Trying to understand inheritance in JavaScript -- what's going on here?

Trying to understand inheritance in JavaScript -- what's going on here? - javascript

Why does this say "Animal" instead of "kitty"?
// create base class Animal
function Animal(animalType) {
this.type = animalType;
this.sayType = function () {
alert(this.type);
};
}
// create derived class Cat
function Cat(myName) {
Animal.call(this, "cat"); // cat calls the Animal base class constructor
this.name = myName;
this.sayName = function () {
alert(this.name);
};
}
Cat.prototype = Object.create(Animal); // set Cat's prototype to Animal
// instantiate a new instance of Cat
var cat = new Cat("kitty");
cat.sayName();
cat.name = "lol";
cat.sayName();
http://jsfiddle.net/dgcoffman/MguUA/5/

Object.create(Animal); // set Cat's prototype to Animal
Yes, but to the Animal constructor function (or, to be exact: to a new object inheriting from that function object - check the docs for Object.create). That's hardly what you want - and explains your curious result of saying "Animal", as that's the Animal function's name property.
Instead, you want to build a prototype chain, so that cat instances inherit from Cat.prototype which inherits from Animal.prototype (which inherits from Object.prototype):
Cat.prototype = Object.create(Animal.prototype);
Also, for prototypical inheritance, you should but the sayName and sayType methods on the prototype object (and only once):
Animal.prototype.sayType = function() {
alert(this.type);
};
Cat.prototype.sayName = function() { // after creation of that object, of course
alert(this.name);
};

You're calling Object.create() in a way that does something different than what I bet you think it does. Try this change:
Cat.prototype = Object.create(new Animal);
The Object.create() function expects its first argument to be the object to be used as the prototype for the returned object. When you pass in the function name "Animal", that means you want the prototype object to be that function object, and not an object constructed by the function.
edit — Bergi's answer involving the direct use of the Animal prototype probably makes more sense, though from what you posted there isn't an Animal prototype object with anything interesting on it. Perhaps that's elsewhere in your code, or will be added later. Anyway if it does exist it's better to do as in his answer.
Since your "Cat" constructor is picking up the "sayType" property from the Animal constructor directly, it's not clear what you're trying to do by setting the Cat prototype.

Related

javascript prototypical inheritance confused

given the standard way of achieving inheritance like this
function BaseClass() {
}
function SubClass() {
BaseClass.call(this);
}
SubClass.prototype = Object.create(BaseClass.prototype);
SubClass.prototype.constructor = SubClass;
why is necesary to do
SubClass.prototype = Object.create(BaseClass.prototype);
and end up with something like
function F(){}
F.prototype = BaseClass.prototype;
SubClass.prototype = new F();
instead of just doing
Subclass.prototype = BaseClass.prototype;

Assigning values to things in JavaScript really just copies a reference (unless working with primitive types). So when you do this:
Subclass.prototype = BaseClass.prototype;
What you're really doing is assigning the prototype of SubClass to the same location in memory as the prototype of BaseClass, therefore any prototype related changes you make to SubClass will also affect BaseClass. Here's a little example:
function BaseClass() {
}
function SubClass() {
BaseClass.call(this);
}
SubClass.prototype = BaseClass.prototype;
SubClass.prototype.constructor = SubClass;
SubClass.prototype.subClassFunction = function(){
console.log("Added this to SubClass");
}
var baseObj = new BaseClass();
baseObj.subClassFunction(); // => "Added this to SubClass"
That's why you want to use
SubClass.prototype = Object.create(BaseClass.prototype);
because it will create a new and unique object with the specified prototype instead.
You can read more about how this function works here.

Object Reference!
Say BaseClass has a method toString:
BaseClass.prototype.toString = function() {
return 'foo'
}
But you redefine the toString method in SubClass:
SubClass.prototype.toString = function() {
return 'bar'
}
You'll expect:
var b = new BaseClass(), s = new SubClass()
b.toString() //=> 'foo'
s.toString() //=> 'bar'
But if you use assignment to create inheritance what you will get is:
var b = new BaseClass(), s = new SubClass()
b.toString() //=> 'bar'
s.toString() //=> 'bar'
Because BaseClass.prototype and SubClass.prototype now reference the same object

Inheritance, the __proto__ :
When an object SubClass inherits from another object BaseClass, in JavaScript that means that there is a special property SubClass.__proto__ = BaseClass.
Code :
function BaseClass() {
}
function SubClass() {
}
var BaseClass = new BaseClass();
var SubClass = new SubClass();
BaseClass.a = 5;
SubClass.b = 10;
SubClass.__proto__ = BaseClass;
console.log(SubClass);
Output :
Here, BaseClass is inherited by SubClass and BaseClass variable is accessable through the SubClass.

The syntax of JavaScript can be somewhat confusing. In JS, there isn't class inheritance but instance-based inheritance. An instance's parent is also called its prototype.
When you write instance.something or instance['something'], the JS engine looks at the instance to see if it has a member called something. If it doesn't, then it looks at the instance's prototype. If that object doesn't have the member something, it looks at the prototype's prototype, again and again until it finds the property or reaches an instance which inherits from null. In that case it will simply return undefined.
Functions have a special property called prototype which is something else. The function's .prototype is a simple object that has a constructor property which refers back to the function itself. When you create an object with the new keyword, that object's prototype will be set to the function's .prototype property. This is where the confusion comes from: the .prototype property of a constructor can be seen as the default prototype of all instances made with said constructor.
So when you add methods to a class by writing something like this:
MyClass.prototype.foo = function() {
alert('foo');
};
...you're actually storing the function in the prototype of all MyClass instances. When the JS engine looks at the instances of MyClass, it will look for the foo member which it won't find. Then it will look at the instance's prototype which happens to be set to MyClass.prototype and it will find the foo member and fetch it.
It's important to make the difference between an instance's prototype and a function's .prototype, but most people don't realize it. When they speak of a class's prototype, they're talking about MyClass.prototype. The instance prototype is accessible via __proto__ in many browsers, but that's not a standard feature of JavaScript and shouldn't be used in your code.
Now let's look at the code you're using to simulate class inheritance.
SubClass.prototype = Object.create(BaseClass.prototype);
SubClass.prototype.constructor = SubClass;
Object.create(parent) can be seen as a function that does this:
return {
__proto__ : parent
};
In other words, it creates a blank Object whose prototype is the passed object. Since all SubClass instances will inherit from Subclass.prototype, replacing SubClass.prototype with an object that inherits from BaseClass.prototype makes sure all SubClass instances also inherit from BaseClass.
However, as I said earlier, the default .prototype attribute of a function is an empty object whose .constructor is set to the function itself. So by manually setting the .constructor again, we're perfectly mimicking default prototype behavior. If we don't do that, then instance.constructor will return the first defined .constructor property down the prototype chain, which will be BaseClass. It doesn't really change anything in terms of behavior unless our code actually depends on the constructor property, but it's safer to have it.
As a final note, like others mentioned before I could finally post this answer, you can't just do SubClass.prototype = BaseClass.prototype; because then you wouldn't be able to add methods to the SubClass without adding them to the BaseClass.

