Javascript Basic Key Concepts

In this blog I am going to discuss some important and basic key concepts of Java Script.

Let’s see them one by one;

In JavaScript we declare variables using var keyword; we may assign the variable or we may not.

Key Capsule 1

So what happens when we declared var name;

Capsule 1

Here name stores undefined by default;

iI we print using console.log(name)

o/p - undefined.

Key Capsule 2

  1. var name=”Surya”  
  2. var name;  
  3. console.log(name)  What you think what will be the out put ?

O/P - “Surya”

Capsule 2

In java script although I declare the variable once again it won’t lose the value.

Key Capsule 3

  1. var result=”sun”+2+3;  
  2. console.log(result)  
  3. result=2+3+”sun”;  
  4. console.log(result)  
What you think the output  is?

O/P - “sun”23


Capsule 3

In java script if any numeric values comesafter the string then the Plus (+) operator works as a concatenation operator whereas if the numeric value comes before the string then plus (+) operator works and it shows addition of number then it concatenates the string.

Key Capsule 4

  1. var studnetName=”Suryakant”;  
  2. var studentName=”Rajanikant” ;  
  3. console.log(studentName);  
  4. var studnetName=”Rahul”;  
  5. var studentname=”Raj”;  
  6. console.log(studnetName);  
  7. console.log(studentname);  

What will be the output?

O/P - “Rajanikant”



Capsule 5

Why so? Because the JavaScript variables are case sensitive; so when I write studnetName and studnetname both are different variables.

Remember all variables in JavaScript are called identifiers and each variable is an unique identifier.

Key Capsule 6

How to print a string like this,

“The name is “Surykant”; he is a blogger.”

I want to print suryakant with the double quot.

To do this in javascript we have different escape characters which can be use to achieve this.

We can write,

var data=”The name is \”Suryakant\”; he is a blogger”.

Same for single quto,

var data=”It\’s looking good.”

Most Important Key Capsule 7

Difference between == and === operator

“== “ operator is used to check directly the value where as “===” equality operator check the equality and types.

See bellow examples;

Test Case 1

var x=”surya”;

var y= “surya”;



o/p- true


Here the output is true for both cases because both are storing the same value and both are of the same type

Test Case 2

var x=”surya”

var y=new string (“surya”)


o/p- true


Here some of us might wonder why it returns true.

This question also came to my mind; because the second one is reference type; as we know inside reference type variable it stores the address of that value i.e “surya”.

So how are both equal then?

Here the important thing to understand is what actually happens when we comparing the value using double equality operator (“==”).

When we using double equality operator (“==”) to compare then this operator performs a type of conversion while comparing two things; so in the above example when we compare x and y, although y is an object type it is converted to string type while comparing so both are getting the  same type with value so it returns as true.

Whereas triple equality operator (“===”) never does any type of conversion while comparing; it only checks the type and value if both are the same then it returns true otherwise it returns false.

Key To Remember

So if we check any two reference/object type variable using triple equality operator (“===”) then it always returns false; same for double equality operator (“==”) also returns false; but if both are reference types pointing to the same location then “===” operator return true.

Confused ????

Let’s clear; both reference type/object type pointing to the same location means;

  1. var obj=new string(“surya”);  
  2. var obj1=obj  
  3. Now if we check; obj===obj then we get output as true.  

Capsule 7 

What we learn form the above?

Double equality operator(“==”) does conversion before checking while triple equal to operator(“===”) never does any conversion before checking it just checks the type and value.


  1. var a=1;  
  2. var b=”surya”;  
  3. var c=new string(“surya”);  
  4. Now; a===a // True ; b===b // True; c===c //True  

Again if i declare a new object type variable;

var d=new string(“surya”);

Now; c===d // False


Although ‘c’ and ‘d’ are the same type still it returns false because after checking the type it will check the value inside it; both are pointing to different locations so both have different values so it returns false.

if we write; c==d


Now what happened is, both are converted to reference type; and due to both pointing to different locations; so it returns false.

Key To Remember

Due to the above reason double equality operator (“==”) is called a loose equality operator whereas triple equality operator (“===”) is called strict equality operator.

Test Case 3-

var x=”surya”

var y=new string (“surya”)


o/p- false

Note - Here the output is false because “===” operator is used to check type and both have different types so it returns false.

Test Case 4-

var x=new string(“surya”);

var y=new string (“surya”);


o/p- false


o/p- false

What is value type and reference type in JavaScript ?

Ans- In java script data types are classified into two types such as value type and reference type.

Under value type we have data type like;

Int, string,number,Boolean,null and etc.

Where as under reference type we have data type like;


Combine this and as we said all these are object types.

Note:- Value types are directly stored values inside the variable whereas reference types are stored in the address of the object and that address points to the actual value location.

Key Capsule 8 :

In Java script we have different global methods for Numbers such as;

  • Number()
  • parseInt()
  • parseFloat()

Important Test Case-

Test Case 1 - parseInt(“10 Year”) //O/P- 10

Test Case 2 - parseInt(“Year 10”) //O/P – NaN

Key Capsule 9-

In JavaScript we can define an array by using bellow syntax;

Var stdArr=[];

Note- stdArr is a special type of object which store different type of object inside it.

Join Method in Array-

In JavaScript the join method is used to convert the array into string but we can specify separator inside join method.

For eg- var arr=[“Ram”,”Hari”,”Madhu”]


so the o/p Ram * Hari * Madhu.

Key Capsule 10

In JavaScript we use a key word called “use strict”.

This indicates that we can not use a variable without declaring it. So if we use a variable without declaring it then it’s gives you an error.

Note - Use strict can be write at the top of the script.

  1. Use strict  
  2. a=2+3;  
  3. console.log(a) // Error because we did not declare the variable a.  

Hope the above concept help you in understanding the basic concept; we will see more in coming days.

If anything is wrong or if anything won't work for you please leave your feedback in the comment section.


Build smarter apps with Machine Learning, Bots, Cognitive Services - Start free.

Start Learning Now