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Top 100 Developer Interview Questions and Answers

Top 100 Developer Interview Questions and Answers

Contents show

1. What is a closure in JavaScript?

Answer:
A closure is a function that retains access to its outer function’s scope even after the outer function has finished executing. This allows the inner function to access variables and parameters from the outer function.

Code Snippet:

function outerFunction() {
  const message = 'Hello, ';
  function innerFunction(name) {
    console.log(message + name);
  }
  return innerFunction;
}

const greet = outerFunction();
greet('John'); // Output: Hello, John

2. Explain the concept of RESTful APIs.

Answer:
RESTful APIs (Representational State Transfer) use standard HTTP methods (GET, POST, PUT, DELETE) to perform operations on resources. They follow a stateless, client-server architecture and are widely used for web services.

Reference: RESTful API Design


3. How do you handle exceptions in Python?

Answer:
In Python, you can use a try block to enclose code that might raise an exception. Use except to handle specific exceptions or a generic except for all exceptions.

Code Snippet:

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")

4. What is the difference between let, const, and var in JavaScript?

Answer:
let and const were introduced in ES6. let allows variable reassignment, while const creates a constant variable. var is function-scoped, while let and const are block-scoped.

Code Snippet:

let x = 10;
const y = 20;
var z = 30;

5. Explain the concept of object-oriented programming (OOP).

Answer:
OOP is a programming paradigm based on the concept of “objects”. Objects can contain data (attributes) and behavior (methods). It promotes modularity, reusability, and extensibility in code.

Reference: Object-Oriented Programming Concepts


6. How do you perform a database query in Node.js using the mysql module?

Answer:

const mysql = require('mysql');

const connection = mysql.createConnection({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'mydatabase'
});

connection.connect();

const query = 'SELECT * FROM users';
connection.query(query, (error, results, fields) => {
  if (error) throw error;
  console.log('Results: ', results);
});

connection.end();

Reference: mysql Node.js Documentation


7. What is a Promise in JavaScript?

Answer:
A Promise is an object representing the eventual completion or failure of an asynchronous operation. It is used to handle asynchronous operations in JavaScript.

Code Snippet:

const myPromise = new Promise((resolve, reject) => {
  if (success) {
    resolve('Success');
  } else {
    reject('Error');
  }
});

myPromise.then(result => console.log(result))
         .catch(error => console.error(error));

8. Explain the concept of version control and give an example of a version control system.

Answer:
Version control is a system that records changes to files over time. It allows you to track, revert, and collaborate on code. An example is Git, a distributed version control system.

Reference: Git Documentation


9. How do you handle form submissions in React?

Answer:
In React, you can handle form submissions by using the useState hook to manage form inputs and the onSubmit event to trigger a function.

Code Snippet:

import React, { useState } from 'react';

function MyForm() {
  const [inputValue, setInputValue] = useState('');

  const handleSubmit = (e) => {
    e.preventDefault();
    console.log('Submitted Value:', inputValue);
  }

  return (
    <form onSubmit={handleSubmit}>
      <input
        type="text"
        value={inputValue}
        onChange={(e) => setInputValue(e.target.value)}
      />
      <button type="submit">Submit</button>
    </form>
  );
}

10. What is the purpose of the map function in JavaScript?

Answer:
The map function in JavaScript is used to iterate over an array and transform its elements. It creates a new array with the results of applying a provided function to each element.

Code Snippet:

const numbers = [1, 2, 3, 4, 

5];
const doubled = numbers.map(num => num * 2);
// doubled will be [2, 4, 6, 8, 10]

11. How do you sort an array of objects based on a specific property in JavaScript?

Answer:

const users = [
  { name: 'John', age: 30 },
  { name: 'Jane', age: 25 },
  { name: 'Bob', age: 35 }
];

users.sort((a, b) => a.age - b.age);

12. Explain the concept of a virtual DOM in React.

Answer:
The virtual DOM is a lightweight copy of the actual DOM. It is used to improve performance by reducing the number of direct manipulations on the actual DOM.

Reference: React Virtual DOM


13. How do you make an AJAX request in jQuery?

Answer:

$.ajax({
  url: '/api/data',
  method: 'GET',
  success: function(data) {
    console.log('Data:', data);
  },
  error: function(error) {
    console.error('Error:', error);
  }
});

Reference: jQuery.ajax()


14. What is the purpose of a CSS preprocessor like Sass or Less?

Answer:
CSS preprocessors extend the capabilities of regular CSS by adding features like variables, nesting, and functions. They improve maintainability and organization of stylesheets.

