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39 Best Object Oriented JavaScript Interview Questions and Answers

Most Popular 37 Key Questions for JavaScript Interviews.
What is Object in JavaScript? What is the Prototype object in JavaScript and how it is used?
What is "this"? What is its value? Explain why "self" is needed instead of "this".
What is a Closure and why are they so useful to us? Explain how to write class methods vs. instance methods.
Can you explain the difference between == and ===? Can you explain the difference between call and apply?
Explain why Asynchronous code is important in JavaScript? Can you please tell me a story about JavaScript performance problems?
Tell me your JavaScript Naming Convention? How do you define a class and its constructor?
What is Hoisted in JavaScript? What is function overloading in JavaScript?
What is the scope variable in JavaScript? What are Associative Arrays in JavaScript?
How to Achieve Inheritance in JavaScript? What are two-way data binding and one-way data flow, and how are they different?
What will be the output of the following code?What will be the output of the JavaScript code?
How to empty an array in JavaScript?What is the difference between the function declarations below?
"1"+2+3 = ? And 1+2+"3" = ? OutPut Will Be?How to show progress bar while loading using Ajax call?
How To Create the Namespace in JavaScript?What Is the Difference Between Undefined and Object?
How To convert JSON Object to String?How To convert JSON String to Object?
How To Convert a string to Lowercase?How To modify the URL of the page without reloading the page?
How To open URL in a new tab in JavaScript?TypeScript Interview Questions and Answers
What is the difference between let, var, and const? Is it possible to use JavaScript to change the meta-tags of the page?

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What is JavaScript?
JavaScript is the most popular scripting language and it was developed by Netscape and used to develop client-side web applications.

Is JavaScript Case Sensitive?
Yes! JavaScript is case-sensitive because the function “str” is not equal to “Str”.

What Is the Type of JavaScript?
There are different of Type as given below.
1.      String, 
2.      Number,
3.      Boolean,
4.      Function,
5.      Object,
6.      Null,
7.      Undefined etc.

What Types of Boolean Operators in JavaScript?
There are three types of Boolean operators as given below.
1.      AND (&&) operator, 
2.      OR (||) operator and 
3.      NOT (!) Operator

What Is the Difference Between “==” and “===”?
The double equal “==” is an auto-type conversion and it checks only value not type.
The three equal “===” is not auto-type conversion and it checks value and types both.

The example looks like–
if(1 == "1")
//Its returns true because it's an auto-type conversion and it checks only value not type.

if(1 === "1")
//Its returns false because it's not auto-type conversion and it checks value and types both.

if(1=== parseInt("1"))
// its returns true.
// alert(0 == false); // return true, because both are same type.
// alert(0 === false); // return false, because both are of a different type.
// alert(1 == "1"); // return true, automatic type conversion for value only.
// alert(1 === "1"); // return false, because both are of a different type.
// alert(null == undefined); // return true.
// alert(null === undefined); // return false.
// alert('0' == false); // return true.
// alert('0' === false); // return false.
// alert(1=== parseInt("1")); // return true.


What Is an Object?
The object is a collection of properties and each property is associated with the name-value pairs. The object can contain any data type (numbers, arrays, objects, etc.)

The example as given below –

var myObject= {empId :001”, empCode :X0091”};


In the above example, there are two properties one is empId and the other is empCode and its values are “001” and “X0091”.

The properties name can me string or number. If a property name is number i.e.
var numObject= {1 :001”, 2 :X0091”};
Console.log(numObject.1); //This line throw an error.

Console.log(numObject[“1”]); // will access to this line not get any error.

As per my thought, the number of property names should be avoided.

Types of creating an object
1.      Object Literals
2.      Object constructor

Object Literals: This is the most common way to create the object with an object literal and the example is given below.

The empty object initialized using object literal i.e.
var emptyObj = {};

This is an object with 4 items using object literal i.e.
var emptyObj ={
empId:Red”,
empCode:X0091”,
empDetail : function(){
   alert(“Hi”);
  };
};

Object Constructor: The second way to create an object using an object constructor and the constructor is a function used to initialize a new object. 

The example as given below -
var obj = new Object();
Obj.empId=”001”;
Obj.empCode=”X0091”;

Obj.empAddressDetai = function(){
console.log(“Hi, I Anil”);
};


What Is Scope Variable in JavaScript?
The scope is a set of objects, variables, and functions and JavaScript can have a global scope variable and a local scope variable.

A global scope is a window object and it’s used out of function and within the functions.
A local scope is a function object and it’s used within the functions.