Javascript prototype constructor - Why should I fix it? [duplicate]

In the section about inheritance in the MDN article Introduction to Object Oriented Javascript, I noticed they set the prototype.constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
Does this serve any important purpose? Is it okay to omit it?

It's not always necessary, but it does have its uses. Suppose we wanted to make a copy method on the base Person class. Like this:
// define the Person Class
function Person(name) {
this.name = name;
}
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
// define the Student class
function Student(name) {
Person.call(this, name);
}
// inherit Person
Student.prototype = Object.create(Person.prototype);
Now what happens when we create a new Student and copy it?
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => false
The copy is not an instance of Student. This is because (without explicit checks), we'd have no way to return a Student copy from the "base" class. We can only return a Person. However, if we had reset the constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
...then everything works as expected:
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => true

Does this serve any important purpose?
Yes and no.
In ES5 and earlier, JavaScript itself didn't use constructor for anything. It defined that the default object on a function's prototype property would have it and that it would refer back to the function, and that was it. Nothing else in the specification referred to it at all.
That changed in ES2015 (ES6), which started using it in relation to inheritance hierarchies. For instance, Promise#then uses the constructor property of the promise you call it on (via SpeciesConstructor) when building the new promise to return. It's also involved in subtyping arrays (via ArraySpeciesCreate).
Outside of the language itself, sometimes people would use it when trying to build generic "clone" functions or just generally when they wanted to refer to what they believed would be the object's constructor function. My experience is that using it is rare, but sometimes people do use it.
Is it okay to omit it?
It's there by default, you only need to put it back when you replace the object on a function's prototype property:
Student.prototype = Object.create(Person.prototype);
If you don't do this:
Student.prototype.constructor = Student;
...then Student.prototype.constructor inherits from Person.prototype which (presumably) has constructor = Person. So it's misleading. And of course, if you're subclassing something that uses it (like Promise or Array) and not using class¹ (which handles this for you), you'll want to make sure you set it correctly. So basically: It's a good idea.
It's okay if nothing in your code (or library code you use) uses it. I've always ensured it was correctly wired up.
Of course, with ES2015 (aka ES6)'s class keyword, most of the time we would have used it, we don't have to anymore, because it's handled for us when we do
class Student extends Person {
}
¹ "...if you're subclassing something that uses it (like Promise or Array) and not using class..." — It's possible to do that, but it's a real pain (and a bit silly). You have to use Reflect.construct.

TLDR; Not super necessary, but will probably help in the long run, and it is more accurate to do so.
NOTE: Much edited as my previous answer was confusingly written and had some errors that I missed in my rush to answer. Thanks to those who pointed out some egregious errors.
Basically, it's to wire subclassing up correctly in Javascript. When we subclass, we have to do some funky things to make sure that the prototypal delegation works correctly, including overwriting a prototype object. Overwriting a prototype object includes the constructor, so we then need to fix the reference.
Let's quickly go through how 'classes' in ES5 work.
Let's say you have a constructor function and its prototype:
//Constructor Function
var Person = function(name, age) {
this.name = name;
this.age = age;
}
//Prototype Object - shared between all instances of Person
Person.prototype = {
species: 'human',
}
When you call the constructor to instantiate, say Adam:
// instantiate using the 'new' keyword
var adam = new Person('Adam', 19);
The new keyword invoked with 'Person' basically will run the Person constructor with a few additional lines of code:
function Person (name, age) {
// This additional line is automatically added by the keyword 'new'
// it sets up the relationship between the instance and the prototype object
// So that the instance will delegate to the Prototype object
this = Object.create(Person.prototype);
this.name = name;
this.age = age;
return this;
}
/* So 'adam' will be an object that looks like this:
* {
* name: 'Adam',
* age: 19
* }
*/
If we console.log(adam.species), the lookup will fail at the adam instance, and look up the prototypal chain to its .prototype, which is Person.prototype - and Person.prototype has a .species property, so the lookup will succeed at Person.prototype. It will then log 'human'.
Here, the Person.prototype.constructor will correctly point to Person.
So now the interesting part, the so-called 'subclassing'. If we want to create a Student class, that is a subclass of the Person class with some additional changes, we'll need to make sure that the Student.prototype.constructor points to Student for accuracy.
It doesn't do this by itself. When you subclass, the code looks like this:
var Student = function(name, age, school) {
// Calls the 'super' class, as every student is an instance of a Person
Person.call(this, name, age);
// This is what makes the Student instances different
this.school = school
}
var eve = new Student('Eve', 20, 'UCSF');
console.log(Student.prototype); // this will be an empty object: {}
Calling new Student() here would return an object with all of the properties we want. Here, if we check eve instanceof Person, it would return false. If we try to access eve.species, it would return undefined.
In other words, we need to wire up the delegation so that eve instanceof Person returns true and so that instances of Student delegate correctly to Student.prototype, and then Person.prototype.
BUT since we're calling it with the new keyword, remember what that invocation adds? It would call Object.create(Student.prototype), which is how we set up that delegational relationship between Student and Student.prototype. Note that right now, Student.prototype is empty. So looking up .species an instance of Student would fail as it delegates to only Student.prototype, and the .species property doesn't exist on Student.prototype.
When we do assign Student.prototype to Object.create(Person.prototype), Student.prototype itself then delegates to Person.prototype, and looking up eve.species will return human as we expect. Presumably we would want it to inherit from Student.prototype AND Person.prototype. So we need to fix all of that.
/* This sets up the prototypal delegation correctly
*so that if a lookup fails on Student.prototype, it would delegate to Person's .prototype
*This also allows us to add more things to Student.prototype
*that Person.prototype may not have
*So now a failed lookup on an instance of Student
*will first look at Student.prototype,
*and failing that, go to Person.prototype (and failing /that/, where do we think it'll go?)
*/
Student.prototype = Object.create(Person.prototype);
Now the delegation works, but we're overwriting Student.prototype with an of Person.prototype. So if we call Student.prototype.constructor, it would point to Person instead of Student. This is why we need to fix it.
// Now we fix what the .constructor property is pointing to
Student.prototype.constructor = Student
// If we check instanceof here
console.log(eve instanceof Person) // true
In ES5, our constructor property is a reference that refers to a function that we've written with the intent to be a 'constructor'. Aside from what the new keyword gives us, the constructor is otherwise a 'plain' function.
In ES6, the constructor is now built into the way we write classes - as in, it's provided as a method when we declare a class. This is simply syntactic sugar but it does accord us some conveniences like access to a super when we are extending an existing class. So we would write the above code like this:
class Person {
// constructor function here
constructor(name, age) {
this.name = name;
this.age = age;
}
// static getter instead of a static property
static get species() {
return 'human';
}
}
class Student extends Person {
constructor(name, age, school) {
// calling the superclass constructor
super(name, age);
this.school = school;
}
}

