console.log( this.name );
}
var d1 = new Dog("Rex", 8);
d1.getName(); // "Rex"
Here's a variant that uses your settings "defaults"
function Dog() {
this.name = "Dog"; // Default name
this.age = 11; // Default age
}
Dog.prototype.getName = function() {
console.log( this.name );
}
var d1 = new Dog();
d1.name = "Rex"; // Override default name
d1.getName(); // "Rex"

You can use class with syntaxic sugar to properly create objects in ES6.
In your exemple that would write like this :
'use strict';
class Dog{
constructor(name, age){
this.name = name;
this.age = age;
}
getName(){
console.log(this.name);
}
}
let doggy = new Dog("krypto", 125);
doggy.getName();

Traditional OO in JavaScript
function Dog(name, age) {
this.name = name || "Dog";// if the name is not given, it defaults to "Dog"
this.age = age || "11";
}
Dog.prototype.getName = function() {
alert(this.name);
}
var d1 = new Dog("Rex", 8);
d1.getName();
More Explicit OO in JavaScript
function createDog(name, age) {
// create a new dog and return it
var dog = {
name: name || "Dog",// if the name is not given, it defaults to "Dog"
age: age || "11"
};
return dog;
}
createDog.getName = function(dog) {
// explicit dog as 1st parameter
alert(dog.name);
}
//createDog is a normal function that returns something, no "new" needed
var d1 = createDog("Rex", 8);
createDog.getName(d1);

Create an instance of object internally

I have tried to create an instance of object internally like the following:
var oo = function(){
return new func();
}
var func = function(){
this.name;
this.age;
};
func.prototype = {
setData: function(name, age){
this.name = name;
this.age = age;
},
getData: function (){
return this.name + " " + this.age;
}
}
When usage, I got an error oo.setData is not a function.
oo.setData("jack", 15);
console.log(oo.getData());
What's wrong in my code?

This happens because oo is not a "func", oo returns a new func. You could set the data using
oo().setData('jack',15);
But then you have no way of accessing it.
You could also use
var newfunc = oo();
newfunc.setData('jack',15);
newfunc.getData();

oo is a function to create a object.
var oo = function(){ //the oo variable is used to create func() objects
return new func();
}
var func = function(){ //function
this.name;
this.age;
};
func.prototype = { //define properties to func
setData: function(name, age){
this.name = name;
this.age = age;
},
getData: function (){
return this.name + " " + this.age;
}
}
//create instance
var myObject = oo();
//or
var myObject = new func();
//Use
myObject.setData("jack", 12);
//Get a property
console.log(myObject.getData())

Add new property to Prototype using contructor argument

If I have:
function Person(name, age){
this.name = name;
this.whoIs = function(){
alert('I am ' + this.name);
}
}
var john = new Person('John');
john.whoIs();
all will work and I will get a nice alert with: "I am John".
Is there a way to add method to the prototype after the constructor and that will have access to the constructor arguments?
Something like:
function Person(name, age){
this.name = name;
this.whoIs = function(){
alert('I am ' + this.name);
}
}
Person.prototype.age = Person.arguments[1];
var john = new Person('John', 20);
john.age; // would give 20
Is there a way to do this? Ie: being able to add a property or method to a prototype that will have access to the arguments when a new instance is created?

It doesn't make sense to have a dynamic property in the prototype. See the prototype as the blueprint of your object.
You can do this:
function Person(name, age){
this.name = name;
this.whoIs = function(){
alert('I am ' + this.name);
}
this.age = age;
}
var john = new Person('John', 20);
john.age; // would give 20
Also, the whoIs function is added for each Person object. You can add it to the prototype instead:
function Person(name, age){
this.name = name;
this.age = age;
}
Person.prototype.whoIs = function () {
return 'I am ' + this.name;
}
var john = new Person('John', 20);
john.whoIs(); // would give "I am John"