let and const in ES6

1. let Declaration:

   • In ES6, the let declaration is introduced to create block-scoped variables. This means that a variable declared with let is only accessible within the block (e.g., inside a function or a pair of curly braces) where it's defined.

   • let variables are not hoisted to the top of their containing function or block. This prevents issues where variables are accessed before they're declared.

   • let allows you to reassign a value to the variable after its initial declaration, making it a mutable variable.

   • Example:

     function exampleFunction() {
       if (true) {
         let x = 10;
         console.log(x); // 10
       }
       console.log(x); // Error: x is not defined
     }
     

2. const Declaration:

   • The const declaration also introduces block-scoped variables, but it comes with an additional constraint: variables declared with const cannot be reassigned after their initial assignment. They are constant.

   • const is often used for values that should not change, such as constants or references to immutable objects.

   • Example:

     const pi = 3.14159;
     pi = 3.14; // Error: Assignment to constant variable.
     

Comparison with ES5:

In ES5, JavaScript primarily used the var keyword for variable declaration. Here are the key differences when comparing let and const in ES6 with var in ES5:

1. Block Scope vs. Function Scope:

   • ES6 (let and const): Variables declared with let and const are block-scoped, meaning they are limited to the block where they are defined, be it a block within a function or a standalone block.

   • ES5 (var): Variables declared with var are function-scoped, meaning they are accessible throughout the entire function containing the var declaration.

2. Hoisting:

   • ES6 (let and const): Variables declared with let and const are not hoisted to the top of their containing block, which prevents the use of variables before they are declared.

   • ES5 (var): Variables declared with var are hoisted to the top of their containing function, which can lead to unexpected behavior if not managed carefully.

3. Reassignment:

   • ES6 (let and const): Variables declared with let can be reassigned after their initial declaration. Variables declared with const cannot be reassigned, making them constants.

   • ES5 (var): Variables declared with var can be reassigned at any point in the function where they are declared.

4. Constants:

   • ES6 (const): ES6 introduces the const keyword, allowing the creation of constants, which cannot be changed after their initial assignment.

In summary, let and const in ES6 provide more predictable and controlled variable scoping compared to var in ES5. They help avoid common issues associated with variable hoisting and provide the flexibility to choose between mutable and immutable variables based on your needs.