I'd disagree. It isn't necessary to set the prototype. Take that exact same code but remove the prototype.constructor line. Does anything change? No. Now, make the following changes:
Person = function () {
this.favoriteColor = 'black';
}
Student = function () {
Person.call(this);
this.favoriteColor = 'blue';
}
and at the end of the test code...
alert(student1.favoriteColor);
The color will be blue.
A change to the prototype.constructor, in my experience, doesn't do much unless you're doing very specific, very complicated things that probably aren't good practice anyway :)
Edit:
After poking around the web for a bit and doing some experimentation, it looks like people set the constructor so that it 'looks' like the thing that is being constructed with 'new'. I guess I would argue that the problem with this is that javascript is a prototype language - there is no such thing as inheritence. But most programmers come from a background of programming that pushes inheritence as 'the way'. So we come up with all sorts of things to try and make this prototypical language a 'classic' language.. such as extending 'classes'. Really, in the example they gave, a new student is a person - it isn't 'extending' from another student.. the student is all about the person, and whatever the person is the student is as well. Extend the student, and whatever you've extended is a student at heart, but is customized to fit your needs.
Crockford is a bit crazy and overzealous, but do some serious reading on some of the stuff that he's written.. it'll make you look at this stuff very differently.

This has the huge pitfall that if you wrote
Student.prototype.constructor = Student;
but then if there was a Teacher whose prototype was also Person and you wrote
Teacher.prototype.constructor = Teacher;
then the Student constructor is now Teacher!
Edit:
You can avoid this by ensuring that you had set the Student and Teacher prototypes using new instances of the Person class created using Object.create, as in the Mozilla example.
Student.prototype = Object.create(Person.prototype);
Teacher.prototype = Object.create(Person.prototype);

So far confusion is still there.
Following the original example, as you have an existing object student1 as:
var student1 = new Student("Janet", "Applied Physics");
Suppose you don't want to know how student1 is created, you just want another object like it, you can use the constructor property of student1 like:
var student2 = new student1.constructor("Mark", "Object-Oriented JavaScript");
Here it will fail to get the properties from Student if the constructor property is not set. Rather it will create a Person object.

Got a nice code example of why it is really necessary to set the prototype constructor..
function CarFactory(name){
this.name=name;
}
CarFactory.prototype.CreateNewCar = function(){
return new this.constructor("New Car "+ this.name);
}
CarFactory.prototype.toString=function(){
return 'Car Factory ' + this.name;
}
AudiFactory.prototype = new CarFactory(); // Here's where the inheritance occurs
AudiFactory.prototype.constructor=AudiFactory; // Otherwise instances of Audi would have a constructor of Car
function AudiFactory(name){
this.name=name;
}
AudiFactory.prototype.toString=function(){
return 'Audi Factory ' + this.name;
}
var myAudiFactory = new AudiFactory('');
alert('Hay your new ' + myAudiFactory + ' is ready.. Start Producing new audi cars !!! ');
var newCar = myAudiFactory.CreateNewCar(); // calls a method inherited from CarFactory
alert(newCar);
/*
Without resetting prototype constructor back to instance, new cars will not come from New Audi factory, Instead it will come from car factory ( base class ).. Dont we want our new car from Audi factory ????
*/

No need for sugared function 'classes' or using 'New' these days. Use object literals.
The Object prototype is already a 'class'. When you define an object literal, it is already an instance of the prototype Object. These can also act as another object's prototype, etc.
const Person = {
name: '[Person.name]',
greeting: function() {
console.log( `My name is ${ this.name || '[Name not assigned]' }` );
}
};
// Person.greeting = function() {...} // or define outside the obj if you must
// Object.create version
const john = Object.create( Person );
john.name = 'John';
console.log( john.name ); // John
john.greeting(); // My name is John
// Define new greeting method
john.greeting = function() {
console.log( `Hi, my name is ${ this.name }` )
};
john.greeting(); // Hi, my name is John
// Object.assign version
const jane = Object.assign( Person, { name: 'Jane' } );
console.log( jane.name ); // Jane
// Original greeting
jane.greeting(); // My name is Jane
// Original Person obj is unaffected
console.log( Person.name ); // [Person.name]
console.log( Person.greeting() ); // My name is [Person.name]
This is worth a read:
Class-based object-oriented languages, such as Java and C++, are
founded on the concept of two distinct entities: classes and
instances.
...
A prototype-based language, such as JavaScript, does not make this
distinction: it simply has objects. A prototype-based language has the
notion of a prototypical object, an object used as a template from
which to get the initial properties for a new object. Any object can
specify its own properties, either when you create it or at run time.
In addition, any object can be associated as the prototype for another
object, allowing the second object to share the first object's
properties

It is necessary when you need an alternative to toString without monkeypatching:
//Local
foo = [];
foo.toUpperCase = String(foo).toUpperCase;
foo.push("a");
foo.toUpperCase();
//Global
foo = [];
window.toUpperCase = function (obj) {return String(obj).toUpperCase();}
foo.push("a");
toUpperCase(foo);
//Prototype
foo = [];
Array.prototype.toUpperCase = String.prototype.toUpperCase;
foo.push("a");
foo.toUpperCase();
//toString alternative via Prototype constructor
foo = [];
Array.prototype.constructor = String.prototype.toUpperCase;
foo.push("a,b");
foo.constructor();
//toString override
var foo = [];
foo.push("a");
var bar = String(foo);
foo.toString = function() { return bar.toUpperCase(); }
foo.toString();
//Object prototype as a function
Math.prototype = function(char){return Math.prototype[char]};
Math.prototype.constructor = function()
{
var i = 0, unicode = {}, zero_padding = "0000", max = 9999;
while (i < max)
{
Math.prototype[String.fromCharCode(parseInt(i, 16))] = ("u" + zero_padding + i).substr(-4);
i = i + 1;
}
}
Math.prototype.constructor();
console.log(Math.prototype("a") );
console.log(Math.prototype["a"] );
console.log(Math.prototype("a") === Math.prototype["a"]);

EDIT, I was actually wrong. Commenting the line out doesn't change it's behavior at all. (I tested it)
Yes, it is necessary. When you do
Student.prototype = new Person();
Student.prototype.constructor becomes Person. Therefore, calling Student() would return an object created by Person. If you then do
Student.prototype.constructor = Student;
Student.prototype.constructor is reset back to Student. Now when you call Student() it executes Student, which calls the parent constructor Parent(), it returns the correctly inherited object. If you didn't reset Student.prototype.constructor before calling it you would get an object that would not have any of the properties set in Student().