Reference: Sass Documentation


15. How do you handle form validation in JavaScript?

Answer:

const form = document.getElementById('myForm');
form.addEventListener('submit', function(event) {
  event.preventDefault();
  const inputValue = document.getElementById('inputField').value;
  if (inputValue === '') {
    alert('Please enter a value.');
  } else {
    // Continue with form submission
  }
});

16. What is the purpose of the useEffect hook in React?

Answer:
The useEffect hook in React is used to perform side effects in function components. It can be used for tasks like data fetching, subscriptions, or manually changing the DOM.

Reference: React useEffect()


17. How do you handle asynchronous code in JavaScript?

Answer:
You can use callbacks, Promises, or async/await to handle asynchronous code in JavaScript.

Code Snippet (Using Promises):

function fetchData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('Data fetched successfully');
    }, 2000);
  });
}

fetchData().then(result => console.log(result));

18. Explain the concept of a closure in Python.

Answer:
In Python, a closure is a nested function that captures and remembers the environment in which it was created. It allows the inner function to access variables from the outer function, even after the outer function has finished executing.

Code Snippet:

def outer_function(message):
    def inner_function():
        print(message)
    return inner_function

my_func = outer_function('Hello')
my_func()  # Output: Hello

19. How do you handle routing in a React application?

Answer:
You can use libraries like React Router to handle routing in a React application.

Code Snippet (Using React Router):

npm install react-router-dom
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

function App() {
  return (
    <Router>
      <Switch>
        <Route exact path="/" component={Home} />
        <Route path="/about" component={About} />
        <Route path="/contact" component={Contact} />
      </Switch>
    </Router>
  );
}

Reference: React Router Documentation


20. How do you handle state management in a large React application?

Answer:
For large React applications, you can use state management libraries like Redux or Context API along with hooks like useReducer.

Reference (Redux): Redux Documentation


21. What is the purpose of the this keyword in JavaScript?

Answer:
The this keyword refers to the context in which a function is called. Its value depends on how and where a function is invoked.

Code Snippet:

const person = {
  name: 'John',
  greet: function() {
    console.log(`Hello, my name is ${this.name}.`);
  }
};

person.greet(); // Output: Hello, my name is John.

22. How do you implement a Singleton pattern in JavaScript?

Answer:

class Singleton {
  constructor() {
    if (!Singleton.instance) {
      Singleton.instance = this;
    }
    return Singleton.instance;
  }
}

const instance1 = new Singleton();
const instance2 = new Singleton();

console.log(instance1 === instance2); // Output: true

23. Explain the concept of Hoisting in JavaScript.

Answer:
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during the compile phase.

Code Snippet (Variable Hoisting):

console.log(myVar); // Output: undefined
var myVar = 10;

24. What are arrow functions in JavaScript and how do they differ from regular functions?

Answer:
Arrow functions are a concise way to write anonymous functions in JavaScript. They do not have their own this and arguments bindings, which can lead to different behavior compared to regular functions.

Code Snippet:

const add = (a, b) => a + b;

25. How do you handle exceptions in Python?

Answer:

try:
    # Code that may raise an exception
    result = 10 / 0
except ZeroDivisionError as e:
    print(f'Error: {e}')

26. Explain the difference between localStorage and sessionStorage in HTML5.

Answer:
localStorage stores data with no expiration date, while sessionStorage stores data for the duration of the page session.

Code Snippet (Using localStorage):

localStorage.setItem('key', 'value');
const storedValue = localStorage.getItem('key');

27. What is a RESTful API?

Answer:
A RESTful API (Representational State Transfer) is an architectural style for designing networked applications. It uses a stateless, client-server communication protocol.

Reference: RESTful API Design


28. How do you handle 404 errors in a Node.js Express application?

Answer:

app.use((req, res) => {
  res.status(404).send('Page not found');
});

29. Explain the concept of Object-Oriented Programming (OOP).

Answer:
OOP is a programming paradigm based on the concept of “objects”, which can contain data and code. It focuses on the organization of code into reusable, self-contained units.

Reference: OOP Concepts


30. How do you optimize a website for performance?

Answer:
Optimizing website performance involves tasks like minimizing HTTP requests, optimizing images, using asynchronous loading, and utilizing browser caching.


