In JavaScript, a variable is a container for storing data values. Variables allow you to store information that can be used and manipulated throughout your code.

let username= 'shubhadip';
let isHappy = true;

let cannot be re-declared in the same scope, but they can be reassigned.

Summary of let in JavaScript

• Reassignment Allowed: You can reassign a let variable, meaning you can change its value after the initial assignment.

  let x = 5;
  x = 10;  // Allowed
  
  

• No Redeclaration in Same Scope: You cannot declare the same let variable twice within the same scope. Attempting to do so will throw an error.

  let y = 20;
  let y = 30;  // SyntaxError: Identifier 'y' has already been declared
  
  

• Block-Scoped: let is confined to the block {} where it’s declared, making it accessible only within that block (such as within an if statement or a loop).

  if (true) {
      let z = 15;
      console.log(z);  // Output: 15
  }
  console.log(z);  // ReferenceError: z is not defined
  
  

• Not Hoisted in the Same Way as var: Although let is technically hoisted, it remains in a "temporal dead zone" until its declaration is reached, meaning it cannot be accessed before declaration.

  console.log(a);  // ReferenceError: Cannot access 'a' before initialization
  let a = 10;
  
  

In short, let is suitable for variables whose values may change and provides a more predictable, block-scoped behavior than var, reducing unintended errors in code.

Summary of const

• Cannot be reassigned: The variable reference is fixed.
• Must be initialized: Requires an initial value at declaration.
• Block-scoped: Only accessible within the block {} it is declared in.
• Mutable contents for objects and arrays: You can modify the properties of an object or the elements of an array but cannot reassign the whole object or array.

Here's a concise comparison of let, var, and const in JavaScript:

In summary: