Type of Undeclared Variable in JavaScript: What is it?


Have you ever thought, what is type of undeclared variable in JavaScript? I know, the first thing that might come to mind is: how can an undeclared variable have a type? Yes, in JavaScript it is possible.

To understand it, let us start with understanding types in JavaScript. There are seven built in types in JavaScript. They are as follows:

  1. null
  2. undefined
  3. boolean
  4. number
  5. string
  6. object
  7. symbol (added on ES6)

Each variable with assigned value has a type. Let us consider the code listed below:

As you can see in the above snippet, if there is no value assigned then type of variable is undefined.

So far so good, we saw that variable with no assigned value is undefined.  Let us consider the next code snippet:

We have created a variable koo and have not assigned any value to it.  Now both value and type of koo are set to undefined.

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Video – Arrow Function in JavaScript

Arrow function was introduced in ECMA 6.  It is different than function statement or expression , as it does not have its own ‘this’ , and can also not be used as constructor.  Some important points about Arrow function are:

  • It does not have prototype property
  • It does not have arguments parameter
  • It does not have its own ‘this’
  • It can not be used with new operator

I have written a detailed article on Arrow function here :


Watch this video to learn more about Arrow function


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Simplifying Object.assign method in JavaScript

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“In JavaScript, Object’s assign method copies source object’s own enumerable properties to a target object, and returns that target object “

There are two important keywords in the above sentence:

  1. Enumerable
  2. Own

Before we go ahead and understand purpose of Object.assign, it is essential that we really understand these two words, enumerable properties and own properties.  Let us see them one by one:

A JavaScript object could have either enumerable or non-enumerable properties. However, by default when you create property on an object, it is enumerable.  Enumerable  means you can enumerate through those properties. Let us understand it through code.



There are two properties in babycat object. By default both are enumerable, hence as the output of for..in loop, you will get both age and name printed.

Now let us change default enumerable behavior of age property using Object.defineProperty method.


We have changed enumerable of age property to false, so as output, you will get only color printed. Hence, age is no longer an enumerable property of babycat object.

Another keyword in above sentence is own properties. To understand it, you need to understand object prototype chain. All JavaScript objects are part of a prototype chain, hence can access properties of its prototype’s also.  So, own properties are those properties, which are particular to the object and not from the prototype chain.  Let us understand own properties through code examples,

In above code snippet, there are two objects cat and babycat. In addition, [[Prototype]] property of babycat object is set to cat object. When you print properties of the babycat object using for..in loop, you will find as output age, color, name printed as shown in below image:

What is happening here? Well, JavaScript prints all properties from the prototype chain. However, only age and color are, own properties of babycat object.

As of now, you should have understood own properties and enumerable properties in context of a JavaScript object.   So let us revisit first statement of this post,

“In JavaScript, Object’s assign method copies source object’s own enumerable properties to a target object, and returns that target object “

You should able to infer what exactly above sentence implies.  Consider code below :






Using Object.assign() method , we are copying cat object own enumerable properties to babycat object. Here, cat object is source and babycat object is target. You will get output printed as below:

The Object.assign() method uses [[Get]] on the source object and [[set]] on the target object and invokes setter and getter to perform the task. Essentially, it assigns properties values from source to target object. It does not create new property in the target object.

As of now you know purpose of the Object.assign() method. Let us examine some variations while copying properties from source object to a target object.

Same Properties in both target and source object

If target object has same properties as of source object, then Object.assign() method will  override target object properties.  To understand it consider code listing below:





There is age property in both target object and source object.  While copying properties in target object the Object.assign() method will override target object age property, hence you will get output as shown in the below image :

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Video – JavaScript object instances does not copy definition from the class

I recently wrote a blog post about Two Problems of a JavaScript Class. In my opinion one gotchas in a JavaScript class is , object instances does not copy definition from the class.

Watch this video to understand it through codes.  f you like the video , do not forget to subscribe to our YouTube Channel for notification about future video tutorials.


