Javascript Prototype

 

what is javascript prototype ?

In JavaScript, a prototype is an object from which other objects inherit properties and methods. When you create an object in JavaScript, it is assigned a prototype, which is used to look up properties and methods that are not defined on the object itself.

The prototype is essentially a template for an object, and it can be used to define default values for properties and methods that all objects created from it will have. By modifying the prototype, you can add new properties and methods to all objects created from it.

In JavaScript, each object has a special property called [[Prototype]] (also called __proto__), which points to its prototype. When you try to access a property or method on an object that is not defined on the object itself, JavaScript will look up the prototype chain until it finds the property or method.

For example, consider the following code:

function Person(name) {

  this.name = name;

}

Person.prototype.greet = function() {

  console.log("Hello, my name is " + this.name);

};

var alex = new Person("Alex");

john.greet(); // logs "Hello, my name is Alex"

In this example, we define a Person constructor function that takes a name parameter and sets it as a property of the object created by the function. We also add a greet method to the prototype of the Person constructor function.

When we create a new object using the Person constructor function (var alex = new Person("Alex");), the [[Prototype]] property of the object will be set to the Person.prototype object. This means that the greet method is available on the alex object, even though it is not defined on the object itself.

By using prototypes, we can create objects that share common functionality without having to duplicate code for each object.

Note:

In the given code, we have three main layers:

Constructor Function: Person(name) - It is a function used to create new objects. It takes a name parameter and sets it as a property of the object created by the function. In this case, it creates a new Person object with a name property.

Prototype Object: Person.prototype - It is an object that defines the properties and methods that will be inherited by all instances of the Person object. In this case, it defines a greet method that logs a message to the console.

Object Instance: alex - It is an instance of the Person object created using the new operator. It has its own name property, which was set using the name parameter passed to the Person constructor function. The [[Prototype]] property of alex points to the Person.prototype object, so it can access the greet method defined on the prototype.

So, to summarize, the constructor function and prototype object are used to define the behavior of the object, while the object instance is used to store the data specific to that instance of the object.

Important Note:

in the constructor function, you can define as many parameters as needed to initialize the object with different values. In the example code you provided, the Person constructor function only takes one parameter (name) and sets it as a property of the object created by the function. However, you can add more parameters to the constructor function to define additional properties of the object.

For example, let's say we want to add an age property to the Person object. We can modify the constructor function as follows:

function Person(name, age) {

  this.name = name;

  this.age = age;

}

Person.prototype.greet = function() {

  console.log("Hello, my name is " + this.name + " and I am " + this.age + " years old.");

};

var john = new Person("John", 30);

john.greet(); // logs "Hello, my name is John and I am 30 years old."

In this modified code, we added an age parameter to the Person constructor function and set it as a property of the object created by the function. We also updated the greet method to include the age property in the message logged to the console. When we create a new Person object (var john = new Person("John", 30);), we pass both a name and an age as arguments to the constructor function, and the resulting john object has both a name and an age property.

What is prototype object?

The prototype object in JavaScript is an object that is inherited by every object. It works as a template, allowing objects to have a set of common properties and methods.

Every JavaScript object has a prototype reference, which is also referred to as __proto__. This reference is part of the prototype chain, where an object inherits properties and methods from its prototype. This chain goes up to Object.prototype.

By using prototype objects, we can define a set of common functionalities in one place, which helps to avoid code duplication. Additionally, by modifying the prototype object, we can change the behavior of an object and apply those changes to all instances of that object.

Here's an example of using a prototype object to define common functionality:

function Person(name) {

  this.name = name;

}

Person.prototype.greet = function() {

  console.log("Hello, my name is " + this.name);

};

var john = new Person("John");

var alex = new Person("Alex");

john.greet(); // logs "Hello, my name is John"

alex.greet(); // logs "Hello, my name is Alex"

In this example, we have defined the greet method in the prototype object of the Person constructor function. The greet method is common for every instance of Person, and we accessed it through the john and alex objects.

What is __proto__ in javascript ?

__proto__ is a reference in JavaScript that points to the prototype of an object. It is a non-standard property and is not recommended to use in production code. Instead, we can use the Object.getPrototypeOf() method or the Object.setPrototypeOf() method to get or set an object's prototype.