The example for global scope variable -
var gblVar = "Anil Singh";

function getDetail() {
     console.log(gblVar);
}

And other example for global -
function demoTest() {
      x = 15;
};
console.log(x); //output is 15

The example for local scope variable -
function getDetail() {
     var gblVar = "Anil Singh";
     console.log(gblVar);
}

And other example for local -
function demoText() {
    var x = 15;
};

console.log(x); //undefined

What Is an Associative Array in JavaScript?
What is the array? 
The array is a collection of index items and it is a number of indexes.

Some programming language support array as named indexes and JavaScript does not support array as name indexes it provides only number indexes but provide this feature using the associative array.

The array with name indexes is called an associative array and the associative array is provide a way to store the information.


The number index array example is given below -

var users = new Object();
users["Name1"] = "Anil 1";
users["Name2"] = "Anil 2";
users["Age"] = 33;

alert(Object.keys(users).length); //output is 3.
var length = Object.keys(users).length;  // 3

   
The name index array example is given below -

var users = [];
users["Name1"] = "Anil 1";
users["Name2"] = "Anil 2";
users["Age"] = 33;

var length = users.length;         // users.length will return 0
var detail = users[0];             // users[0] will return undefined

Where To Use the Associate Array?
I am going to explain the associate array over the database table columns. A table has some columns and their type i.e.

The empName is the text type, empAge is the number type, and enpDOB is the date type.

If we need to find the type of a column that time we can create the associate array i.e.
var empDetailType = new Array();

empDetailType["empName"] = "ANil";
empDetailType["empAge"] = 30;
empDetailType["empDOB"] = "10/03/1984";

console.log("Find the emp age type :" + empDetailType["empAge"]);



How To Achieve Inheritance in JavaScript?
In JavaScript, we can implement the inheritance using some alternative ways and we cannot define a class keyword but we create a constructor function and use a new keyword to achieve it.  

Some alternative ways as given below –
1.      Pseudo-classical inheritance
2.      Prototype inheritance

Pseudo-classical inheritance is the most popular way. In this way, create a constructor function using the new operator and add the members function with the help of the constructor function prototype.

Prototype-based programming is a technique of object-oriented programming. In this mechanism, we can reuse the existing objects as prototypes. The prototype inheritance also is known as prototypal, Classless, or instance-based inheritances.



The Inheritance example for prototype based as given below –
//create a helper function.
if (typeof Object.create !== 'function') {

Object.create = function (obj) {
function fun() { };

fun.prototype = obj;
return new fun();
};
}

//This is a parent class.
var parent = {
sayHi: function () {
alert('Hi, I am parent!');
},

sayHiToWalk: function () {
alert('Hi, I am parent! and going to walk!');
}
};

//This is child class and the parent class is inherited in the child class.
var child = Object.create(parent);
child.sayHi = function () {
alert('Hi, I am a child!');
};

HTML –
<button type="submit" onclick="child.sayHi()"> click to oops</button>
//The output is: Hi, I am a child!


What Is the typeof Operator?
The type of operator is used to find the type of variables.

The example as given below -
typeof "Anil Singh"   // Returns string
typeof 3.33           // Returns number
typeof true          // Returns Boolean
typeof { name: 'Anil', age: 30 } // Returns object
typeof [10, 20, 30, 40] // Returns object


What Is Public, Private, and Static Variables in JavaScript?
I am going to explain strongly typed object-oriented languages (OOPs) like(C#, C++, Java, etc.).

First I am creating a conductor class and trying to achieve to declare the public, private and static variables and detail as given below –
function myEmpConsepts() { // This myEmpConsepts is a constructor  function.
var empId = "00201"; //This is a private variable.
this.empName = "Anil Singh"; //This is a public variable.

this.getEmpSalary = function () {  //This is a public method
console.log("The getEmpSalary method is a public method")
}
}

//This is an instance method and its call at the only one time when the call is instantiate.
myEmpConsepts.prototype.empPublicDetail = function () {
console.log("I am calling public vaiable in the istance method :" + this.empName);
}

//This is a static variable and it’s shared by all instances.
myEmpConsepts.empStaticVaiable = "Department";
var instanciateToClass = new myEmpConsepts();



How To Create the Namespace in JavaScript?
Please see the below example for how to create the namespace in JavaScript.

//Create the namespace.
var nameSpace = nameSpace || {};

nameSpace.myEmpConsepts = function () {
var empId = "00201"; //This is a private variable.
this.empName = "Anil Singh"; //This is a public variable.

//This is public function
this.getEmp = function () {
return "Anil Singh"
}

//This is private function
var getEmp = function () {
return "Anil Singh"
}

return {

getEmp: getEmp,// work as public
getEmp: getEmp // work as public
}
}();


How to Add/Remove Properties to an Object in run-time in JavaScript?
I am going to explain by example to add and remove properties from JavaScript objects as given below.


