“Top Frontend Interview Questions : (Part 2) — Boost Your Knowledge and Ace Your Next Job Interview!”

Ashwini Paraye
8 min readMay 4, 2023

Welcome back! In our previous article, we covered some common frontend interview questions that can help you prepare for your next job interview. In this article, we will continue our discussion and dive into some additional interview questions that you might encounter during your interview process.

1. How many types of scope are there in JavaScript? :

Scope is a term used in programming to describe the accessibility of variables, functions, and objects within a particular section of code.

  • Global Scope: This is the outermost scope in JavaScript, and any variable declared outside of a function or block is considered a global variable. Global variables are accessible from anywhere in the code, but they can also be overwritten accidentally, leading to bugs and unintended consequences.
  • Local Scope or Function Scope: Local scope is created by functions and blocks, and any variable declared inside a function or block is considered a local variable. Local variables are only accessible within the function or block in which they are declared, which helps to avoid naming conflicts and unintended changes to variables.
  • Block Scope: Block scope refers to variables that are declared inside a block of code, such as inside an if statement or for loop. These variables are only accessible within that block and are not visible outside it.
  • Lexical Scope: Lexical scope is the ability of a function to access variables from its outer function or global scope. This means that a function can access variables from its parent function or the global scope, but not from its child functions or other scopes. Lexical scope allows for the creation of private variables and functions, which can only be accessed from within the scope they were defined in. This is useful for encapsulating data and functionality, and helps to prevent naming collisions and other bugs.

2. What is “this” keyword in JavaScript :

  • In JavaScript, this is a keyword that refers to the current execution context, which could be an object, a function, or the global context. The value of this depends on how a function is called or executed.

// in the browser: Window object, in Node.js: global object
  • When a function is called as a method of an object, this refers to the object that the method is called on.
  • When a function is called as a standalone function, this refers to the global object (window in the browser or global in Node.js) in non-strict mode, and undefined in strict mode.
  • You can use call, apply, or bind methods to explicitly set the value of this when calling a function.
  • When this is used in an arrow function, it inherits the value of this from its surrounding context (lexical scope) rather than having its own value.
const person = {
name: 'John',
sayHello: () => {
console.log(`Hello, my name is ${this.name}`);

person.sayHello(); // Hello, my name is undefined
  • In the above example, this inside the arrow function sayHello refers to the global object, not the person object.

3. What is Call, Apply and Bind :

call() method :

  • call() is a method that allows you to invoke a function with a specific this value and arguments.
  • The first argument to call() is the value that this should refer to when the function is executed.
  • Subsequent arguments to call() are the arguments that should be passed to the function.

apply() method :

  • apply() is similar to call(), but it takes an array of arguments instead of individual arguments.
  • The first argument to apply() is the value that this should refer to when the function is executed.
  • The second argument to apply() is an array of arguments that should be passed to the function.

bind() method :

  • bind() returns a new function with a specific this value and any arguments that are passed to it.
  • The first argument to bind() is the value that this should refer to when the new function is called.
  • Subsequent arguments to bind() are arguments that will be passed to the new function when it is called.

Here’s an example that demonstrates the use of these methods:

const person = {
name: 'John'

function sayHello(greeting) {
console.log(`${greeting}, my name is ${this.name}.`);

sayHello.call(person, 'Hello'); // Hello, my name is John.

sayHello.apply(person, ['Hello']); // Hello, my name is John.

const boundHello = sayHello.bind(person, 'Hello');
boundHello(); // Hello, my name is John.
  • In this example, we have an object person with a name property, and a function sayHello() that takes a greeting parameter and logs a message to the console using the this keyword.
  • We use call() and apply() to invoke the sayHello() function with person as the this value and pass a greeting argument. We use bind() to create a new function boundHello with person as the this value and a greeting argument, and then call the new function.

4. Difference between regular function and arrow function :

  • Syntax: Arrow functions have a shorter and more concise syntax than regular functions. Regular functions are created using the “function” keyword, while arrow functions use the “=>” syntax.
  • this binding: Arrow functions do not have their own ‘this’ binding and instead inherit it from the parent scope, whereas regular functions have their own ‘this’ binding.
  • Usage: Arrow functions are generally used for simpler functions, while regular functions are used for more complex functionality.
  • Return statement: Arrow functions can implicitly return a value without the use of the ‘return’ keyword, while regular functions require an explicit ‘return’ statement to return a value.
  • Constructor: Arrow functions cannot be used as constructors to create new objects, while regular functions can be used in this way.

