Javascript Interview Questions

Basics

Question 1 . What is JavaScript, and what is its primary use in web development?

Ans. JavaScript is a high-level, interpreted programming language used primarily for enhancing the interactivity and functionality of web pages. It allows developers to manipulate the Document Object Model (DOM) of a web page, handle user interactions, validate input, and make asynchronous requests to servers, making web applications dynamic and responsive.

Question 2. Explain the difference between let, const, and var when declaring variables.

Ans. 1.var is function-scoped and hoisted to the top of the function or global context. It can be reassigned and used before declaration. 2. let is block-scoped, meaning it’s limited to the block it’s declared in (e.g., inside a loop or an if statement). It’s not hoisted and can be reassigned. 3.const is also block-scoped but represents a constant value that cannot be reassigned after initialization. It’s not hoisted and should be used when the variable’s value won’t change.

Question 3. How do you write a comment in JavaScript?

Ans. You can write comments in JavaScript using two methods: Single-line comments using //, e.g., // This is a comment. Multi-line comments using /* */

Question 4. What is the purpose of the console.log() method in JavaScript?

Ans. The console.log() method is used for debugging and logging messages to the browser’s console. It’s a helpful tool for developers to inspect values, variables, and the flow of their code during development.

Question 5. What is the significance of the typeof operator in JavaScript?

Ans. The typeof operator is used to determine the data type of a value or expression in JavaScript. It returns a string representing the data type, such as “number”, “string”, “object”, “function”, or “undefined”.

Question 6. How can you create a function in JavaScript?

Ans. “You can create a function in JavaScript using the function keyword or by using arrow functions (introduced in ES6). For example: Function declaration:

function add(a, b) {
return a + b;
}
Arrow function:const add = (a, b) => a + b;

Question 7. What is a JavaScript event handler, and how do you use it?

Ans. A JavaScript event handler is a function that responds to specific events, such as user interactions (e.g., clicks, keypresses). You can attach event handlers to HTML elements using event attributes (e.g., onclick, onmouseover) or through JavaScript code using methods like addEventListener().

Question 8.How do you add an element to an array in JavaScript?

Ans. You can add an element to an array in JavaScript using various methods, such as: push(): Adds elements to the end of the array. unshift(): Adds elements to the beginning of the array. splice(): Adds elements at a specific position by specifying the index. concat(): Combines arrays to create a new array.

Question 9. Explain the difference between == and === in JavaScript for equality comparison.

Ans. (== loose equality) compares values for equality after performing type coercion if necessary. For example, 1 == ‘1’ is true, (===strict equality) compares both values and types. It only returns true if both the value and type are the same. For example, 1 === ‘1’ is false

Question 10. What is the DOM (Document Object Model) in web development?

Ans. The Document Object Model (DOM) is a programming interface provided by browsers that represents the structure of a web page as a hierarchical tree of objects. It allows JavaScript to interact with and manipulate the content and structure of a web page dynamically. Developers can access, modify, or create HTML elements and attributes using the DOM.

Question 11. How do you select an HTML element in JavaScript using its id attribute?

Ans. You can select an HTML element by its id attribute using the getElementById() method, like this: const element = document.getElementById(‘myElementId’);

Question 12. What is an anonymous function in JavaScript?

Ans. An anonymous function is a function that does not have a name. It can be defined and used without assigning it a name. Anonymous functions are often used as callbacks or for immediately invoked function expressions (IIFE).

Question 13. How do you use setTimeout() in JavaScript to delay code execution?

Ans. “The setTimeout() function is used to execute a specified function or code block after a specified delay (in milliseconds). Here’s an example of its usage:

setTimeout(function () {
console.log(‘Delayed code executed.’);
}, 2000); // Executes after a 2-second delay”

Question 14. Explain the purpose of the null value in JavaScript.

Ans. The null value in JavaScript represents the intentional absence of any object value or the absence of a value. It is often used to indicate that a variable or property should have no value or is not initialized with any meaningful data.

Question 15. What are the benefits of using strict mode in JavaScript?

Ans. Strict mode, enabled by adding “use strict”; at the beginning of a script or function, offers several benefits, including:1.Catches common coding mistakes and “unsafe” actions. 2.Prevents the use of undeclared variables. 3. Disallows reserved words as variable or function names. 4. Restricts certain features, such as assigning to read-only properties. 5. Makes code more optimized for modern JavaScript engines.

