JavaScript — A Brush up on Prototypal Inheritance

Vilva Athiban P B
4 min readOct 14, 2018


Prototypal Inheritance is still a not much explored space in JavaScript by many developers and holds a important place in the learning curve of the language. This article will be a brush up on this space covering various topics on high level.

Topics to be covered:

  1. Prototype chain
  2. Prototype Delegation through New Keyword
  3. For in Loop with Prototype
  4. Static methods
  5. Instanceof Operator
  6. defineProperty
  7. OLOO (Object Linking to Other Objects)

1 — Prototype Chain:

Prototype chaining is used to build new types of objects based on existing ones. Constructor functions have a property called prototype. Adding properties and methods to the prototype property will automatically add the method or property to all objects created by the constructor function. These prototype method or properties are visible under __proto__ object found in every object.

Lets see an example of how to inherit properties using prototype chaining.

In the above example, we have create two objects namely a, b. Each of the above two objects has its own property and own prototype methods. Now we have used Object.setPrototypeOf method to replace prototype of object — ‘a’ with properties of object — ‘b’ along with its default prototype methods. Both properties from ‘a’ as well ‘b’ are retained in object ‘a’ and hence inheritance is achieved through prototype chaining.

2-Prototype Delegation through New Keyword:

Prototype Delegation is an other interesting topic in the world of Javascript. When a class/function (es5) is instantiated using new keyword, the following three things happen:

  1. A new object is created out of the class
  2. Prototype is linked to the object
  3. ‘this’ binding for the instance is made.

Refer to the below example:

A class named Car is created. When it is instantiated using the new keyword, a new object is created (newCar), prototype is linked to the object (`__proto__`) and is bound with “BMW”. This explains Prototypal delegation using new keyword

3- For in Loop with Prototype:

Unlike other loops, the for in loop doesn't just iterate only through the properties of the object but also the prototype properties of the object. Though it ignores repeated properties, it comes handy when we have to operate on prototype properties as well.

Lets examine through the following example:

Lets consider an object Foo, which has properties name, firstName and a prototype property lastName. When we run the for in loop through Foo, ideally we should get 2 iterations, but the image below proves that its 3 iterations including the prototype property.

We can still restrict for in loop from looping through prototype properties by having a check for hasOwnProperty as below:

Using HasOwnProperty

4- Static Methods:

In ES6, the methods we create in a class is always linked to prototype and hence, they cant be called directly and always needs an instance of the class to be called. We have created a class Bar, which has a method printHi. When the method is tried to be accessed directly, it throws an error, as the method is linked to prototype and will not be available for direct access.

In order to execute such methods directly, use Static keyword during declaration of the method and it does the magic as below:

To avoid making this post further longer, I am splitting it into two parts. The remaining four topics can be found here in the continuation.

Thanks for reading through pretty long article.



Vilva Athiban P B

6+ years of Experience in React-Redux / NodeJS / GraphQL / TypeScript.