Given simple constructor function:
function Person(){
this.name = 'test';
}
console.log(Person.prototype.constructor) // function Person(){...}
Person.prototype = { //constructor in this case is Object
sayName: function(){
return this.name;
}
}
var person = new Person();
console.log(person instanceof Person); //true
console.log(person.sayName()); //test
console.log(Person.prototype.constructor) // function Object(){...}
By default (from the specification https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor), all prototypes automatically get a property called constructor that points back to the function on which it is a property.
Depending on the constructor, other properties and methods might be added to the prototype which is not a very common practice but still it is allowed for extensions.
So simply answering: we need make sure that the value in prototype.constructor is correctly set as it is supposed by the specification to be.
Do we have to always set correctly this value? It helps with debugging and makes internal structure consistent against specification. We should definitely when our API is being used by the thirdparties, but not really when the code is finally executed in the runtime.

Here's one example from MDN which I found very helpful to understand its uses.
In JavaScript, we have async functions which returns AsyncFunction object. AsyncFunction is not a global object but one may retrieve it by using constructor property and utilize it.
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
// AsyncFunction constructor
var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor
var a = new AsyncFunction('a',
'b',
'return await resolveAfter2Seconds(a) + await resolveAfter2Seconds(b);');
a(10, 20).then(v => {
console.log(v); // prints 30 after 4 seconds
});

It is necessary. Any class in class inheritance must has its own constructor, so as in prototype inheritance.It is also convenient for object construction. But the question is unnecessary and what is necessary is understanding in JavaScript world effect of calling function as constructor and rule of resolving object property.
Effect of executing function as constructor with expression new <function name>( [ parameters] )
a object whose type name is the function name is created
inner properties in the function attaches to the created object
property prototype of the function attaches automatically to the created object as prototype
Rule of resolving property of object
The property will not only be sought on the object but on the prototype of the object, the prototype of the prototype, and so on until either a property with a matching name is found or the end of the prototype chain is reached.
Basing on these underlying mechanisms, statement <constructor name>.prototype.constructor = <constructor name> equals in term of effect to attach constructor in constructor body with expression this.constructor = <constructor name>. The constructor will be resolved on the object if second utterance while on object's prototype if first utterance.

It is not necessary. It is just one of the many things traditional, OOP champions do to try to turn JavaScript's prototypical inheritance into classical inheritance. The only thing that the following
Student.prototype.constructor = Student;
does, is that you now have a reference of the current "constructor".
In Wayne's answer, that has been marked as correct, you could the exact same thing that the following code does
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
with the code below (just replace this.constructor with Person)
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new Person(this.name);
};
Thank God that with ES6 classical inheritance purists can use language's native operators like class, extends and super and we don't have to see like prototype.constructor corrections and parent refereces.

Inheritance using Object.create [duplicate]

In the section about inheritance in the MDN article Introduction to Object Oriented Javascript, I noticed they set the prototype.constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
Does this serve any important purpose? Is it okay to omit it?

It's not always necessary, but it does have its uses. Suppose we wanted to make a copy method on the base Person class. Like this:
// define the Person Class
function Person(name) {
this.name = name;
}
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
// define the Student class
function Student(name) {
Person.call(this, name);
}
// inherit Person
Student.prototype = Object.create(Person.prototype);
Now what happens when we create a new Student and copy it?
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => false
The copy is not an instance of Student. This is because (without explicit checks), we'd have no way to return a Student copy from the "base" class. We can only return a Person. However, if we had reset the constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
...then everything works as expected:
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => true

TLDR; Not super necessary, but will probably help in the long run, and it is more accurate to do so.
NOTE: Much edited as my previous answer was confusingly written and had some errors that I missed in my rush to answer. Thanks to those who pointed out some egregious errors.
Basically, it's to wire subclassing up correctly in Javascript. When we subclass, we have to do some funky things to make sure that the prototypal delegation works correctly, including overwriting a prototype object. Overwriting a prototype object includes the constructor, so we then need to fix the reference.
Let's quickly go through how 'classes' in ES5 work.
Let's say you have a constructor function and its prototype:
//Constructor Function
var Person = function(name, age) {
this.name = name;
this.age = age;
}
//Prototype Object - shared between all instances of Person
Person.prototype = {
species: 'human',
}
When you call the constructor to instantiate, say Adam:
// instantiate using the 'new' keyword
var adam = new Person('Adam', 19);
The new keyword invoked with 'Person' basically will run the Person constructor with a few additional lines of code:
function Person (name, age) {
// This additional line is automatically added by the keyword 'new'
// it sets up the relationship between the instance and the prototype object
// So that the instance will delegate to the Prototype object
this = Object.create(Person.prototype);
this.name = name;
this.age = age;
return this;
}
/* So 'adam' will be an object that looks like this:
* {
* name: 'Adam',
* age: 19
* }
*/
If we console.log(adam.species), the lookup will fail at the adam instance, and look up the prototypal chain to its .prototype, which is Person.prototype - and Person.prototype has a .species property, so the lookup will succeed at Person.prototype. It will then log 'human'.
Here, the Person.prototype.constructor will correctly point to Person.
So now the interesting part, the so-called 'subclassing'. If we want to create a Student class, that is a subclass of the Person class with some additional changes, we'll need to make sure that the Student.prototype.constructor points to Student for accuracy.
It doesn't do this by itself. When you subclass, the code looks like this:
var Student = function(name, age, school) {
// Calls the 'super' class, as every student is an instance of a Person
Person.call(this, name, age);
// This is what makes the Student instances different
this.school = school
}
var eve = new Student('Eve', 20, 'UCSF');
console.log(Student.prototype); // this will be an empty object: {}
Calling new Student() here would return an object with all of the properties we want. Here, if we check eve instanceof Person, it would return false. If we try to access eve.species, it would return undefined.
In other words, we need to wire up the delegation so that eve instanceof Person returns true and so that instances of Student delegate correctly to Student.prototype, and then Person.prototype.
BUT since we're calling it with the new keyword, remember what that invocation adds? It would call Object.create(Student.prototype), which is how we set up that delegational relationship between Student and Student.prototype. Note that right now, Student.prototype is empty. So looking up .species an instance of Student would fail as it delegates to only Student.prototype, and the .species property doesn't exist on Student.prototype.
When we do assign Student.prototype to Object.create(Person.prototype), Student.prototype itself then delegates to Person.prototype, and looking up eve.species will return human as we expect. Presumably we would want it to inherit from Student.prototype AND Person.prototype. So we need to fix all of that.
/* This sets up the prototypal delegation correctly
*so that if a lookup fails on Student.prototype, it would delegate to Person's .prototype
*This also allows us to add more things to Student.prototype
*that Person.prototype may not have
*So now a failed lookup on an instance of Student
*will first look at Student.prototype,
*and failing that, go to Person.prototype (and failing /that/, where do we think it'll go?)
*/
Student.prototype = Object.create(Person.prototype);
Now the delegation works, but we're overwriting Student.prototype with an of Person.prototype. So if we call Student.prototype.constructor, it would point to Person instead of Student. This is why we need to fix it.
// Now we fix what the .constructor property is pointing to
Student.prototype.constructor = Student
// If we check instanceof here
console.log(eve instanceof Person) // true
In ES5, our constructor property is a reference that refers to a function that we've written with the intent to be a 'constructor'. Aside from what the new keyword gives us, the constructor is otherwise a 'plain' function.
In ES6, the constructor is now built into the way we write classes - as in, it's provided as a method when we declare a class. This is simply syntactic sugar but it does accord us some conveniences like access to a super when we are extending an existing class. So we would write the above code like this:
class Person {
// constructor function here
constructor(name, age) {
this.name = name;
this.age = age;
}
// static getter instead of a static property
static get species() {
return 'human';
}
}
class Student extends Person {
constructor(name, age, school) {
// calling the superclass constructor
super(name, age);
this.school = school;
}
}

