The Topic Which is Covered in This Article

• Variable

• Variable name convention Rules

• Hoisting

• Variable Scope

  • 1. variable scope
  • 1. block scope
  • 1. global scope

• Datatypes in Javascript

• What Is NaN?

• Type Of Operators

• Difference With Null vs Undefined

• Expression And Operators

  • 1 Assignment operators

  • 2 Arithmetic operators

  • 3Logical operators

  • 4 Comparison operators

  • 5 String Concatenation(operators)

• What Is the Difference between == vs ===?

Variable

📌Variable declaration

🪢 var

🪢let

🪢const

📌Hoisting

📌Variable Naming Rules

📌Variable Scopes

🪢Block

🪢Function

🪢Global

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📌Variable declaration

📌There are 3 Types of Javascript Variable

• ➡️Variable simply store value

  🪢var

  🪢let

  🪢const

  📌Let

  

•      let language = "javascript";
       console.log(language); //JAVASCRIPT
  
       let language;
       language = "javascript";
       console.log(language); //JAVASCRIPT
  

📌const

• ➡️once declare cant change

• ➡️its good because in a real application mutations less (change in value) then also bug will less

• 

•      const secondsInMinute = 60;
       secondsInMinute = 500;
       console.log(secondsInMinute);   //error :signment to constant variable.
  

  📌Variable name convention Rules

  • ➡️The first character must be a letter or an underscore ( - ) or an dollar ( $ . You can't use a number as the first character .

  • ➡️The rest of the variable name can include any letter , any number , or the underscore . Can use any other characters , but not spaces .

  • ➡️ Variable names are case sensitive . No limit to the length of the variable name .

  • ➡️You can't use any of JavaScript's reserved words as a variable name

  • ➡️we cant use special character as name

📌What is Hoisting

• Hoisting is JavaScript's default behaviour of moving declarations to the top.

• Hoisting is a JavaScript mechanism where variables, function declarations and classes are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting,

•   console.log(message); //output : undefined
    var message = "The variable Has been hoisted";
  

The above code looks like as below to the interpreter.

var message;
console.log(message);
message = "The variable Has been hoisted";

• In the same fashion, function declarations are hoisted too

    message("Good morning"); //Good morning
  
    function message(name) {
      console.log(name);
    }
  

This hoisting makes functions to be safely used in code before they are declared.

📌Variable Scopes

🪢Block Scopes

🪢Function Scopes

🪢 Global Scopes

📌BLOCK SCOPE

• as per name block scope means the Variable value is only limit in that block scope

• 🪢let

• it is a block scope => {...............} the code betwween curly brackets

           if (true) {         
                age = 25;     
                console.log(age); //answer get 25 
            }
         console.log(age); //error
  

• 🪢const

• it is a block scope =>{...............} the code between curly brackets

 if (true) {
   const age = 25;
   console.log(age); //answer get 25
 }
 console.log(age); //error

📌Function Scope

• var

• ➡️ It is a function scope

     function years() {
       var age = 25;
    }
     console.log(age); //error => age is not defined
  

  • if ‘var ‘ is in the function then it can't get outside function

  • if it's any other blocks then it gets the output

    if (true) {
       var age = 25;
       console.log(age); //answer get 25
     }
     console.log(age); //25

• if variables we create and we can store any kind of data string, object, numeric value, array function

📌Global Scope

• if age variabel value is 25 in the function then it can run in that only

• if its outside function and simply alone in the file then its global scope. then alone value will be given as answer

• while its is normal scope

•    var age = 32;
    if (true) {
       var age = 25;
       console.log(age); //answer get 25
     }
     console.log(age); //answer get 25
  

• while it is in function

  var age = 32;
  function xyz()
     {
  var age = 25;
  console.log(age); //answer get 25
    }
    xyz();
    console.log(age); //answer get 32

📌Datatypes in Javascript

Six Data Types that are primitives

• Boolean : typeof instance === " boolean "

• undefined : typeof instance === " undefined "

• Number : typeof instance === " number "

• String : typeof instance === " string "