In JavaScript, every object has a prototype, and this prototype is shared among all instances created by the same constructor function. This is known as the prototype chain. When a property or method is called on an object, JavaScript first looks for that property or method in the object itself. If it doesn't find it, it then looks for it in the object's prototype. If it doesn't find it there, it keeps going up the prototype chain until it reaches the end, which is the Object.prototype object.

Using the prototype chain, we can define a set of common properties and methods for all instances of an object. We can modify an object's prototype to add or remove properties or methods, which will affect all instances of that object.

Here is an example of using the __proto__ reference to access an object's prototype:

function Person(name) {

  this.name = name;

}

Person.prototype.greet = function() {

  console.log("Hello, my name is " + this.name);

};

var john = new Person("John");

console.log(john.__proto__ === Person.prototype); // logs true

In this example, we are accessing the __proto__ reference of the john object, which points to the Person.prototype object. We then compare this reference to Person.prototype to confirm that they are the same object

here's another example that demonstrates how the __proto__ reference works:

function Animal() {}

Animal.prototype.makeSound = function() {

  console.log("This animal makes a sound.");

};

function Dog() {}

Dog.prototype = Object.create(Animal.prototype);

Dog.prototype.constructor = Dog;

Dog.prototype.makeSound = function() {

  console.log("Woof!");

};

var myDog = new Dog();

myDog.makeSound(); // logs "Woof!"

console.log(myDog.__proto__ === Dog.prototype); // logs true

console.log(Dog.prototype.__proto__ === Animal.prototype); // logs true

console.log(Animal.prototype.__proto__ === Object.prototype); // logs true

In this example, we have defined an Animal constructor function and a Dog constructor function. We have also defined a makeSound method on the Animal.prototype object, which is inherited by every instance of Animal and Dog.

To create a Dog object that inherits from the Animal prototype, we use Object.create(Animal.prototype) to set the Dog.prototype object's prototype to Animal.prototype. We also set the Dog.prototype.constructor property to Dog to ensure that it correctly identifies itself as a Dog.

We then define a new makeSound method on the Dog.prototype object, which overrides the makeSound method inherited from Animal.prototype.

Finally, we create a new myDog object using the Dog constructor function and call its makeSound method. This logs "Woof!" to the console, demonstrating that the Dog prototype's makeSound method was called.

We also use __proto__ to check the prototype chain and confirm that myDog.__proto__ points to Dog.prototype, Dog.prototype.__proto__ points to Animal.prototype, and Animal.prototype.__proto__ points to Object.prototype.

What is javascript prototype inheritance?

Prototype inheritance is a way of sharing properties and methods between objects in JavaScript. When an object is created in JavaScript, it is assigned a prototype. The prototype is an object that provides a set of default properties and methods that the object can inherit.

In JavaScript, inheritance is achieved through the prototype chain. Each object has a reference to its prototype, and if a property or method is not found in the object itself, JavaScript looks for it in the object's prototype. If it's not found there, it continues up the prototype chain until it reaches the Object.prototype object, which is the top of the chain.

To demonstrate prototype inheritance, let's use an example:

function Person(name, age) {

  this.name = name;

  this.age = age;

}

Person.prototype.greet = function() {

  console.log("Hello, my name is " + this.name);

};

function Employee(name, age, salary) {

  Person.call(this, name, age);

  this.salary = salary;

}

Employee.prototype = Object.create(Person.prototype);

Employee.prototype.constructor = Employee;

Employee.prototype.work = function() {

  console.log("Working...");

};

var john = new Employee("John", 35, 50000);

john.greet(); // logs "Hello, my name is John"

john.work(); // logs "Working..."

In this example, we have defined a Person constructor function that takes a name and age parameter and a greet method. We have also defined an Employee constructor function that extends Person and takes a salary parameter and a work method.

To inherit the Person properties and methods in Employee, we use Object.create(Person.prototype) to set the Employee.prototype object's prototype to Person.prototype. We also set the Employee.prototype.constructor property to Employee to ensure that it correctly identifies itself as an Employee.