5. What is Memoization :

  • Memoization is a technique used to improve the performance of a function by caching its results.
  • When a function is memoized, the first time it is called with a specific set of arguments, it computes the result and stores it in a cache.
  • If the function is called again with the same arguments, it returns the cached result instead of recomputing it.
  • Memoization can be useful when a function is computationally expensive or time-consuming, and its results don’t change often.
  • Memoization can be implemented using an object, where the keys are the function arguments and the values are the computed results.
  • When a function is called, its arguments are used to look up the cached result in the object. If a cached result is found, it is returned. Otherwise, the function is computed, and its result is stored in the cache for future use.
  • Memoized functions can be implemented using higher-order functions, decorators, or closures.
  • It’s important to note that memoization should be used with caution, as it can lead to memory leaks if not implemented properly.
  • In React.js when a component is memoized, it will only re-render it if its props have changed, avoiding unnecessary re-renders.
  • Memoized components should be pure components, meaning they don’t have any side effects and always produce the same output for the same input.
  • Memoization can also be achieved using the useCallback() hook, which memoizes a function and returns a memoized version of it.
  • Memoization can also be achieved using the useMemo() hook, which memoizes a value and returns a memoized version of it.

6. What is spread operator and rest operator :

In javascript, both the spread operator and rest parameter have the same syntax which is three dots(…). Even though they have the same syntax they differ in functions.

Spread Operator :

  • The spread operator has a syntax of three dots (…).
  • It can expand an iterable such as an array or object.
  • It can be used to merge two or more arrays into one.
  • It can also be used to add an element to an array.
  • The spread operator can copy properties from one object to another object.

Rest Operator :

  • The rest operator is opposite to the spread operator.
  • While the spread operator expands elements of an iterable, the rest operator compresses them. It collects several elements.
  • Both the spread and rest operator are denoted by three dots (…).
  • In functions when we require to pass arguments but were not sure how many we have to pass, the rest parameter makes it easier.
  • The rest parameter must always be the last parameter in a function declaration & there must be only one rest parameter in javascript functions.

Example :


const numbers = [1, 2, 3];

// Spread the array into a new array
const newNumbers = [...numbers];
console.log(newNumbers); // Output: [1, 2, 3]

// Add new items to the array using spread
const moreNumbers = [...numbers, 4, 5];
console.log(moreNumbers); // Output: [1, 2, 3, 4, 5]

//Collecting multiple arguments into an array:
function myFunc(...args) {

myFunc(1, 2, 3); // Output: [1, 2, 3]
myFunc('a', 'b', 'c', 'd'); // Output: ['a', 'b', 'c', 'd']

//Extracting specific elements from an array:
const arr = [1, 2, 3, 4, 5];

const [first, second, ...rest] = arr;

console.log(first); // Output: 1
console.log(second); // Output: 2
console.log(rest); // Output: [3, 4, 5]

//Calculating the sum of an arbitrary number of arguments:
function sum(...args) {
return args.reduce((acc, val) => acc + val);

console.log(sum(1, 2, 3)); // Output: 6
console.log(sum(4, 5, 6, 7)); // Output: 22

7. What is Virtual DOM :

  • The Virtual DOM is a programming concept used in React, a popular JavaScript library for building user interfaces.
  • It is a lightweight copy of the real DOM, which is the web browser’s representation of the HTML elements on a web page.
  • The Virtual DOM is a tree-like structure that represents the current state of the UI components in memory.
  • When a user interacts with a React application and triggers a state change, the Virtual DOM is updated, and React compares it to the previous version of the Virtual DOM.
  • React then uses a diffing algorithm which calculates the difference between the two versions of the Virtual DOM and updates the real DOM with the minimal number of changes required to make it match the new state of the application.
  • This process is called “reconciliation,” and it is the key to React’s high performance and efficiency.
  • In summary, the Virtual DOM is not an actual implementation of the DOM, but rather a concept that helps developers optimize the rendering process of web applications.

8. Advantages of React.js :

  • React.js is a popular front-end JavaScript library that is used for building user interfaces (UI).
  • One of the main advantages of React.js is that it allows for the creation of reusable UI components, which can save time and reduce code complexity.
  • React.js uses a virtual DOM, which can improve performance by minimizing the number of updates required to the actual DOM.
  • React.js provides a one-way data binding approach, which can make code easier to understand and maintain.
  • React.js can be used with other libraries and frameworks, making it highly adaptable and flexible.
  • React.js is scalable, which means it can be used to build complex and large-scale applications.

9. What is one-way data binding :

  • One-way data binding is a data flow architecture used in React.js.
  • One-way data binding allows the data to flow in one direction only, from the parent component to the child component.
  • The parent component controls the state of the child component by passing down props (properties) as an argument.
  • The child component can receive these props and use them to render its content.
  • The child component can’t directly modify the props passed to it by the parent component.
  • One-way data binding helps to keep the application state more predictable and easier to manage by enforcing a unidirectional flow of data.
  • It is typically used in situations where you want to display data to the user, but not allow the user to modify that data directly.

That’s it for today! In our next part of the article, we will continue to cover more interview questions to help you prepare for your next job interview. Stay tuned & Good Luck!

I hope you found this article helpful. Thank you for reading! 😀

Happy Coding !



Ashwini Paraye

Fullstack Developer | 👨‍💻 Writing about programming concepts | React.js | JavaScript | Vue.js | Golang