Javascript Introduction
Comments in Javascript
Javascript Variables
Difference between var, let and const in Javascript
Datatypes
Assignment Operators
Logical Operators
Comparison Operators
Actions alert, prompt & confirm

Javascript Flow Control
Conditional Statements in Javascript
Nullish coalescing operator in Javascript
Break and Continue in Javascript
Loops in javascript
for loop in Javascript
while loop

Javascript Functions
Javascript Functions
JavaScript Arrow Function

Javascript Objects
Javascript Objects

Javascript Array
Javascript Arrays

Javascript Error Handling
Error Handling in Javascript

Javascript ES6
Set
Javascript Spread Operator
Rest Parameter in Javascript

Javascript Asynchronous
Javascript Callback Function
Promises in Javascript
Promise Chaining
Async-Await in Javascript

Javascript DOM
DOM Document
DOM Elements
DOM Attributes
DOM Manipulation
DOM Node Lists
Javascript Events
DOM Traversing Elements
DOM EventListener

Javascript Class
Classes in javascript
Class Inheritance

Javascript Design Principles
SOLID - The Single responsibility principle in javascript
SOLID - Open-Closed principle in Javascript

Other Topics
Javascript Hoisting
Filter, Map and Reduce functions in Javascript
Closure in Javascript
Optional chaining operator in Javascript
Consoling in table format in javascript
Fetch API in Javascript
HTML web Storage API
Checking the online or offline status of the browser

Difference between var, let and const in Javascript

In javascript in order to create a variable we use 'var' keyword. But in ES6 they have introduced two more ways to create a variable. They are 'let' and 'const' keyword.

The 'var' and 'let' keywords are used to create a variable whose value can be changed at any point of time in the program but 'const' keyword allows us to create a variable whose value cannot be changed.

    var x = 10;
x = 5;
console.log(x);

let y = 10;
y = 5;
console.log(y);

const z = 10;
z = 5;
console.log(z);

  

Output

5
5
TypeError: Assignment to constant variable.

In the above program we have created 3 variables. Variable 'x' is declared with var keyword and assigned a value of 10. In the next line we change the value of 'x' to 5 and console the value of the variable 'x' which will print 5. Similarly we declare a variable 'y' and assign a value 10 and in the next line we change it to 5 and console the value of variable 'y' which will print 5.

Next we declare variable 'z' and assign value 10. Then in the next line when trying to change the value of z to 5 it would throw an error stating

'TypeError: Assignment to constant variable.'.

This is because constant variable once assigned a value cannot be changed later in the program.

Variable declared using const keyword should be assigned a value during its declaration itself. If a const variable is declared and if value is not assigned to it then it would throw an error.

Example

    const z;
z = 5;
console.log(z);	

  

Output:

const z;
SyntaxError: Missing initializer in const declaration

Now its important to know the difference between var and let keywords. 'const' and 'let' keywords has block scope but 'var' keywords has function scope.

    function checkEligible(role) {
  if (role === "admin") {
    var isEligible = true;
  }
  console.log(isEligible);
}
checkEligible("admin");

  

Output:

true

In the above code we declared a function checkEligible which get role as parameter. If the role is 'admin' then we declare a variable 'isEligible' and assign it to true. Then we console 'isEligible' variable. In the code, even though we declare a variable inside if block we can access the variable all through the function. This is because a variable declared with 'var' keyword can be used anywhere inside the function.

Now lets rewrite the above code with let keyword

          function checkEligible(role) {
        if (role === "admin") {
          let isEligible = true;
        }
        console.log(isEligible);
      }
      checkEligible("admin");
       
  

This will throw an error ' ReferenceError: isEligible is not defined'. This is because 'let' keyword is block scoped. So it can be used within the if block alone. Its scope is not extended everywhere inside the function.

Difference between var, let and const in Javascript

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