Two Problems of a JavaScript Class


Starting ECMA 6, JavaScript has class keyword to create a class. I have written in detail about class here. There is no second question that class simplifies the way object is created, inheritance is implemented etc. JavaScript class has,

  • Constructors
  • Methods
  • Extends etc.

Above features of a class helps to easily write Object Oriented JavaScript. As a developer, you do not need to know complexities of prototype chains, relationship between function constructor and its prototype object, and value of object’s __proto__ properties etc. to write effective Object Oriented JavaScript.  So, class keyword  is good addition to JavaScript language, however it is not gospel perfect. It has some problems, which may restrict you to write full-fledged Object Oriented JavaScript. In this post, I am going to share two such problems. Read along,

No Static Member Properties in class

A static member property is shared by all object instance of the class. JavaScript class does not allow creating it inside the class.

You cannot declare properties directly in the class. You can only have it through class’s constructors, and properties created inside constructor is local to the object instances and not shared by all of them.

Above code will throw error “unexpected identifier”.  There is a work around to create static property using the class prototype.

Now on the instances of Speaker class, you can access static property count.

Therefore, you are able to create a static property, but not without help of understanding of prototype.  In my opinion class should have a way to create static property directly inside the class like a method or constructor.

Object instances does not copy definitions from class

To understand this problem, let us first revise Constructor Invocation Pattern and prototype chain. You have a function constructor Speaker.

Using new operator, you can create object instances,

In this approach, a and b object instances both have their own copy of hello method.  Now if you add a hello method to Speaker.prototype, still a and b object instances will access their own copy of hello method. Consider below code:

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How to add a static member property in a JavaScript class?

Recently while solving a problem, I came across a requirement to create a property in a JavaScript class, which needs to be shared by all the object instances. In the programming world, these types of properties are called Static Properties.

There are various scenarios when you need a static member property:

  • When counting number of object instances created from a particular class
  • When logging some information at the class level instead of object instance level, etc.

To create a static property, you should know two important things:

  1. A JavaScript class cannot have a member property. You can only create it using a constructor
  2. Like the function constructor, a JavaScript class also has a prototype

Well, if you are a champion of prototypes, you must have guessed the answer in your mind by this time.  Any way, let us move forward to see the implementation,

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How to Create Basic Inheritance in JavaScript Constructors

There are four ways to create an object in JavaScript. They are as follows:

  1. Object as literal
  2. Constructor Invocation Pattern
  3. Object.create() method
  4. Using class after ES6

Implementation of Inheritance varies according to the object creation method. In this post, I am going to explain creating inheritance in between a function constructor.

Let’s say you have a function:

If you call the animal function using new operator, an object will be created. This way of object creation is also known as “Constructor Invocation Pattern

Object dog and cat both have their own names and age properties. If you want a property or method to be shared across all objects, add that to the prototype of the function.

Using the JavaScript prototype chain, both dog and cat objects can access the canRun method.

Next, let us create another constructor – human:

At this point in time, human and animal functions do not have any relationship. However, we know that human is also an animal. There are two problems with the human constructor.

  1. It has duplicate codes for name and age initialization. It should use animal constructor for this purpose.
  2. It does not have any link with animal constructor

The above said two problems can be removed by creating inheritance in between animal and human function constructors.

You can solve problem 1 of code duplication by modifying the human function as below:

Now, in human function, we are using the call method to manually pass a current object as a value of ‘this’ in animal function. This approach is also called Indirect Invocation Pattern. Now, an object instance for human can be created as shown below:

So far, we have solved problem 1 of code duplication; however, human function is still not linked to animal function. If you try to call canRun method on h1 object, JavaScript will throw you an error.

You can fix this problem by linking the prototype of the human function with the prototype of the animal function constructor. There are two ways to do that.

  1. Using __proto__
  2. Using Object.create() method

You can link prototype of function constructors using Object.create() as shown below:

You can link prototype of function constructors using __proto__ as shown below:

I would prefer the Object.create() method because__proto__ may not be supported in many browsers. After linking prototypes, in one way, you have created inheritance in between animal and human function constructors. Object instance of human can read all properties of animal function and can execute animal function methods.

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