Difference Between Variable and Constant in JavaScript

Variables and constants are used to store data values in JavaScript. The main differences between them are:

• Variables can change: Variables can have their value reassigned. Once a variable is declared, you can assign it a new value.

let x = 5; 
x = 10; // x is now 10

• Constants cannot change: Constants are immutable - once declared and defined, their value cannot be changed.

const y = 5;
y = 10; // TypeError: Assignment to constant variable.

• Scope: Variables can have global, local, or block scope depending on where they are declared. Constants also have the same scope - global, local, or block.

• Initialization: Variables can be declared without initializing them with a value. Constants must be initialized with a value when declared.

let x; // Valid
const y; // SyntaxError: Missing initializer in const declaration

• Naming: It is good practice to name constants with all uppercase letters and underscores between words (snake case), while variables can use camelCase.

const PI = 3.14;
let radius = 5;

• Performance: Since constants cannot be reassigned, some JavaScript engines may optimize code using constants more aggressively, resulting in better performance.

In summary, use variables when the value needs to change, and constants when the value is fixed. This makes your code more predictable and easier to reason about.

Hope this helps! Let me know if you have any other questions.