This example for delete property -
//This is the JSON object.
var objectJSON = {
id: 1,
name: "Anil Singh",
dept: "IT"
};

//This is the process to delete
delete objectJSON.dept;


//Delete property by the array collection
MyArrayColection.prototype.remove = function (index) {
this.splice(index, 3);
}

This example for add property -
//This is used to add the property.
objectJSON.age = 30;
console.log(objectJSON.age); //The result is 30;

//This is the JSON object.
var objectJSON = {
id: 1,
name: "Anil Singh",
dept: "IT",
age: 30
};

How To Extend built-in Objects in JavaScript?
JavaScript support built-in objects which use to develop the flexibility of JavaScript. The built-in object is a date, string, math, array, and object.

It's very similar to other languages and it’s available in the window content and works independently when brewers are loaded.

Example as give below -
var date = new Date(); //This is date built-in object.
var math = Math.abs(10); // this is math built-in object.

var string = "my string" // this is string built-in object.

Why Never Use New Array in JavaScript?
We have some fundamental issues with the new Array () the example in detail for the array constructor function is given below.

When Array Have more the one Integer?
var newArray = new Array(10, 20, 30, 40, 50);
console.log(newArray[0]); //returns 10.
console.log(newArray.length); //returns 5.

When Array Have Only One Integer?
var newArray = new Array(10);
console.log(newArray[0]); //returns undefined
console.log(newArray.length); //returns 10 because it has an error "array index out of bound";

//This is the fundamental deference to need to avoid the new array ();

What are eval() and floor() functions in JavaScript?
The eval() function is used to execute an argument as an expression or we can say that evaluates a string as an expression and is used to parse the JSON.

The example over eval() function as given below -
var x = 14;
eval('x + 10'); //The output is 24.

Another over eval() function example -
eval('var myEval = 10');
console.log(myEval); // The output is 10.


The floor () function is a static method of Math and we can write it as Math.floor() and used to round the number of downwards i.e.
Math.floor(1.6);//The output is 1.


What are join() and isNaN() functions in JavaScript?
The join() function is used to join the separator in the array.

Syntax - 
myArray.join(mySeparator);

The example as given below -
var alfabets = ["A", "B", "C", "D"];

//Join without seperator
var result1 = alfabets.join();//The output is A B C D.

//Join with seperator.
var result2 = alfabets.join(','); //The output is A, B, C, D.

The isNaN() function is used to check the value is not a number.

The example as given below -
var var1 = isNaN(-1.23);//The output is false.
var var2 = isNaN(3);//The output is false.
var var3 = isNaN(0);//The output is false.
var var3 = isNaN("10/03/1984"); //The output is true.

What Is Closure in JavaScript?
While you create the JavaScript function within another function and the inner function freely access all the variable of the outer function i.e.

function ourterFun(i) {
var var1 = 3;

function innerFun(j) {
console.log(i + j + (++var1)); // It will return the 16.
}
innerFun(10);
}
ourterFun(2); // Pass an argument 2


The output will get 16 because the innerFun() function can access the argument "i" & variable "var1" but both are defined in the outerFun() function that is closure.


That means simply accessing variables outside of your scope creates a closure.

//OR Other WAYS
function ourterFun(i) {
var var1 = 3;

return function (j) {
   console.log(i + j + (++var1)); // It will return the 16.
  }
}

var innerFun = ourterFun(2); // innerFun() function is now a closure.

innerFun(10);


What is JavaScript Hoisted?
In JavaScript, the variables can be used before they are declared, these kinds of mechanisms are called Hoisted. It's the default behavior of JavaScript.


You can easily be understanding in the below example in detail -
//The variable declaration looks like.
var emp;

//The variable initialization looks like.
emp = "Anil Singh";

var emp; //The declaration of emp is hoisted but the value of emp is undefined.
emp = 10; //The Assignment still occurs where we intended (The value of emp is 10)

function getEmp() {
var emp; //The declaration of a different variable name emp is hoisted but the value of emp is  undefined.
console.log(emp); //The output is undefined
emp = 20; //The assignment values is 20.
console.log(emp); //The output is 20.
}

getEmp();

console.log(emp); //The variable named emp in the outer scope still contains 10.

What Is Function Overloading in JavaScript?
There is no real function overloading in JavaScript and it allows passing any number of parameters of any type.

You have to check inside the function how many arguments have been passed and what is a type of arguments using typeof.

The example for function overloading not supported in JavaScript is gives below -
function sum(a, b) {
    alert(a + b);
}
function sum(c) {
    alert(c);
}
sum(3);//The output is 3.
sum(2, 4);//The output is 2.

In JavaScript, when we write more than one function with the same name that time JavaScript considers the last defined function and overrides the previous functions. You can see the above example output for the same.

