JavaScript Data Types

  

JavaScript is a single-threaded language 

JavaScript is a single-threaded language because while running code on a single thread, it can be really easy to implement as we don't have to deal with the complicated scenarios that arise in the multi-threaded environment like a deadlock. Since JavaScript is a single-threaded language, it is synchronous in nature

What does mean by declaration- assignment- initialization !! in java script ?? Initialization: When you declare a variable it is automatically initialized, which means memory is allocated for the variable by the JavaScript engine.

Assignment: This is when a specific value is assigned to the variable.

And Hoisting available only for declaration , its not available for assignment and initialization.

     
        // Hoisting in JavaScript

        // debugger;

        // console.log(a,b,c);

        // d();

        // e();

        var a = "A";

        let b = "B";

        const c ="C";

        function d(){

            console.log("D");

        }

        var e = function(){

            console.log("E");

        }

        console.log(a,b,c);

        d();

        e(); Scope in Variables :: let , Var , Const .

JavaScript variables can hold different data types: numbers, strings, objects and more:

they are ..

var x = "Hellow World";                        // String
var x = 25;                                    // Number
var x = true ;                                 // Boolean

var x = ["HTML","CSS","JS"] ;                  // Array

var x = {firstName:"John", lastName:"Doe"};    // Object

var x = null ;                                 // Null

var x;                                         // Undefined

Meaning  of DataTypes =
Different types of variable value is a data types, they are..

JavaScript Strings

A string (or a text string) is a series of characters like "John Doe". Strings are written with quotes. You can use single or double quotes:

let carName1 = "Volvo XC60";   // Using double quotes
let carName2 = 'Volvo XC60';   // Using single quotes

document.write(carName1);

JavaScript Numbers

JavaScript has only one type of numbers.
Numbers can be written with, or without decimals:

let x1 = 34.00;     // Written with decimals
let x2 = 34;        // Written without decimals

document.write(x1);

JavaScript Arrays

JavaScript arrays are written with square brackets. Array items are separated by commas. The following code declares (creates) an array called cars, containing three items (car names):

const cars = ["Saab", "Volvo", "BMW"];

document.write(cars);

array with index

var ary = [10,20,30,40,50];

document.write(ary[2]);

array with for loop..

var ary = [10,20,30,40,50];

for (var a = 0; a <= 4; a++){

    document.write(ary[a]+"<br>");

 }      

nested array 

<script>  

var user =["ramesh", 22, "pune", ["Male",25000]];

var [name, age = 20, city , [gender, salary]] = user;

console.log(name);

console.log(age);

console.log(city);

console.log(gender);

console.log(salary);

</script>

array with function..

<script>  

  function user([name,age=20, city]){

    console.log(name);

    console.log(age);

    console.log(city);

  }

  user(["code guruva",22, "Delhi"]);

</script> 

       

     

JavaScript Objects

JavaScript objects are written with curly braces {}.
Object properties are written as name:value pairs, separated by commas.

               

        var a = {

            fname : 'product',

            lname : "one",

            age : 25,

            email : 'ramesh@gmail.com',

        }

        // document.write(a);

        // console.log(a);

        console.log(a.email);

       

Destructing Objects

               

       var user ={

        name:"ramesh",

        age:"29",

        city:"banglore"

       }

       var {name, age, city} = user;

       console.log(name);

       console.log(age);

       console.log(city);

Object Literals 

    <script>

        var n = "student";

        var obj ={

            [n + "name"] : "Ram Babu",

            course : "MTech",

            detail : function(){

                return `${this.studentname} is student of ${this.course}`

            }

        };

        console.log(obj);

        console.log(obj.detail());

    </script>

       

Undefined

In JavaScript, a variable without a value, has the value undefined. The type is also undefined. Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

let car;    // Value is undefined, type is undefined
car = undefined;    // Value is undefined, type is undefined

Empty Values

An empty value has nothing to do with undefined.An empty string has both a legal value and a type.

let car = "";    // The value is "", the typeof is "string"

Ex : 

        // undefined variable value

        var abc ;

        console.log(abc)

        // undefined function value

        function msg(y){

            console.log(y)

        }

        msg();

Null Value :

        // null value with variable

        var abc = null ;

        console.log(abc);

        // null value with variable object

        var student ={

            sname:"kishore",

            course:null

        }

        console.log(student.course);

JavaScript Booleans

Booleans can only have two values: true or false. Booleans are often used in conditional testing.

let x = 5;
let y = 5;
let z = 6;
(x == y)       // Returns true
(x == z)       // Returns false

Ex :

        //variable with boolean

       

        var abc = 0;

        var msg = 1;

        var value = Boolean(abc);

        document.write(value);