I'd disagree. It isn't necessary to set the prototype. Take that exact same code but remove the prototype.constructor line. Does anything change? No. Now, make the following changes:
Person = function () {
this.favoriteColor = 'black';
}
Student = function () {
Person.call(this);
this.favoriteColor = 'blue';
}
and at the end of the test code...
alert(student1.favoriteColor);
The color will be blue.
A change to the prototype.constructor, in my experience, doesn't do much unless you're doing very specific, very complicated things that probably aren't good practice anyway :)
Edit:
After poking around the web for a bit and doing some experimentation, it looks like people set the constructor so that it 'looks' like the thing that is being constructed with 'new'. I guess I would argue that the problem with this is that javascript is a prototype language - there is no such thing as inheritence. But most programmers come from a background of programming that pushes inheritence as 'the way'. So we come up with all sorts of things to try and make this prototypical language a 'classic' language.. such as extending 'classes'. Really, in the example they gave, a new student is a person - it isn't 'extending' from another student.. the student is all about the person, and whatever the person is the student is as well. Extend the student, and whatever you've extended is a student at heart, but is customized to fit your needs.
Crockford is a bit crazy and overzealous, but do some serious reading on some of the stuff that he's written.. it'll make you look at this stuff very differently.

This has the huge pitfall that if you wrote
Student.prototype.constructor = Student;
but then if there was a Teacher whose prototype was also Person and you wrote
Teacher.prototype.constructor = Teacher;
then the Student constructor is now Teacher!
Edit:
You can avoid this by ensuring that you had set the Student and Teacher prototypes using new instances of the Person class created using Object.create, as in the Mozilla example.
Student.prototype = Object.create(Person.prototype);
Teacher.prototype = Object.create(Person.prototype);

So far confusion is still there.
Following the original example, as you have an existing object student1 as:
var student1 = new Student("Janet", "Applied Physics");
Suppose you don't want to know how student1 is created, you just want another object like it, you can use the constructor property of student1 like:
var student2 = new student1.constructor("Mark", "Object-Oriented JavaScript");
Here it will fail to get the properties from Student if the constructor property is not set. Rather it will create a Person object.

Got a nice code example of why it is really necessary to set the prototype constructor..
function CarFactory(name){
this.name=name;
}
CarFactory.prototype.CreateNewCar = function(){
return new this.constructor("New Car "+ this.name);
}
CarFactory.prototype.toString=function(){
return 'Car Factory ' + this.name;
}
AudiFactory.prototype = new CarFactory(); // Here's where the inheritance occurs
AudiFactory.prototype.constructor=AudiFactory; // Otherwise instances of Audi would have a constructor of Car
function AudiFactory(name){
this.name=name;
}
AudiFactory.prototype.toString=function(){
return 'Audi Factory ' + this.name;
}
var myAudiFactory = new AudiFactory('');
alert('Hay your new ' + myAudiFactory + ' is ready.. Start Producing new audi cars !!! ');
var newCar = myAudiFactory.CreateNewCar(); // calls a method inherited from CarFactory
alert(newCar);
/*
Without resetting prototype constructor back to instance, new cars will not come from New Audi factory, Instead it will come from car factory ( base class ).. Dont we want our new car from Audi factory ????
*/

No need for sugared function 'classes' or using 'New' these days. Use object literals.
The Object prototype is already a 'class'. When you define an object literal, it is already an instance of the prototype Object. These can also act as another object's prototype, etc.
const Person = {
name: '[Person.name]',
greeting: function() {
console.log( `My name is ${ this.name || '[Name not assigned]' }` );
}
};
// Person.greeting = function() {...} // or define outside the obj if you must
// Object.create version
const john = Object.create( Person );
john.name = 'John';
console.log( john.name ); // John
john.greeting(); // My name is John
// Define new greeting method
john.greeting = function() {
console.log( `Hi, my name is ${ this.name }` )
};
john.greeting(); // Hi, my name is John
// Object.assign version
const jane = Object.assign( Person, { name: 'Jane' } );
console.log( jane.name ); // Jane
// Original greeting
jane.greeting(); // My name is Jane
// Original Person obj is unaffected
console.log( Person.name ); // [Person.name]
console.log( Person.greeting() ); // My name is [Person.name]
This is worth a read:
Class-based object-oriented languages, such as Java and C++, are
founded on the concept of two distinct entities: classes and
instances.
...
A prototype-based language, such as JavaScript, does not make this
distinction: it simply has objects. A prototype-based language has the
notion of a prototypical object, an object used as a template from
which to get the initial properties for a new object. Any object can
specify its own properties, either when you create it or at run time.
In addition, any object can be associated as the prototype for another
object, allowing the second object to share the first object's
properties

It is necessary when you need an alternative to toString without monkeypatching:
//Local
foo = [];
foo.toUpperCase = String(foo).toUpperCase;
foo.push("a");
foo.toUpperCase();
//Global
foo = [];
window.toUpperCase = function (obj) {return String(obj).toUpperCase();}
foo.push("a");
toUpperCase(foo);
//Prototype
foo = [];
Array.prototype.toUpperCase = String.prototype.toUpperCase;
foo.push("a");
foo.toUpperCase();
//toString alternative via Prototype constructor
foo = [];
Array.prototype.constructor = String.prototype.toUpperCase;
foo.push("a,b");
foo.constructor();
//toString override
var foo = [];
foo.push("a");
var bar = String(foo);
foo.toString = function() { return bar.toUpperCase(); }
foo.toString();
//Object prototype as a function
Math.prototype = function(char){return Math.prototype[char]};
Math.prototype.constructor = function()
{
var i = 0, unicode = {}, zero_padding = "0000", max = 9999;
while (i < max)
{
Math.prototype[String.fromCharCode(parseInt(i, 16))] = ("u" + zero_padding + i).substr(-4);
i = i + 1;
}
}
Math.prototype.constructor();
console.log(Math.prototype("a") );
console.log(Math.prototype["a"] );
console.log(Math.prototype("a") === Math.prototype["a"]);

