What is the difference between const and let in JavaScript

In JavaScript, both const and let are used to declare variables, but they have some key differences:

1. Mutability:
   • const: Variables declared with const are constant and cannot be reassigned after they are initially assigned a value. This means that once you assign a value to a const variable, you cannot change that value.
   • let: Variables declared with let are mutable, which means you can reassign them with a new value at any time.
2. Block Scoping:
   • Both const and let are block-scoped, which means they are only accessible within the block (enclosed by curly braces) where they are defined. This is different from var, which is function-scoped.
3. Temporal Dead Zone (TDZ):
   • Variables declared with const and let are hoisted to the top of their containing block or function, but they are not initialized until they reach the actual declaration. This results in a “Temporal Dead Zone” where you cannot access the variable before it is declared.

Here’s an example to illustrate the differences:

// Using const
const a = 10;
a = 20; // Error: Cannot reassign a constant variable

// Using let
let b = 30;
b = 40; // Valid: You can reassign a variable declared with let

// Block scope
if (true) {
  const x = 50; // x is only accessible within this block
  let y = 60;   // y is only accessible within this block
}

// Temporal Dead Zone
console.log(c); // Error: Cannot access 'c' before initialization
const c = 70;