Variables: It holds the data or information which can be changed anytime. JavaScript use reserved keyword var to declare variables. In JavaScript, there are two types of variable and also it tells you where in your program you are allowed to use the variables and functions that you’ve defined.

In JavaScript, variables can be either global or local depending on where they are declared and used within a program. Global variables are those that are declared outside of any function and can be accessed from anywhere in the program, while local variables are those that are declared within a function and can only be accessed within that function.

Global variables can be useful in certain situations, such as when you need to share data between different functions or modules. However, overuse of global variables can lead to a number of issues, including naming conflicts and unexpected behavior.

Local variables, on the other hand, are generally preferred because they help to encapsulate code and prevent unintended side effects. By declaring variables within a function, you can ensure that they are only accessible within that function and that their values are not accidentally modified by other parts of the program.

In JavaScript, it's important to use variable declaration keywords like "var", "let", and "const" to properly define and scope your variables. "var" is used to declare global variables or local variables within a function, while "let" and "const" are used to declare block-scoped local variables. Block-scoped variables are declared within curly braces ("{}") and are only accessible within that block of code.