EDIT, I was actually wrong. Commenting the line out doesn't change it's behavior at all. (I tested it)
Yes, it is necessary. When you do
Student.prototype = new Person();
Student.prototype.constructor becomes Person. Therefore, calling Student() would return an object created by Person. If you then do
Student.prototype.constructor = Student;
Student.prototype.constructor is reset back to Student. Now when you call Student() it executes Student, which calls the parent constructor Parent(), it returns the correctly inherited object. If you didn't reset Student.prototype.constructor before calling it you would get an object that would not have any of the properties set in Student().

Given simple constructor function:
function Person(){
this.name = 'test';
}
console.log(Person.prototype.constructor) // function Person(){...}
Person.prototype = { //constructor in this case is Object
sayName: function(){
return this.name;
}
}
var person = new Person();
console.log(person instanceof Person); //true
console.log(person.sayName()); //test
console.log(Person.prototype.constructor) // function Object(){...}
By default (from the specification https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor), all prototypes automatically get a property called constructor that points back to the function on which it is a property.
Depending on the constructor, other properties and methods might be added to the prototype which is not a very common practice but still it is allowed for extensions.
So simply answering: we need make sure that the value in prototype.constructor is correctly set as it is supposed by the specification to be.
Do we have to always set correctly this value? It helps with debugging and makes internal structure consistent against specification. We should definitely when our API is being used by the thirdparties, but not really when the code is finally executed in the runtime.

Here's one example from MDN which I found very helpful to understand its uses.
In JavaScript, we have async functions which returns AsyncFunction object. AsyncFunction is not a global object but one may retrieve it by using constructor property and utilize it.
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
// AsyncFunction constructor
var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor
var a = new AsyncFunction('a',
'b',
'return await resolveAfter2Seconds(a) + await resolveAfter2Seconds(b);');
a(10, 20).then(v => {
console.log(v); // prints 30 after 4 seconds
});

It is necessary. Any class in class inheritance must has its own constructor, so as in prototype inheritance.It is also convenient for object construction. But the question is unnecessary and what is necessary is understanding in JavaScript world effect of calling function as constructor and rule of resolving object property.
Effect of executing function as constructor with expression new <function name>( [ parameters] )
a object whose type name is the function name is created
inner properties in the function attaches to the created object
property prototype of the function attaches automatically to the created object as prototype
Rule of resolving property of object
The property will not only be sought on the object but on the prototype of the object, the prototype of the prototype, and so on until either a property with a matching name is found or the end of the prototype chain is reached.
Basing on these underlying mechanisms, statement <constructor name>.prototype.constructor = <constructor name> equals in term of effect to attach constructor in constructor body with expression this.constructor = <constructor name>. The constructor will be resolved on the object if second utterance while on object's prototype if first utterance.

It is not necessary. It is just one of the many things traditional, OOP champions do to try to turn JavaScript's prototypical inheritance into classical inheritance. The only thing that the following
Student.prototype.constructor = Student;
does, is that you now have a reference of the current "constructor".
In Wayne's answer, that has been marked as correct, you could the exact same thing that the following code does
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
with the code below (just replace this.constructor with Person)
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new Person(this.name);
};
Thank God that with ES6 classical inheritance purists can use language's native operators like class, extends and super and we don't have to see like prototype.constructor corrections and parent refereces.

What is the advantage of using prototype in combination with new?

From a book I got this example of inheritance:
var Animal = function(){};
Animal.prototype.sleep = function(){
// ...
};
var Cat = function(){};
// Cat is an animal
Cat.prototype = new Animal;
Cat.prototype.meow = function(){
// ...
};
var scratchy = new Cat();
scratchy.sleep();
scratchy.meow();
But this also works and seems to be more intuitive to me. Why don't you do it? Or do you? Does it create a reference instead of copying the prototype properties?
Cat.prototype = Animal.prototype;

If you do Cat.prototype = Animal.prototype; and then try to add to Cat.prototype, you are also adding to Animal.prototype. The current best practice is actually
Cat.prototype = Object.create(Animal.prototype);

This means that any change to Cat.prototype will be reflected to Animal, i.e. subclass mehods appearing in the superclass!
Any Animal member defined in the constructor, will not be available when doing new Cat(). E.g. suppose the Animal constructor was:
function Animal(birthDate) {
this.birthDate = birthDate;
}
The Cat of the former method will contain the birthDate property, the second method's Cat will not.

In the previous code with Cat.prototype = new Animal you are invoking the Animal constructor function var Animal = function() {}; which is going to create a new object, run that constructor function and any code within it, assign this to the new object, the prototype property is attached to the constructor function, and return the new object. The use of new looks familiar to how you make a 'class' with classical inheritance but Javascript uses PROTOTYPAL Inheritence to make new objects that inherit from other objects.
By doing Cat.protoype = Animal.prototype you are not doing what was previously mentioned you are assigning the prototypes to each other so if you add something to one it will affect the other.

Why is it necessary to set the prototype constructor?

In the section about inheritance in the MDN article Introduction to Object Oriented Javascript, I noticed they set the prototype.constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
Does this serve any important purpose? Is it okay to omit it?