31. What is the purpose of the use strict directive in JavaScript?

Answer:
'use strict' is used to enable strict mode in JavaScript, which enforces more stringent parsing and error handling. It helps catch common coding mistakes and prevent the use of certain error-prone features.

Code Snippet:

'use strict';

// Code here is in strict mode

32. How do you handle asynchronous code in JavaScript?

Answer:
You can use callbacks, promises, or async/await for handling asynchronous code. Promises provide a cleaner and more structured way to deal with asynchronous operations.

Code Snippet (Using Promises):

function fetchData() {
  return new Promise((resolve, reject) => {
    // Asynchronous operation
    if (success) {
      resolve(data);
    } else {
      reject(error);
    }
  });
}

33. What is the purpose of the map function in JavaScript?

Answer:
The map function creates a new array by applying a provided function to each element in the original array. It does not mutate the original array.

Code Snippet:

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(num => num * 2);

34. Explain the concept of SQL Joins.

Answer:
SQL Joins are used to combine rows from two or more tables based on a related column between them. Common types include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN.

Code Snippet (SQL Query):

SELECT *
FROM table1
INNER JOIN table2 ON table1.column = table2.column;

35. How do you handle form submission in React?

Answer:
In React, you can use the useState hook to manage form inputs and the handleSubmit function to handle form submission.

Code Snippet (React Component):

import React, { useState } from 'react';

const MyForm = () => {
  const [formData, setFormData] = useState({});

  const handleChange = (e) => {
    setFormData({ ...formData, [e.target.name]: e.target.value });
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    // Handle form submission
  };

  return (
    <form onSubmit={handleSubmit}>
      <input type="text" name="username" onChange={handleChange} />
      <button type="submit">Submit</button>
    </form>
  );
};

36. Explain the concept of virtual DOM in React.

Answer:
The virtual DOM is a lightweight copy of the actual DOM. It allows React to efficiently update the real DOM by minimizing the number of updates needed.

Reference: React’s Virtual DOM


37. How do you implement a stack data structure in Python?

Answer:

class Stack:
    def __init__(self):
        self.items = []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def is_empty(self):
        return not bool(self.items)

    def peek(self):
        return self.items[-1] if self.items else None
}

38. What is a closure in JavaScript?

Answer:
A closure is a function that has access to its own scope, the outer function’s scope, and the global scope. It can access variables even after the outer function has finished execution.

Code Snippet:

function outerFunction() {
  const message = 'Hello';

  return function innerFunction() {
    console.log(message);
  }
}

39. How does Git differ from SVN?

Answer:
Git is a distributed version control system, while SVN (Subversion) is centralized. Git allows for local commits and branching, making it more flexible and efficient for collaborative development.

Reference: Git vs. SVN


40. What is the purpose of a Docker container?

Answer:
A Docker container is a lightweight, portable, and self-sufficient environment that can run applications in isolation. It ensures consistent deployment across different environments.

Reference: Docker Documentation


41. How do you handle exceptions in Python?

Answer:
In Python, you can use try-except blocks to handle exceptions. This allows you to catch and respond to errors gracefully.

Code Snippet:

try:
    # Code that may raise an exception
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")

42. Explain the concept of RESTful APIs.

Answer:
RESTful APIs follow the principles of Representational State Transfer (REST). They use standard HTTP methods (GET, POST, PUT, DELETE) to perform CRUD operations on resources.

Reference: RESTful API Design


43. How do you optimize the performance of a website?

Answer:
Optimizing website performance involves techniques like minimizing HTTP requests, using asynchronous loading, optimizing images, and leveraging browser caching.

Reference: Website Performance Optimization


44. What is the purpose of the this keyword in JavaScript?

Answer:
this refers to the context in which a function is called. In global scope, it refers to the global object. In a method, it refers to the object that owns the method.

Code Snippet:

const obj = {
  name: 'John',
  sayHello: function() {
    console.log(`Hello, ${this.name}!`);
  }
};
obj.sayHello();

45. How do you create a responsive web design?

Answer:
Use CSS media queries to adjust the layout based on the user’s device or screen size. Design with relative units like percentages and ems, and use flexible grids.

Reference: Responsive Web Design Basics


46. Explain the concept of Object-Oriented Programming (OOP).

Answer:
OOP is a programming paradigm that organizes code around objects and their interactions. It emphasizes concepts like encapsulation, inheritance, and polymorphism.

