Prototype inheritance in Javascript

Introduction

Prototype inheritance lets one object use properties and actions from another object. It helps share features without copying them.

When you want many objects to share the same behavior without repeating code.

When creating objects that are variations of a base object, like different types of animals sharing common actions.

When you want to add new features to existing objects without changing them directly.

Syntax

Javascript

function Parent() {
  this.property = 'value';
}

Parent.prototype.method = function() {
  // code here
};

function Child() {
  Parent.call(this); // inherit properties
}

Child.prototype = Object.create(Parent.prototype);
Child.prototype.constructor = Child;

Parent.call(this) runs the parent function to copy properties to the child.

Object.create sets up the child to inherit methods from the parent's prototype.

Examples

This example shows a Dog inheriting from Animal. Dog changes the speak method.

Javascript

function Animal(name) {
  this.name = name;
}

Animal.prototype.speak = function() {
  console.log(this.name + ' makes a noise.');
};

function Dog(name) {
  Animal.call(this, name);
}

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

Dog.prototype.speak = function() {
  console.log(this.name + ' barks.');
};

Here, child inherits greet from parent using Object.create.

Javascript

const parent = { greet() { return 'Hello'; } };
const child = Object.create(parent);
console.log(child.greet());

Sample Program

This program shows a Student inheriting from Person. Student can use introduce from Person and also has its own study method.

Javascript

function Person(name) {
  this.name = name;
}

Person.prototype.introduce = function() {
  console.log('Hi, I am ' + this.name);
};

function Student(name, subject) {
  Person.call(this, name);
  this.subject = subject;
}

Student.prototype = Object.create(Person.prototype);
Student.prototype.constructor = Student;

Student.prototype.study = function() {
  console.log(this.name + ' studies ' + this.subject);
};

const alice = new Student('Alice', 'Math');
alice.introduce();
alice.study();

Important Notes

Always reset the constructor property after setting the prototype to keep the right reference.

Prototype inheritance shares methods efficiently, so all objects use the same function in memory.

Summary

Prototype inheritance lets objects share properties and methods.

Use Object.create to link prototypes.

Call the parent function inside the child to copy properties.

Practice

1. What does prototype inheritance allow in JavaScript?

easy

A. Arrays to store only numbers

B. Functions to run faster

C. Variables to be private

D. Objects to share properties and methods

Solution

1. Step 1: Understand prototype inheritance concept

   Prototype inheritance allows one object to use properties and methods from another object.

2. Step 2: Match concept to options

   Only Objects to share properties and methods correctly describes sharing properties and methods between objects.

3. Final Answer:

   Objects to share properties and methods -> Option D

4. Quick Check:

   Prototype inheritance = share properties and methods [OK]

Hint: Prototype inheritance means sharing properties/methods [OK]

Common Mistakes:

• Thinking it speeds up functions
• Confusing with variable privacy
• Assuming it limits array types

2. Which syntax correctly creates an object that inherits from another object parentObj?

easy

A. const child = parentObj.create();

B. const child = new Object(parentObj);

C. const child = Object.create(parentObj);

D. const child = Object.inherit(parentObj);

Solution

1. Step 1: Recall correct syntax for prototype inheritance

   Use Object.create(parentObj) to create a new object inheriting from parentObj.

2. Step 2: Check options for correct method

   Only const child = Object.create(parentObj); uses Object.create correctly; others are invalid or incorrect.

3. Final Answer:

   const child = Object.create(parentObj); -> Option C

4. Quick Check:

   Object.create() creates inherited object [OK]

Hint: Use Object.create(parent) to inherit prototype [OK]

Common Mistakes:

• Using new Object(parentObj) which doesn't inherit
• Using non-existent Object.inherit method
• Calling create on parentObj instead of Object

3. What is the output of this code?

function Parent() { this.name = 'Parent'; }
Parent.prototype.greet = function() { return 'Hello from ' + this.name; };

function Child() { Parent.call(this); this.name = 'Child'; }
Child.prototype = Object.create(Parent.prototype);
Child.prototype.constructor = Child;

const c = new Child();
console.log(c.greet());

medium

A. 'Hello from Parent'

B. 'Hello from Child'

C. undefined

D. TypeError: c.greet is not a function

Solution

1. Step 1: Understand property assignment in Child constructor

   Child calls Parent constructor to set this.name, then overwrites it with 'Child'.

2. Step 2: Check prototype chain and method call

   Child's prototype inherits from Parent.prototype, so greet() is available and uses this.name which is 'Child'.

3. Final Answer:

   'Hello from Child' -> Option B

4. Quick Check:

   Child instance uses own name 'Child' in greet() [OK]

Hint: Child overwrites name, greet uses Child's name [OK]

Common Mistakes:

• Assuming greet uses Parent's name
• Forgetting to call Parent.call(this)
• Not resetting Child.prototype.constructor

4. Find the error in this code that tries to set up prototype inheritance:

function Animal() { this.alive = true; }
Animal.prototype.breathe = function() { return 'breathing'; };

function Dog() { Animal.call(this); this.bark = function() { return 'woof'; } }
Dog.prototype = Animal.prototype;

const d = new Dog();
console.log(d.breathe());

medium

A. Dog.prototype should be Object.create(Animal.prototype)

B. Animal.call(this) should be Dog.call(this)

C. Dog.prototype.bark should be defined outside constructor

D. No error, code works fine

Solution

1. Step 1: Identify prototype assignment mistake

   Assigning Dog.prototype = Animal.prototype links both constructors to same prototype object, causing shared mutations.

2. Step 2: Correct way to inherit prototype

   Use Dog.prototype = Object.create(Animal.prototype) to create a new object inheriting from Animal.prototype without sharing it directly.

3. Final Answer:

   Dog.prototype should be Object.create(Animal.prototype) -> Option A

4. Quick Check:

   Use Object.create for prototype inheritance [OK]

Hint: Use Object.create to avoid shared prototype object [OK]

Common Mistakes:

• Assigning prototype directly instead of using Object.create
• Confusing call() usage
• Defining methods inside constructor unnecessarily

5. You want to create a Car object that inherits from Vehicle and adds a drive method. Which code correctly sets this up and calls drive on a Car instance?

hard

A. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { Vehicle.call(this); }
Car.prototype = Object.create(Vehicle.prototype);
Car.prototype.constructor = Car;
Car.prototype.drive = function() { return 'driving'; };

const myCar = new Car();
console.log(myCar.drive());

B. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { this.wheels = 4; }
Car.prototype = Vehicle.prototype;
Car.prototype.drive = function() { return 'driving'; };

const myCar = new Car();
console.log(myCar.drive());

C. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { Vehicle.call(this); }
Car.prototype.drive = function() { return 'driving'; };

const myCar = new Car();
console.log(myCar.drive());

D. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { Vehicle.call(this); }
Car.prototype = Object.create(Vehicle.prototype);

const myCar = new Car();
console.log(myCar.drive());

Solution

1. Step 1: Set up inheritance correctly

   Use Vehicle.call(this) inside Car constructor to copy properties, then set Car.prototype = Object.create(Vehicle.prototype) to inherit methods.

2. Step 2: Add drive method and create instance

   Define drive on Car.prototype, then create instance and call drive().

3. Final Answer:

   Code in option A correctly sets inheritance and calls drive() -> Option A

4. Quick Check:

   Call parent, use Object.create, add method, then call [OK]

Hint: Call parent, Object.create prototype, add method, then use instance [OK]

Common Mistakes:

• Assigning prototype directly instead of Object.create
• Not calling Vehicle.call(this) in Car
• Forgetting to set constructor property
• Defining methods before setting prototype