It's not always necessary, but it does have its uses. Suppose we wanted to make a copy method on the base Person class. Like this:
// define the Person Class
function Person(name) {
this.name = name;
}
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
// define the Student class
function Student(name) {
Person.call(this, name);
}
// inherit Person
Student.prototype = Object.create(Person.prototype);
Now what happens when we create a new Student and copy it?
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => false
The copy is not an instance of Student. This is because (without explicit checks), we'd have no way to return a Student copy from the "base" class. We can only return a Person. However, if we had reset the constructor:
// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;
...then everything works as expected:
var student1 = new Student("trinth");
console.log(student1.copy() instanceof Student); // => true

TLDR; Not super necessary, but will probably help in the long run, and it is more accurate to do so.
NOTE: Much edited as my previous answer was confusingly written and had some errors that I missed in my rush to answer. Thanks to those who pointed out some egregious errors.
Basically, it's to wire subclassing up correctly in Javascript. When we subclass, we have to do some funky things to make sure that the prototypal delegation works correctly, including overwriting a prototype object. Overwriting a prototype object includes the constructor, so we then need to fix the reference.
Let's quickly go through how 'classes' in ES5 work.
Let's say you have a constructor function and its prototype:
//Constructor Function
var Person = function(name, age) {
this.name = name;
this.age = age;
}
//Prototype Object - shared between all instances of Person
Person.prototype = {
species: 'human',
}
When you call the constructor to instantiate, say Adam:
// instantiate using the 'new' keyword
var adam = new Person('Adam', 19);
The new keyword invoked with 'Person' basically will run the Person constructor with a few additional lines of code:
function Person (name, age) {
// This additional line is automatically added by the keyword 'new'
// it sets up the relationship between the instance and the prototype object
// So that the instance will delegate to the Prototype object
this = Object.create(Person.prototype);
this.name = name;
this.age = age;
return this;
}
/* So 'adam' will be an object that looks like this:
* {
* name: 'Adam',
* age: 19
* }
*/
If we console.log(adam.species), the lookup will fail at the adam instance, and look up the prototypal chain to its .prototype, which is Person.prototype - and Person.prototype has a .species property, so the lookup will succeed at Person.prototype. It will then log 'human'.
Here, the Person.prototype.constructor will correctly point to Person.
So now the interesting part, the so-called 'subclassing'. If we want to create a Student class, that is a subclass of the Person class with some additional changes, we'll need to make sure that the Student.prototype.constructor points to Student for accuracy.
It doesn't do this by itself. When you subclass, the code looks like this:
var Student = function(name, age, school) {
// Calls the 'super' class, as every student is an instance of a Person
Person.call(this, name, age);
// This is what makes the Student instances different
this.school = school
}
var eve = new Student('Eve', 20, 'UCSF');
console.log(Student.prototype); // this will be an empty object: {}
Calling new Student() here would return an object with all of the properties we want. Here, if we check eve instanceof Person, it would return false. If we try to access eve.species, it would return undefined.
In other words, we need to wire up the delegation so that eve instanceof Person returns true and so that instances of Student delegate correctly to Student.prototype, and then Person.prototype.
BUT since we're calling it with the new keyword, remember what that invocation adds? It would call Object.create(Student.prototype), which is how we set up that delegational relationship between Student and Student.prototype. Note that right now, Student.prototype is empty. So looking up .species an instance of Student would fail as it delegates to only Student.prototype, and the .species property doesn't exist on Student.prototype.
When we do assign Student.prototype to Object.create(Person.prototype), Student.prototype itself then delegates to Person.prototype, and looking up eve.species will return human as we expect. Presumably we would want it to inherit from Student.prototype AND Person.prototype. So we need to fix all of that.
/* This sets up the prototypal delegation correctly
*so that if a lookup fails on Student.prototype, it would delegate to Person's .prototype
*This also allows us to add more things to Student.prototype
*that Person.prototype may not have
*So now a failed lookup on an instance of Student
*will first look at Student.prototype,
*and failing that, go to Person.prototype (and failing /that/, where do we think it'll go?)
*/
Student.prototype = Object.create(Person.prototype);
Now the delegation works, but we're overwriting Student.prototype with an of Person.prototype. So if we call Student.prototype.constructor, it would point to Person instead of Student. This is why we need to fix it.
// Now we fix what the .constructor property is pointing to
Student.prototype.constructor = Student
// If we check instanceof here
console.log(eve instanceof Person) // true
In ES5, our constructor property is a reference that refers to a function that we've written with the intent to be a 'constructor'. Aside from what the new keyword gives us, the constructor is otherwise a 'plain' function.
In ES6, the constructor is now built into the way we write classes - as in, it's provided as a method when we declare a class. This is simply syntactic sugar but it does accord us some conveniences like access to a super when we are extending an existing class. So we would write the above code like this:
class Person {
// constructor function here
constructor(name, age) {
this.name = name;
this.age = age;
}
// static getter instead of a static property
static get species() {
return 'human';
}
}
class Student extends Person {
constructor(name, age, school) {
// calling the superclass constructor
super(name, age);
this.school = school;
}
}

I'd disagree. It isn't necessary to set the prototype. Take that exact same code but remove the prototype.constructor line. Does anything change? No. Now, make the following changes:
Person = function () {
this.favoriteColor = 'black';
}
Student = function () {
Person.call(this);
this.favoriteColor = 'blue';
}
and at the end of the test code...
alert(student1.favoriteColor);
The color will be blue.
A change to the prototype.constructor, in my experience, doesn't do much unless you're doing very specific, very complicated things that probably aren't good practice anyway :)
Edit:
After poking around the web for a bit and doing some experimentation, it looks like people set the constructor so that it 'looks' like the thing that is being constructed with 'new'. I guess I would argue that the problem with this is that javascript is a prototype language - there is no such thing as inheritence. But most programmers come from a background of programming that pushes inheritence as 'the way'. So we come up with all sorts of things to try and make this prototypical language a 'classic' language.. such as extending 'classes'. Really, in the example they gave, a new student is a person - it isn't 'extending' from another student.. the student is all about the person, and whatever the person is the student is as well. Extend the student, and whatever you've extended is a student at heart, but is customized to fit your needs.
Crockford is a bit crazy and overzealous, but do some serious reading on some of the stuff that he's written.. it'll make you look at this stuff very differently.

This has the huge pitfall that if you wrote
Student.prototype.constructor = Student;
but then if there was a Teacher whose prototype was also Person and you wrote
Teacher.prototype.constructor = Teacher;
then the Student constructor is now Teacher!
Edit:
You can avoid this by ensuring that you had set the Student and Teacher prototypes using new instances of the Person class created using Object.create, as in the Mozilla example.
Student.prototype = Object.create(Person.prototype);
Teacher.prototype = Object.create(Person.prototype);

So far confusion is still there.
Following the original example, as you have an existing object student1 as:
var student1 = new Student("Janet", "Applied Physics");
Suppose you don't want to know how student1 is created, you just want another object like it, you can use the constructor property of student1 like:
var student2 = new student1.constructor("Mark", "Object-Oriented JavaScript");
Here it will fail to get the properties from Student if the constructor property is not set. Rather it will create a Person object.