We can achieve this using several different techniques as given below -
1.      You can check the declared argument name value is undefined.
2.      We can check the total arguments with the arguments.length.
3.      Checking the type of passing arguments.
4.      Using a number of arguments
5.      Using optional arguments like x=x || 'default'
6.      Using a different name in the first place
7.   We can use the arguments array to access any given argument by using arguments[i]

What Is the Prototype in JavaScript?
The prototype is a fundamental concept of JavaScript and it must be known by JavaScript developers.

All the JavaScript objects have an object and its property called prototype and it's used to add custom functions and properties.


The example without a prototype is given below -

var employee = function () {
//This is a constructor function.
}

//Crate the instance of above constructor function and assign in a variable
var empInstance = new employee();
empInstance.deportment = "IT";

console.log(empInstance.deportment);//The output of above is IT.

//The example with prototype as given below-
var employee = function () { //This is a constructor  function.}

employee.prototype.deportment = "IT";//Now, for every instance employee will have a department.

//Crate the instance of above constructor functions and assign in a variable
var empInstance = new employee();
empInstance.deportment = "HR";

console.log(empInstance.deportment);//The output of above is IT not HR.


What is the difference between let, var, and const?

I’m explaining the importance of these three keywords in JavaScript and TypeScript I also provided various examples to deeply understand and use them and what happened when?

Let's see the detail about the - var vs. let vs. const

var: -
1.      var is function-scoped
2.      var returns undefined when accessing a variable before it's declared

let: -
1.      let is block-scoped
2.      let throw ReferenceError when accessing a variable before it's declared

const:-
1.      Const is block-scoped
2.      Const throw ReferenceError when accessing a variable before it's declared
3.      Const cannot be reassigned

Let's see the Examples to understand what happens when -

What will happen when the following code is executed?
var x = 5;
console.log(x);
if (true) {
    var x = 6;
    console.log(x);
}
console.log(x);
//The Output will be -
5
6
6

What will happen when the following code is executed?
var x = 5;
console.log(x);
if (false) {
    var x = 6;
    console.log(x);
}
console.log(x);
//The Output will be -
5
5

What will happen when the following code is executed?
var x = 5;
function a() {
    var x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
5
6

What will happen when the following code is executed?
var x = 5;
function a() {
    x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
5
6

What will happen when the following code is executed?
var x = 5;
function a() {
    let x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
5
6

What will happen when the following code is executed?
let x = 5;
function a() {
    let x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
Uncaught SyntaxError: Identifier 'x' has already been declared
at<anonymous>: 1: 1

What will happen when the following code is executed?
const x = 5;
function a() {
    let x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
Uncaught SyntaxError: Identifier 'x' has already been declared
at<anonymous>: 1: 1

What will happen when the following code is executed?
const x = 5;
function a() {
    x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
Uncaught SyntaxError: Identifier 'x' has already been declared
at<anonymous>: 1: 1

What will happen when the following code is executed?
const x = 5;
function a() {
    const x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
Uncaught SyntaxError: Identifier 'x' has already been declared
at<anonymous>: 1: 1

What will happen when the following code is executed?
const x = 5;
function a() {
    var x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
Uncaught SyntaxError: Identifier 'x' has already been declared
at<anonymous>: 1: 1

What will happen when the following code is executed?
var x = 5;
function a() {
    const x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
5
6

What will happen when the following code is executed?
let x = 5;
function a() {
    var x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
Uncaught SyntaxError: Identifier 'x' has already been declared
at<anonymous>: 1: 1

What will happen when the following code is executed?
var x = 5;
function a() {
    var x = 6;
    return x;
}
x;
a();
//The Output will be -
6

What will happen when the following code is executed?
var x = 5;
function a() {
    var x = 6;
    return x;
}
x = a();
//The Output will be -
6

What will happen when the following code is executed?
var x = 5;
function a() {s
    var x = 6;
    return x;
}
x
//The Output will be -
5

What will happen when the following code is executed?
var x = 5;
function a() {
    var x = 6;
    return x;
}
console.log(x);
console.log(a());
//The Output will be -
5
6

What will happen when the following code is executed?
var x = 5;
function a() {
    var x = 6;
}
console.log(x);
console.log(a());
//The Output will be -
5

What will happen when the following code is executed?
var x = 5;
function a() {
    let x = 6; return x;
}
console.log(x);
console.log(a());
//The Output will be -
5
6


We hope you enjoy the short explanations provided by our experts and know you feel more familiar with the main terms of JavaScript programming. Answering the FAQs is commonly needed to deal with your homework assignment or complete your working task. We offer to pay attention to the practical JavaScript help provided by first-class developers and STEM scientists. If you stumble upon a task that you can’t complete for a long time, please, address it to such specialists, and they’ll surely assist you. The main thing is to never stop doing your work, even if you think that it’s impossible! 

By Anil Singh | Rating of this article (*****)

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