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Advanced JavaScript Interview Questions

Last update: 02 Jul 2021, 23 Questions

Over the years, JavaScript has become one of the most influential programming languages in the software industry. Due to this, there are a number of job applications and applicants in this industry. Due to this, we decided to come up with the actual Advanced JavaScript Interview Questions, questions that actually matter while you create codes for your company. Have a read on these questions to cement your knowledge in JavaScript and get that dream job. But, over time, we have found out that many developers lack a basic understanding of JavaScript such as working on objects and working on obsolete patterns like deeply layered inheritance hierarchies.

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Advanced JavaScript Questions And Answers

Here in this article, we will be listing frequently asked Advanced JavaScript Interview Questions and Answers with the belief that they will be helpful for you to gain higher marks. Also, to let you know that this article has been written under the guidance of industry professionals and covered all the current competencies.

1. What is the meaning of prototype in JavaScript?

A special type of enumerable object to which we can attach additional properties is called a prototype. JavaScript is a dynamic language and at any time we can add new properties to an object and can be shared among all the instances. Hence to add new properties and implement inheritance prototypes are used.

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2. What is a callback function?

A function passed as an argument to another function is called a callback function and this technique allows a function to call another function. A callback function gets executed after the execution of another function gets finished.

Example

                                                    

function myDisplayer(some) {
      document.getElementById("demo").innerHTML = some;
}

function myCalculator(num1, num2, myCallback) {
      let sum = num1 + num2;
      myCallback(sum);
}

myCalculator(5, 5, myDisplayer);

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3. What is JavaScript Hoisting?

A JavaScript default behavior that moves the declaration of variables and functions at the top of the current scope is called JavaScript hosting. You can use hosting for declaration, not for initialization.

Example

                                                    

1. Variable hoisting

console.log(x); // undefined
var x = 1;

2. Function hoisting

let x = 20, y = 10;
let result = add(x,y);
console.log(result);
function add(a, b) {
     return a + b;
}

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4. What is the difference between let, var & const?
Let Var Const
Let variables can be updated within its scope. Var variables can be updated. The value of const is cannot be updated.
Redeclaration is possible within its scope. Redeclaration is possible. Redeclaration is not possible.
For example:
jeepName ="Saab";
let carName="Ford";
For example:
jeepName = "Ford";
var jeepName;
For example:
const jeep = {type:"Open", model:"700", color:"black"};
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5. What is promise in JavaScript with example?

An object that shows the eventual completion or failure of asynchronous options along with its resultant value is called promise in JavaScript. The possible three states of a promise are pending, fulfilled, or rejected. It allows handling multiple asynchronous operations in JavaScript.

A Promise has 3 states:

  • Pending: initial state, neither fulfilled nor rejected.
  • Fulfilled: meaning that the operation was completed successfully.
  • Rejected: meaning that the operation failed.
promise
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6. What is Callback Hell?

When a JavaScript developer tries to execute multiple asynchronous operations then sometimes issues arise and it is termed as callback hell.

How to escape from a callback hell?

JavaScript gives an easy way of avoiding callback hell. This is performed by event queue and promises.

  • Promises
  • Async/Await

Example

                                                    

getDataFromFunction1(function(x){
    getDataFromFunction2(x, function(y){
        getDataFromFunction3(y, function(z){
            ...
        });
    });
});

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7. What is the difference between arrow functions & normal functions?

1) Using the arrow function, you can get the same results as normal functions by writing a few lines of code as shown in the example below.

//Example of Regular function:
var add = function(c, d) { return c + d;};

// Example of Arrow function
let add = (c, d) => { return c + d}; //or
let add = (c, d) => c + d;

2) In normal functions, argument binding is possible and other hand arrow functions have no argument binding.

3) Regular functions are construable but arrow functions are not constructible.

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8. How to remove duplicate values in an array with JavaScript?

There are two methods to remove duplicate content from an array such as using a temporary array and a separate index.