Reference: OOP Concepts


47. How do you handle CORS (Cross-Origin Resource Sharing) in a web application?

Answer:
You can configure the server to include appropriate CORS headers or use middleware to handle CORS. In JavaScript, you can also use JSONP or a proxy server.

Reference: MDN Web Docs – CORS


48. What is the purpose of a package manager?

Answer:
A package manager automates the process of installing, upgrading, configuring, and removing software packages. It helps manage dependencies and keeps software up-to-date.

Reference: npm – About npm


49. How do you implement a binary search algorithm in Python?

Answer:

def binary_search(arr, target):
    left, right = 0, len(arr) - 1

    while left <= right:
        mid = (left + right) // 2
        if arr[mid] == target:
            return mid
        elif arr[mid] < target:
            left = mid + 1
        else:
            right = mid - 1

    return -1

50. Explain the difference between synchronous and asynchronous programming.

Answer:
In synchronous programming, tasks are executed one after the other in a blocking manner. In asynchronous programming, tasks can be executed concurrently, allowing for non-blocking operations.


51. What is a closure in JavaScript?

Answer:
A closure is a function that retains access to variables from its containing (enclosing) function’s scope even after the enclosing function has finished executing.

Code Snippet:

function outer() {
  const message = "Hello";
  function inner() {
    console.log(message);
  }
  return inner;
}
const myFunction = outer();
myFunction(); // Outputs: "Hello"

52. Explain the concept of version control and its importance in software development.

Answer:
Version control is a system that tracks and manages changes to code over time. It allows multiple developers to collaborate, revert to previous states, and maintain a history of changes.

Reference: Git – About Version Control


53. What is the difference between == and === in JavaScript?

Answer:
== checks for equality after type coercion, while === checks for strict equality without type coercion.

Code Snippet:

console.log(5 == "5");   // true (coerced)
console.log(5 === "5");  // false (strict)

54. How can you optimize SQL queries for better performance?

Answer:
Optimizing SQL queries involves using indexes, minimizing the use of SELECT *, avoiding unnecessary joins, and using efficient aggregation functions.

Reference: SQL Query Performance Tuning


55. Explain the purpose of the DOCTYPE declaration in HTML.

Answer:
The DOCTYPE declaration specifies the document type and version of HTML being used. It helps the browser render the page correctly by following the specified standards.

Reference: HTML DOCTYPE Declaration


56. What is the difference between cookies and localStorage in web development?

Answer:
Cookies are small pieces of data sent from a server and stored on the client’s device, while localStorage is a client-side storage mechanism. Cookies have limitations on size and are sent with every HTTP request, while localStorage can store larger amounts of data and is accessible only on the client side.

Reference: Cookies vs. Web Storage


57. How do you handle concurrency in multi-threaded programming?

Answer:
Concurrency issues can be addressed through synchronization mechanisms like mutexes and semaphores. These ensure that multiple threads don’t access shared resources simultaneously, preventing data corruption.

Reference: Java Concurrency Basics


58. Explain the concept of responsive images in web design.

Answer:
Responsive images adapt to different screen sizes and resolutions. You can use attributes like srcset and sizes in HTML to provide different image sources based on the user’s device.

Reference: Responsive Images in HTML


59. What is the purpose of the finally block in a try-catch-finally statement in Java?

Answer:
The finally block is used to specify code that should be executed regardless of whether an exception is thrown or not. It’s often used for cleanup tasks like closing files or releasing resources.

Reference: Java try-catch-finally


60. How does the “box model” work in CSS?

Answer:
The CSS box model describes how elements are displayed as rectangular boxes with content, padding, borders, and margins. It defines the layout and spacing of elements on a web page.


61. Explain the concept of RESTful APIs.

Answer:
REST (Representational State Transfer) is an architectural style for designing networked applications. RESTful APIs use HTTP methods like GET, POST, PUT, DELETE to perform operations on resources. They are stateless and can be accessed via URLs.

Reference: RESTful API Design


62. What is the purpose of the async and await keywords in JavaScript?

Answer:
async is used to declare that a function returns a promise. await is used inside an async function to pause the execution until the promise is resolved, making asynchronous code look more like synchronous.

Code Snippet:

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  return data;
}

63. How does a binary search algorithm work?

Answer:
Binary search is a divide-and-conquer algorithm. It works by repeatedly dividing the search space in half. If the value of the search key is less than the item in the middle, the search continues in the lower half; otherwise, it continues in the upper half.

