JAVASCRIPT

Variable and Data:

• Just like we follow some rule while speaking english (the grammar) we have some rules to follow while writing a javascript program.
• The set of those rules is called syntax in javascript.

Variable:

• A variable is a container that stores a value.
• This is very similar to the containers used to store rice, water and oats (treat this as an analogy).
• The value of a javascript variable can be changed during the execution.
• In javascript, we have two types of variables: var and let.

var:

• var is a global variable and can be used anywhere in the program.
• It is declared using the keyword var followed by the variable name and the value.
• It can be updated and re-declared within its scope.
• It is initialized with undefined.

var a = 7; 

let:

• let is a block-scoped variable and can be used only within the block it is defined.
• It is declared using the keyword let followed by the variable name and the value.
• It can be updated but not re-declared within its scope.
• It is not initialized with undefined.

let a = 7;

const:

• const is a block-scoped variable and can be used only within the block it is defined.
• It is declared using the keyword const followed by the variable name and the value.
• It can be updated but not re-declared within its scope.
• It is not initialized with undefined.

const a = 7;

Rules of Assigning a Varibale:

• Letters, digits, underscore and $ sign is allowed.
• Must begin with a $, _ or a letter.
• Javascript reserved words cannot be used as a variable name.
• Saad and sAAd are different variables (case sensitive).

Data Types:

• In javascript, there are several data types such as string, number, boolean, object, array, function, etc.
• We can use the typeof operator to check the data type of a variable.

typeof "Hello World" // "string"
typeof 10 // "number"
typeof true // "boolean"
typeof {} // "object"
typeof [] // "object"
typeof function() {} // "function"

var VS let in Javascript:

• var is glovaly scoped While let & const are block scoped.
• Var can be updated and re-declared within its scope.
• let can be updated but not re-declared.
• const can neither nor be re-declared.
• var variables are initialized with undefined whereas let and const variables are not initialized.
• const must be initialized during declaration unlike let and var.

Interview Questions:

1. What is the difference between var, let, and const in JavaScript?
2. What is the difference between undefined and null in JavaScript?