Example

                                                    

function getUnique(arr){     
    let uniqueArr = [];    
     for(let i of arr) {
        if(uniqueArr.indexOf(i) === -1) {
            uniqueArr.push(i);
        }
    }
    console.log(uniqueArr);
}

const array = [1, 2, 3, 2, 3,4,5];
getUnique(array);

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9. What is Closure with example?

A function that can access its parent scope, even after the parent function has closed is called JavaScript closure.

Example

                                                    


function makeFunc() {
  var  type = 'circle';
  function displaytype() {
    alert(type);
  }
  return displayType;
}

var myFunc = makeFunc();
myFunc();

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10. What are the data types used in JavaScript?

The following are the different types of data types used in JavaScript.

  • Strings
  • Numbers
  • Booleans
  • Null
  • Undefined
  • Object
  • Array
  • RegExP
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11. How do you use the spread Operator & Spread operators?

Using the rest operator we can call a function using any number of arguments and can be accessed as an array. It allows the destruction of an array of objects.

Example of Rest operator in JavaScript

function sum(...theArgs) {
   return theArgs.reduce((last, now) => {
      return last + now;
   });
}
console.log(sum(1, 2, 3, 4,5)); // expected output 15

 

Speare operator is just the reverse of rest operators and allows to expand an iterable such as an array expression can be expanded using by dividing the array into individual elements.

For example

let array 1 = [3,4];
let array 2 = [5,6,7];
array= [...array 1,...array 2];
console.log(array); // [ 3, 4, 5,6,7 ]

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12. How do you use map filter function in JavaScript?

The map function returns the same number of elements as present in the original array but the value of elements will be transformed in some way.

On the other hand, the filter function can return fewer or more elements than the original array but the value of the original elements will not change.

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13. What is the significance and benefit of including 'use strict' at the beginning of a JavaScript source file?

The "use strict" is not a statement, rather a literal expression that is vital to your code as it presents a way for voluntarily enforcing a stricter parsing and error handling process on your JavaScript code files or functions during runtime. Most importantly, it just makes your code very easy to manage.

Here are some of the benefits of using use strict expression at the beginning of a JS source file:
  • It makes debugging a lot easier by making the code errors that are ignored generate an error or throw exceptions.
  • It prevents the most common error of making an undeclared variable as a global variable if it is assigned a value.
  • It does not allow the usage of duplicate property names or parameter values.
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14. What is asynchronous programming and why is it important in JavaScript?

Asynchronous programming means that the program engine runs in an event loop. Only when blocking operation is required, a request is started, and the code runs without blocking the result.

This is vital in JavaScript because it is a very natural fit for the user interface code and very efficient performance-wise on the server end.

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15. What is the use of the Weakmap object in JavaScript?

The WeakMap object is basically a collection of key or value pairs where the keys are weakly referenced. It provides a way for extending objects from the outside without meddling into the garbage collection.

Here are some use cases of Weakmap objects:
  • Maintaining private data about a specific object while only giving access to people having a reference to the Map.
  • Safekeeping of data related to the library objects without making any changes to them or incurring any overhead.
  • Keeping of data related to the host objects such as DOM nodes in the browser.
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16. Give an example and explain the main difference between ES6 class and ES5 function constructors?

The main difference between these two is when you are using inheritance. It is much more complicated using inheritance in ES5, while the ES6 version is simple and easy to remember.

ES6 Class:

class Person {
   constructor(name) {
      this.name = name;
   }
}

ES5 function constructor:

function Person(name) {
   this.name = name;
}

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17. What is a generator in JavaScript and when to use it?

In JavaScript, generators are those functions which can be exited and re-entered later on. Their variable bindings shall be saved across the re-entrances. They are written using the function* syntax.

The Generator function should be used when:
  • You can choose to jump out of a function and let the outer code determine when to jump back in the function.
  • The control of the asynchronous call can be executed outside of your code.

JavaScript is a high-level, multi-paradigm programming language which conforms to the ECMAScript specification. Read our list of advanced javascript interview questions to get a deep understanding of this language.