Code Snippet (in Python):

def binary_search(arr, target):
    low, high = 0, len(arr) - 1
    while low <= high:
        mid = (low + high) // 2
        if arr[mid] == target:
            return mid
        elif arr[mid] < target:
            low = mid + 1
        else:
            high = mid - 1
    return -1

64. Explain the concept of Object-Oriented Programming (OOP) and its pillars.

Answer:
OOP is a programming paradigm based on the concept of “objects”. It emphasizes the organization of code around data, rather than just functions and logic. The four pillars of OOP are encapsulation, inheritance, polymorphism, and abstraction.

Reference: Object-Oriented Programming Concepts


65. How does a linked list differ from an array?

Answer:
An array is a collection of elements accessed by an index, while a linked list is a collection of nodes where each node points to the next one in the list. Arrays have fixed sizes, while linked lists can dynamically grow and shrink.

Code Snippet (in C++ for linked list insertion):

struct Node {
    int data;
    Node* next;
};

void insert(Node* &head, int data) {
    Node* newNode = new Node{data, head};
    head = newNode;
}

66. What is the purpose of the this keyword in JavaScript?

Answer:
this refers to the context in which a function is called. In a method, this refers to the object on which the method is called. In a simple function, this refers to the global object.

Code Snippet:

const obj = {
  value: 42,
  getValue: function() {
    return this.value;
  }
};

67. Explain the concept of Big O notation in algorithm analysis.

Answer:
Big O notation is used to describe the performance or complexity of an algorithm. It provides an upper bound on the time or space complexity in the worst-case scenario.

Reference: Big O Notation


68. How does the event loop work in JavaScript?

Answer:
The event loop is a mechanism that allows JavaScript to perform non-blocking I/O operations. It continuously checks the call stack for functions to execute, and queues tasks to be processed in the callback queue.

Reference: Concurrency model and the event loop


69. Explain the concept of design patterns in software development.

Answer:
Design patterns are reusable solutions to common problems in software design. They provide templates for how to structure code to solve specific design issues.

Reference: Design Patterns


70. What is the purpose of a Docker container in software development?

Answer:
A Docker container is a lightweight, portable, and self-sufficient unit that can run any application and its dependencies in isolation. It provides a consistent environment across different systems.

Reference: Docker Overview


71. What is a closure in JavaScript?

Answer:
A closure is a function that remembers the variables from the outer scope in which it was created. It has access to its own scope, the outer function’s variables, and the global variables.

Code Snippet:

function outerFunction() {
  const message = 'Hello, ';
  function innerFunction(name) {
    console.log(message + name);
  }
  return innerFunction;
}

const greet = outerFunction();
greet('John'); // Output: Hello, John

72. Explain the concept of a callback function in JavaScript.

Answer:
A callback function is a function passed as an argument to another function, which is then invoked inside the outer function. It is used to ensure that a function is executed only after the previous function has finished.

Code Snippet:

function doSomething(callback) {
  // Do something here
  callback();
}

function onComplete() {
  console.log('Operation complete');
}

doSomething(onComplete);

73. What is the purpose of a try…catch block in JavaScript?

Answer:
A try…catch block is used for error handling in JavaScript. Code that may throw an error is placed inside the try block, and if an error occurs, it’s caught in the catch block. This prevents the program from crashing.

Code Snippet:

try {
  // Code that may throw an error
  const result = 10 / 0;
  console.log(result);
} catch (error) {
  console.error('An error occurred:', error.message);
}

74. Explain the concept of CORS (Cross-Origin Resource Sharing) in web development.

Answer:
CORS is a security feature implemented by web browsers that restricts web pages from making requests to a different domain. It prevents a web page from making requests to a different domain than the one that served the web page.

Reference: MDN Web Docs – Same-origin policy


75. What is the purpose of the localStorage and sessionStorage objects in web browsers?

Answer:
localStorage and sessionStorage are Web API objects used to store key-value pairs in a web browser. localStorage persists even when the browser is closed, while sessionStorage is cleared when the session ends.

Code Snippet (localStorage):

// Set value
localStorage.setItem('username', 'JohnDoe');

// Get value
const username = localStorage.getItem('username');
console.log(username); // Output: JohnDoe

76. Explain the concept of Hoisting in JavaScript.

Answer:
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during the compile phase. However, only the declarations are hoisted, not the initializations.

