Variables

• In programming, you use a variable to store data temporarily in the computer's memory. Your data is stored somewhere in the memory, and you give that memory location a name. Using this name, you can read the data from the given location in the future.
• Think of the boxes you use to organize your stuff. You put your stuff in various boxes and put a label on each box. With this, you can easily find your stuff. A variable is like a box. what you put inside the box is the value/data that you assign to a variable and the label that you put on the box is the name of our variable.
• In simple terms variables are containers for storing data.

Declare Variables

• You can declare variables using var, let, const keyword.
• Before ES6, var keyword was used to declare a variable, but there are issues with var
• From ES6, the best practice is to use the let keyword to declare a variable.
• Give the variable a name or identifier

let name;
name = "ganesh"

• By default, variable declared without a value will have the value undefined.
• You can initialize the variable with a value during declaration

let name = "ganesh";

Identifiers

• All JavaScript variables must be identified with unique names. These unique names are called identifiers.
• The general rules for constructing names for variables (unique identifiers) are:
  • Can contain letters, digits, underscores, and dollar signs.
  • Must begin with a letter.
  • Cannot start with a number.
  • Cannot contain a space or hyphen
  • Are case sensitive (firstName and FirstName are different variables).
  • Reserved words (like JavaScript keywords) cannot be used.

Declare Multiple Variables

• You can declare multiple variables in one statement separated by a comma.

let firstName, lastName;
let firstName = 'Ganesh', lastName;
let firstName = 'Ganesh', lastName = "Kumar";

• Always declare each variable in new line

let firstName = "Ganesh";
let lastName = "Kumar";