• BigInt : typeof instance === " bigint "

• Symbol : typeof instance === " symbol "

Example(Practise)

//10 + "20" // number + string so here 1020
//9 - "5" // number - string so here = 4 but its a bug
//"Java" + "Script" //string  + string = concate and its javascript
//" " + " " //empty
//" " + 0 //0
//"vinod" - " thapa " //NaN = not a number

//js is
// true = 1
// false = 0

// true +  true   bool+bool = 1+1  =  2
// true +  false  bool+bool = 1+0  =  1
// false + true   bool-bool = 0+1  =  1
// false - true   bool-bool = 0-1  =  1

IQ: DIFFERENCE BETWEEN NULL VS UNDEFINED

IQ: WHAT IS NaN?

• console.log(”dipesh” - “joshi”) // NaN

• NaN is a property of the global object .

• In other words , it is a variable in global scope .

• The initial value of NaN is Not - A - Number

  

Example(Practise)

Expression And Operators

1 Assignment operators

• An assignment operator assigns a value to its left operand based on the value of its right operand .

The simple assignment operator is equal ( = )

 var x = 5 ; var y = 5 ; 

I will tell you when we will see es6 console.log

 ( Is both the x and y are equal : $ { x || = y } ) ;

2 Arithmetic operators

• An arithmetic operator takes numerical values ( either literals or variables ) as their operands and returns a single numerical value .

 console.log ( 3 + 3 ) ;
 console.log ( 10-5 ) ;
 console.log ( 20/5 ) ; |
 console.log ( 5 * 6 ) ;
- the modulus operator(%)returns  the division remainder
 console.log ( " Remainder Operator " + 81 % 8 ) ;

Increment and Decrement operator 
Operator : x++ x-- or ++x --x
- If used postfix , with operator after operand ( for example , x ++ ) , 
the increment operator increments and returns the value before incrementing .
- If used prefix , with operator before operand ( for example , + x ) ,
the increment operator increments and returns the value after incrementing

3 Logical operators

• Logical operators are typically used with Boolean ( logical ) values ; when they are , they return a Boolean value .

     var a = 30 ;
     var b = -20 ;
  
     Logical AND ( && )
     The logical AND ( && ) operator ( logical conjunction ) for a set of
     operands is true if and only if all of its operands are true .
  
     Logical OR ( || )
     The logical OR ( || ) operator ( logical disconjunction ) for a set of
     operands is true if and only if one or more of its operands is true .
  
     Logical NoT (!)
     the logical not(!) operator (logical complement,negattion)
     takes truth to falsity and vice versa
  

  4 Comparison operators

•    var a = 30 ;
     var b = 10 ;
  
     // Equal ( == )
     console.log ( a == b ) ;
     // Not equal ( != )
     console.log ( a != b ) ;
     // Greater than ( > )
     console.log ( a > b ) ;
     // Greater than or equal ( >= )
     console.log ( a >= b ) ;
     // Less than (<)
     console.log ( a <= b ) ;
     // Less than or equal (<=)
     console.log(a <= b);
  

5 String Concatenation(operators)

• The concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings

Example(Practise)

//Exponentiation Operator
// What will be the output of 3**3?
// console.log(3**3); //27
// console.log(3**5); // 3*3*3*3*3 =243
//console.log(10 ** -1); // 1/10 =0.1

// What will be the output, when we add a number and a string ?
// console.log(5 + "thapa"); //5thapa
// console.log(5 - "thapa"); //NaN

// Write a program to swap two numbers?
var a = 5;
var b = 10;
//output b=5 and a=10
var c = b; //c=10
b = a; //b = 5
a = c;

console.log("the value of a is " + a);
console.log("the value of b is " + b);

// Write a program to swap two numbers without using third variable
 var a = 5;
var b = 10;
//output b=5 and a=10

a = a + b; //a = 15
b = a - b; //b = 5
a = a - b; //a = 10

console.log("the value of a is " + a);
console.log("the value of b is " + b);

IQ: WHAT IS the difference between == vs ===?

• \== check only value === check value and its datatype