Code Snippet:

console.log(myVar); // Output: undefined
var myVar = 10;

77. What is a package manager in software development?

Answer:
A package manager is a tool used to automate the process of installing, upgrading, configuring, and removing software packages or libraries. It helps manage dependencies and ensures consistent development environments.

Reference: npm (Node Package Manager)


78. Explain the purpose of the useState hook in React.

Answer:
useState is a hook in React that allows functional components to have state. It returns an array with two elements: the current state value and a function that lets you update it.

Code Snippet (React):

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

79. Explain the purpose of the useEffect hook in React.

Answer:
The useEffect hook in React allows you to perform side effects in your functional components. It is similar to lifecycle methods in class components. It’s commonly used for tasks like data fetching, subscriptions, or manually changing the DOM.

Code Snippet (React):

import React, { useState, useEffect } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    document.title = `Count: ${count}`;
  }, [count]);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

80. What is a RESTful API?

Answer:
A RESTful API (Representational State Transfer) is an architectural style for designing networked applications. It uses standard HTTP methods (GET, POST, PUT, DELETE) to perform CRUD (Create, Read, Update, Delete) operations on resources identified by URLs.

Reference: RESTful API – Wikipedia


81. Explain the difference between let, const, and var in JavaScript.

Answer:

  • var is function-scoped and can be redeclared within the same scope. It’s also hoisted to the top of its scope.
  • let is block-scoped, can’t be redeclared in the same scope, and isn’t hoisted.
  • const is also block-scoped and cannot be reassigned after initialization. It must be assigned a value when declared.

Code Snippet:

var x = 10;
let y = 20;
const z = 30;

82. What is the purpose of the map function in JavaScript?

Answer:
The map function in JavaScript is used to iterate over an array and apply a function to each element, creating a new array with the modified elements. It doesn’t modify the original array.

Code Snippet:

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // Output: [2, 4, 6, 8]

83. What is a callback hell in JavaScript?

Answer:
Callback hell, also known as the pyramid of doom, is a situation in JavaScript programming where callback functions are nested within each other, making the code hard to read and maintain. It arises from heavy use of asynchronous operations.

Code Snippet:

asyncFunction1(() => {
  asyncFunction2(() => {
    asyncFunction3(() => {
      // ...
    });
  });
});

84. Explain the concept of memoization in programming.

Answer:
Memoization is a technique used to optimize the performance of functions by caching the results of expensive function calls. If the same inputs occur again, the cached result is returned, avoiding redundant computations.

Code Snippet:

const memoize = (fn) => {
  const cache = {};
  return (...args) => {
    const key = JSON.stringify(args);
    if (!cache[key]) {
      cache[key] = fn(...args);
    }
    return cache[key];
  };
};

85. What is a closure in Python?

Answer:
A closure in Python is a nested function that has access to a free variable from an enclosing function. It remembers the environment in which it was created, even after the outer function has finished execution.

Code Snippet (Python):

def outer_function(message):
    def inner_function():
        print(message)
    return inner_function

my_closure = outer_function("Hello, World!")
my_closure()  # Output: Hello, World!

86. What is the purpose of a constructor in object-oriented programming?

Answer:
A constructor is a special method in a class that is automatically called when an instance of the class is created. It is used to initialize the object’s properties and perform any necessary setup.

Code Snippet (Python):

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

person1 = Person("John Doe", 30)

87. Explain the concept of method overloading.

Answer:
Method overloading is a feature in object-oriented programming that allows a class to have multiple methods with the same name but with different parameter lists. The correct method to call is determined at compile time based on the number or type of arguments.

Code Snippet (Python):

class Calculator:
    def add(self, a, b):
        return a + b

    def add(self, a, b, c):
        return a + b + c

calc = Calculator()
result = calc.add(1, 2, 3)  # Calls the second add method

88. What is a design pattern? Provide an example.

Answer:
A design pattern is a general repeatable solution to a commonly occurring problem in software design. One example is the Singleton pattern, which ensures a class has only one instance and provides a global point of access to it.

Code Snippet (Python – Singleton):

class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super().__new__(cls, *args, **kwargs)
        return cls._instance

singleton1 = Singleton()
singleton2 = Singleton()

print(singleton1 == singleton2)  # Output: True

89. What is the purpose of version control systems like Git?

Answer:
Version control systems like Git are used to track changes in a project’s codebase over time. They allow multiple contributors to collaborate on a project, keep track of changes, and revert to previous versions if needed.