Got a nice code example of why it is really necessary to set the prototype constructor..
function CarFactory(name){
this.name=name;
}
CarFactory.prototype.CreateNewCar = function(){
return new this.constructor("New Car "+ this.name);
}
CarFactory.prototype.toString=function(){
return 'Car Factory ' + this.name;
}
AudiFactory.prototype = new CarFactory(); // Here's where the inheritance occurs
AudiFactory.prototype.constructor=AudiFactory; // Otherwise instances of Audi would have a constructor of Car
function AudiFactory(name){
this.name=name;
}
AudiFactory.prototype.toString=function(){
return 'Audi Factory ' + this.name;
}
var myAudiFactory = new AudiFactory('');
alert('Hay your new ' + myAudiFactory + ' is ready.. Start Producing new audi cars !!! ');
var newCar = myAudiFactory.CreateNewCar(); // calls a method inherited from CarFactory
alert(newCar);
/*
Without resetting prototype constructor back to instance, new cars will not come from New Audi factory, Instead it will come from car factory ( base class ).. Dont we want our new car from Audi factory ????
*/

No need for sugared function 'classes' or using 'New' these days. Use object literals.
The Object prototype is already a 'class'. When you define an object literal, it is already an instance of the prototype Object. These can also act as another object's prototype, etc.
const Person = {
name: '[Person.name]',
greeting: function() {
console.log( `My name is ${ this.name || '[Name not assigned]' }` );
}
};
// Person.greeting = function() {...} // or define outside the obj if you must
// Object.create version
const john = Object.create( Person );
john.name = 'John';
console.log( john.name ); // John
john.greeting(); // My name is John
// Define new greeting method
john.greeting = function() {
console.log( `Hi, my name is ${ this.name }` )
};
john.greeting(); // Hi, my name is John
// Object.assign version
const jane = Object.assign( Person, { name: 'Jane' } );
console.log( jane.name ); // Jane
// Original greeting
jane.greeting(); // My name is Jane
// Original Person obj is unaffected
console.log( Person.name ); // [Person.name]
console.log( Person.greeting() ); // My name is [Person.name]
This is worth a read:
Class-based object-oriented languages, such as Java and C++, are
founded on the concept of two distinct entities: classes and
instances.
...
A prototype-based language, such as JavaScript, does not make this
distinction: it simply has objects. A prototype-based language has the
notion of a prototypical object, an object used as a template from
which to get the initial properties for a new object. Any object can
specify its own properties, either when you create it or at run time.
In addition, any object can be associated as the prototype for another
object, allowing the second object to share the first object's
properties

It is necessary when you need an alternative to toString without monkeypatching:
//Local
foo = [];
foo.toUpperCase = String(foo).toUpperCase;
foo.push("a");
foo.toUpperCase();
//Global
foo = [];
window.toUpperCase = function (obj) {return String(obj).toUpperCase();}
foo.push("a");
toUpperCase(foo);
//Prototype
foo = [];
Array.prototype.toUpperCase = String.prototype.toUpperCase;
foo.push("a");
foo.toUpperCase();
//toString alternative via Prototype constructor
foo = [];
Array.prototype.constructor = String.prototype.toUpperCase;
foo.push("a,b");
foo.constructor();
//toString override
var foo = [];
foo.push("a");
var bar = String(foo);
foo.toString = function() { return bar.toUpperCase(); }
foo.toString();
//Object prototype as a function
Math.prototype = function(char){return Math.prototype[char]};
Math.prototype.constructor = function()
{
var i = 0, unicode = {}, zero_padding = "0000", max = 9999;
while (i < max)
{
Math.prototype[String.fromCharCode(parseInt(i, 16))] = ("u" + zero_padding + i).substr(-4);
i = i + 1;
}
}
Math.prototype.constructor();
console.log(Math.prototype("a") );
console.log(Math.prototype["a"] );
console.log(Math.prototype("a") === Math.prototype["a"]);

EDIT, I was actually wrong. Commenting the line out doesn't change it's behavior at all. (I tested it)
Yes, it is necessary. When you do
Student.prototype = new Person();
Student.prototype.constructor becomes Person. Therefore, calling Student() would return an object created by Person. If you then do
Student.prototype.constructor = Student;
Student.prototype.constructor is reset back to Student. Now when you call Student() it executes Student, which calls the parent constructor Parent(), it returns the correctly inherited object. If you didn't reset Student.prototype.constructor before calling it you would get an object that would not have any of the properties set in Student().

Given simple constructor function:
function Person(){
this.name = 'test';
}
console.log(Person.prototype.constructor) // function Person(){...}
Person.prototype = { //constructor in this case is Object
sayName: function(){
return this.name;
}
}
var person = new Person();
console.log(person instanceof Person); //true
console.log(person.sayName()); //test
console.log(Person.prototype.constructor) // function Object(){...}
By default (from the specification https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor), all prototypes automatically get a property called constructor that points back to the function on which it is a property.
Depending on the constructor, other properties and methods might be added to the prototype which is not a very common practice but still it is allowed for extensions.
So simply answering: we need make sure that the value in prototype.constructor is correctly set as it is supposed by the specification to be.
Do we have to always set correctly this value? It helps with debugging and makes internal structure consistent against specification. We should definitely when our API is being used by the thirdparties, but not really when the code is finally executed in the runtime.

Here's one example from MDN which I found very helpful to understand its uses.
In JavaScript, we have async functions which returns AsyncFunction object. AsyncFunction is not a global object but one may retrieve it by using constructor property and utilize it.
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
// AsyncFunction constructor
var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor
var a = new AsyncFunction('a',
'b',
'return await resolveAfter2Seconds(a) + await resolveAfter2Seconds(b);');
a(10, 20).then(v => {
console.log(v); // prints 30 after 4 seconds
});

It is necessary. Any class in class inheritance must has its own constructor, so as in prototype inheritance.It is also convenient for object construction. But the question is unnecessary and what is necessary is understanding in JavaScript world effect of calling function as constructor and rule of resolving object property.
Effect of executing function as constructor with expression new <function name>( [ parameters] )
a object whose type name is the function name is created
inner properties in the function attaches to the created object
property prototype of the function attaches automatically to the created object as prototype
Rule of resolving property of object
The property will not only be sought on the object but on the prototype of the object, the prototype of the prototype, and so on until either a property with a matching name is found or the end of the prototype chain is reached.
Basing on these underlying mechanisms, statement <constructor name>.prototype.constructor = <constructor name> equals in term of effect to attach constructor in constructor body with expression this.constructor = <constructor name>. The constructor will be resolved on the object if second utterance while on object's prototype if first utterance.

It is not necessary. It is just one of the many things traditional, OOP champions do to try to turn JavaScript's prototypical inheritance into classical inheritance. The only thing that the following
Student.prototype.constructor = Student;
does, is that you now have a reference of the current "constructor".
In Wayne's answer, that has been marked as correct, you could the exact same thing that the following code does
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new this.constructor(this.name);
};
with the code below (just replace this.constructor with Person)
Person.prototype.copy = function() {
// return new Person(this.name); // just as bad
return new Person(this.name);
};
Thank God that with ES6 classical inheritance purists can use language's native operators like class, extends and super and we don't have to see like prototype.constructor corrections and parent refereces.

Develop Reference

JavaScript is the programming language of the Web.