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18. What is the drawback of creating a true private in JavaScript?

One of the drawbacks of creating a true private method in JavaScript is that they are highly memory consuming. A new copy for each method is created for every instance.

Example:

var Employee = function (name, company, salary) {
    this.name = name || "";
    this.company = company || "";
    this.salary = salary || 5000;
    var increaseSlary = function () {
        this.salary = this.salary + 1000;
    };
    this.dispalyIncreasedSalary = function() {
       increaseSalary();
       console.log(this.salary);
   };
};
var emp1 = new Employee("Amar","Mars",3000);

Here, while creating each variable, each instance makes a new copy of emp1, emp2, and emp3 in IncreaseSalary. Thus, making it inefficient for your server end.

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19. What do you mean by the Temporal Dead Zone in ES6?

In ES6, Temporal Dead Zone is a behavior occurring in JavaScript while declaring a variable with the let and const keywords. The period between entering the scope and being declared is the one when these keywords cannot be accessed and enter the Temporal Dead Zone.

Also Read: JQuery Questions
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20. Is JavaScript as Pass by value or pass by reference language?

Interestingly enough, it is both. Primitive types like number, string, etc. are passed by value, but objects can be passed-by-value (when we consider a variable holding an object is a reference to the object) and also as a pass-by-reference (when we consider variable to the object is holding the object itself).

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21. How to "deep-freeze" an object in JavaScript?

To ensure that an object is deep-frozen, you will have to create a recursive function for freezing each property of type object: Without deep freeze.

Example

                                                    

let person = {
    name: "Ankit",
    profession: {
        name: "content writer"
    }
};
Object.freeze(person);
person.profession.name = "content writer";
console.log(person);

Output { name: 'Ankit', profession: { name: 'content writer' } }

Now, using Deep Freeze for the object,

function deepFreeze(object) {
    let propNames = Object.getOwnPropertyNames(object);
    for (let name of propNames) {
        let value = object[name];
        object[name] = value && typeof value === "object" ?
            deepFreeze(value) : value;
    }
    return Object.freeze(object);
}
let person = {
    name: "Ankit",
    profession: {
        name: "content writer"
    }
};
deepFreeze(person);
person.profession.name = "content writer";

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22. Why is the "this" operator inconsistent In JavaScript?

In JavaScript, this operator always refers to the object, which is invoking the function being executed. So, if the function is currently being used as an event handler, this operator will refer to the node which fired the event.

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23. How to calculate the Fibonacci series in JavaScript?

Here's the code to calculate the Fibonacci series in JS code:

Example

                                                    

var the_fibonacci_series = function (n)
{
  if (n===1)
  {
    return [0, 1];
  }
  else
  {
    var a = the_fibonacci_series(n - 1);
    a.push(a[a.length - 1] + a[a.length - 2]);
    return a;
  }
};

console.log(the_fibonacci_series(9));

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Top 10 Advanced JavaScript Interview Questions

Here you will find the list of Top Advanced JavaScript Interview Questions, which were written under the supervision of industry experts. These are the most common questions and usually asked in every interview for the role of the JavaScript Developers.

  • What is prototype?
  • What is Callback hell?
  • What is callback?
  • What are the difference between arrow functions & normal functions?
  • What is JavaScript Hoisting?
  • What are the difference between let, var & const?
  • What is Promises?
  • What is the difference between Map & filter function?
  • How to remove duplicate values in array?
  • What are Closure?
  • What is rest & spread operators?
  • What are the data types used in JavaScript?
About Umesh Singh
Umesh Singh
Full Stack Developer

My name is Umesh Singh having 7+ experience in Node.JS, React JS, PHP, Laravel, WordPress, MySQL, JavaScript, HTML and CSS etc. I have worked on around 20+ projects. I love programming, coding and most of my time goes in learning the best ways to write clean code. When I am not found coding, I can be seen helping out tech geeks. I am always open to help you.