Reference: Git – Official Website


90. Explain the concept of a foreign key in database design.

Answer:
A foreign key is a field in a relational database table that refers to the primary key in another table. It establishes a relationship between the two tables, allowing for data integrity and enabling the creation of joins.

Code Snippet (SQL):

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    ProductID INT,
    FOREIGN KEY (ProductID) REFERENCES Products(ProductID)
);

91. What is the purpose of the super() function in Python?

Answer:
The super() function in Python is used to call a method in a superclass. It allows you to access inherited methods and properties of a parent class from a subclass.

Code Snippet (Python):

class Parent:
    def __init__(self, name):
        self.name = name

class Child(Parent):
    def __init__(self, name, age):
        super().__init__(name)
        self.age = age

92. Explain the concept of a virtual function in C++.

Answer:
A virtual function is a member function in a base class that is declared using the virtual keyword. It is intended to be overridden by a derived class. When a virtual function is called through a pointer or reference to a base class, the actual implementation in the derived class is executed.

Code Snippet (C++):

class Base {
public:
    virtual void display() {
        cout << "Displaying from Base class" << endl;
    }
};

class Derived : public Base {
public:
    void display() override {
        cout << "Displaying from Derived class" << endl;
    }
};

93. What is the purpose of a callback function in JavaScript?

Answer:
A callback function is a function that is passed as an argument to another function and is executed after the completion of that function. It is used to handle asynchronous operations, allowing code to continue running while waiting for a response.

Code Snippet (JavaScript):

function fetchData(callback) {
    setTimeout(function() {
        const data = 'Some fetched data';
        callback(data);
    }, 2000);
}

function displayData(data) {
    console.log(data);
}

fetchData(displayData);

94. Explain the difference between == and === in JavaScript.

Answer:
== is a loose equality operator in JavaScript that compares values after converting them to a common type. === is a strict equality operator that checks both the value and the type. It does not perform type coercion.

Code Snippet (JavaScript):

console.log(1 == '1');  // Output: true
console.log(1 === '1'); // Output: false

95. What is the purpose of the this keyword in JavaScript?

Answer:
The this keyword in JavaScript refers to the context in which a function is executed. Its value is determined by how a function is called. In a method, this refers to the object that the method is called on.

Code Snippet (JavaScript):

const person = {
    name: 'John Doe',
    greet: function() {
        console.log('Hello, ' + this.name);
    }
};

person.greet(); // Output: Hello, John Doe

96. Explain the concept of a promise in JavaScript.

Answer:
A promise in JavaScript represents the eventual completion or failure of an asynchronous operation. It is an object that may produce a single value some time in the future. Promises are used to handle asynchronous code in a more readable and manageable way.

Code Snippet (JavaScript):

const fetchData = new Promise((resolve, reject) => {
    setTimeout(() => {
        const data = 'Some fetched data';
        resolve(data);
    }, 2000);
});

fetchData.then(data => console.log(data));

97. What is the purpose of the localStorage and sessionStorage objects in web browsers?

Answer:
localStorage and sessionStorage are Web API objects that allow you to store key-value pairs in a web browser. localStorage persists even after the browser is closed, while sessionStorage is limited to the session and is cleared when the browser is closed.

Code Snippet (JavaScript – localStorage):

localStorage.setItem('username', 'JohnDoe');
const username = localStorage.getItem('username');

98. Explain the concept of a RESTful API.

Answer:
A RESTful API (Representational State Transfer) is an architectural style for designing networked applications. It uses a stateless, client-server communication where resources are represented as URIs, and operations (GET, POST, PUT, DELETE) are performed on these resources.

Reference: RESTful API – Wikipedia


99. What is a closure in JavaScript?

Answer:
A closure is a combination of a function and the lexical environment in which it was declared. It allows a function to access variables from its outer scope even after that scope has finished execution.

Code Snippet (JavaScript):

function outerFunction() {
    const message = 'Hello';

    function innerFunction() {
        console.log(message);
    }

    return innerFunction;
}

const closure = outerFunction();
closure(); // Output: Hello

100. Explain the purpose of the __init__ method in Python.

Answer:
The __init__ method is a special method in Python classes that is called when a new instance of the class is created. It is used to initialize the object’s attributes or perform any necessary setup.

Code Snippet (Python):

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

person = Person("John Doe", 30)