We then call the Person constructor function in the Employee constructor function using Person.call(this, name, age) to set the name and age properties on the Employee object.

Finally, we create a new john object using the Employee constructor function and call its greet and work methods. This logs "Hello, my name is John" and "Working..." to the console, demonstrating that the Employee object has inherited the Person properties and methods.

Most important Question: What if this prototype didn't exist?

If the prototype object does not exist for a constructor function in JavaScript, it means that instances created from that constructor function will not inherit any properties or methods. In other words, the instances will not have access to any shared properties or methods, and each instance will have its own set of properties and methods.

For example, let's consider the following code without using the prototype object:

function Person(name, age) {

  this.name = name;

  this.age = age;

  

  this.greet = function() {

    console.log("Hello, my name is " + this.name);

  };

}

var john = new Person("John", 35);

var jane = new Person("Jane", 28);

console.log(john.greet === jane.greet); // logs false

In this example, we have defined a Person constructor function that creates a new greet function for each instance of the Person object. When we create new instances of Person using the new keyword, each instance gets its own greet function.

Because each instance has its own greet function, the john.greet === jane.greet expression logs false. This means that the greet function is not shared between the john and jane objects.

If we had used the prototype object instead, the greet function would be shared between all instances of the Person object, resulting in the expression john.greet === jane.greet logging true.

What is prototype chaining?

Prototype chaining is the mechanism by which objects in JavaScript inherit properties and methods from their parent objects through the prototype chain.

In JavaScript, every object has a prototype property that refers to its parent object. When a property or method is accessed on an object, JavaScript first looks for the property or method on the object itself. If the property or method is not found on the object, JavaScript then looks for it on the object's prototype, and continues up the prototype chain until the property or method is found or until the end of the chain is reached.

For example, let's consider the following code:

function Animal() {}

Animal.prototype.eat = function() {

  console.log("The animal is eating.");

};

function Dog() {}

Dog.prototype = Object.create(Animal.prototype);

Dog.prototype.bark = function() {

  console.log("The dog is barking.");

};

var myDog = new Dog();

myDog.bark(); // logs "The dog is barking."

myDog.eat(); // logs "The animal is eating."

In this example, we have defined an Animal constructor function that creates an eat method on its prototype. We have also defined a Dog constructor function that inherits from the Animal prototype using the Object.create() method.

When we create a new Dog object using the new keyword, the Dog object inherits the eat method from the Animal prototype through the prototype chain. We can then call the bark and eat methods on the myDog object.

This is an example of prototype chaining in action. By setting the Dog prototype to be an instance of the Animal prototype, we create a prototype chain that allows Dog objects to inherit properties and methods from the Animal prototype.

Use cases of javascript prototype

JavaScript prototypes have several real-life use cases that are crucial in modern web development. Here are some examples:

Object-oriented programming: Prototypes are an essential part of object-oriented programming (OOP) in JavaScript. In OOP, you can create objects with properties and methods that are shared across instances. By using prototypes, you can create a blueprint for an object and share its properties and methods across instances, thereby reducing the amount of code needed to create and maintain objects.

Code reusability: Prototypes allow you to reuse code across multiple instances of an object. For example, if you have multiple buttons on a web page, you can use a prototype to create a shared set of properties and methods for all buttons. This reduces the amount of code you need to write and makes it easier to maintain.

Inheritance: Prototypes are used in JavaScript for inheritance, which is the ability of an object to inherit properties and methods from a parent object. By using prototypes, you can create a hierarchy of objects that share common functionality, which makes it easier to maintain and modify your code.

Frameworks and libraries: JavaScript frameworks and libraries such as React and Angular rely heavily on prototypes. By using prototypes, these libraries can create efficient and reusable components that are easy to maintain and modify.

Performance: Prototypes can improve the performance of your code by reducing the amount of memory used by your program. By sharing properties and methods across instances of an object, you can reduce the amount of memory needed to store those properties and methods.

In summary, JavaScript prototypes have a wide range of real-life use cases that are crucial in modern web development. They are used for object-oriented programming, code reusability, inheritance, frameworks and libraries, and performance optimization.