Question 16. How can you prevent the default behavior of an HTML form submit event?

Ans. To prevent the default behavior of an HTML form submit event, you can use the event.preventDefault() method within an event handler. This prevents the form from being submitted, allowing you to perform custom actions instead.

Question 17. How do you concatenate strings in JavaScript?

Ans. “You can concatenate strings in JavaScript using the + operator or the concat() method. Here are examples of both methods: 1. Using the + operator:

const str1 = ‘Hello’;
const str2 = ‘World’;
const concatenated = str1 + ‘ ‘ + str2; // Concatenates “”Hello”” and “”World”” 2. Using the concat() method: const str1 = ‘Hello’;
const str2 = ‘World’;
const concatenated = str1.concat(‘ ‘, str2); // Concatenates “”Hello”” and “”World”””

Question 18. Explain the purpose of the JSON.stringify() and JSON.parse() methods in JavaScript.

Ans. “JSON.stringify(): This method is used to convert a JavaScript object into a JSON-formatted string. It serializes the object, including its properties and values, into a string representation. This is commonly used when sending data to a server or storing data in a file. 1.

const person = { name: ‘Alice’, age: 30 };
const jsonString = JSON.stringify(person);
// jsonString is ‘{“”name””:””Alice””,””age””:30}’

JSON.parse(): This method is used to parse a JSON-formatted string and convert it into a JavaScript object. It takes a JSON string as input and returns a corresponding JavaScript object.

const jsonString = ‘{“”name””:””Bob””,””age””:25}’;
const person = JSON.parse(jsonString);
// person is { name: ‘Bob’, age: 25 }”

Question 19. What is the difference between null and undefined in JavaScript?

Ans. null is a value in JavaScript that represents the intentional absence of any object value or a value that indicates no value or no object. It is explicitly assigned to indicate the absence of a value. undefined is a primitive value in JavaScript that indicates a variable has been declared but has not been assigned a value. It is the default value for function parameters that are not provided and for uninitialized variables.

Question 20. What will be the output of the following code?

function foo() {
setTimeout(() => {
console.log(‘Timeout’);
}, 0);
Promise.resolve().then(() => {
console.log(‘Promise’);
});
console.log(‘Function’);
}
foo();
console.log(‘End’);

Ans. Expected Output:

Function

End

Promise

Timeout

Explanation: This demonstrates JavaScript’s event loop behavior. The order of execution is such that the synchronous code (Function and End) is executed first, then microtasks (Promise), and finally the setTimeout callback (Timeout).”

Intermediate

Question 1.What is scope in JavaScript, and how does it work?

Ans. Scope refers to the context in which variables are declared and accessed. JavaScript has function scope and block scope. Function scope means variables declared inside a function are only accessible within that function. Block scope (introduced with let and const in ES6) restricts variable access to the block in which they are defined.

Question 2. How do you handle asynchronous operations in JavaScript?

Ans. Asynchronous operations in JavaScript can be handled using callbacks, promises, or the async/await syntax (ES6+). Callbacks are functions passed as arguments to be executed when an asynchronous task completes. Promises provide a structured way to handle async operations, allowing you to chain .then() and .catch() for handling success and errors. async/await is a syntax for working with promises in a more synchronous-like manner.

Question 3. Explain the concept of closures in JavaScript.

Ans. Closures occur when an inner function has access to the variables and scope of an outer (enclosing) function, even after the outer function has finished executing. Closures are commonly used for data encapsulation, maintaining private data, and creating factory functions. They preserve the scope chain of the enclosing function.

Question 4. What is a callback function in JavaScript, and when would you use one?

Ans. A callback function is a function that is passed as an argument to another function and is executed after the completion of that function. Callbacks are used for asynchronous operations, event handling, and to ensure that certain code runs after a specific task completes. They are a fundamental building block for handling async code in JavaScript.

Question 5. How do you declare a JavaScript class and create instances of it?

Ans. Classes can be declared in JavaScript using the class keyword. You can define a constructor and methods within the class. To create instances, use the new keyword followed by the class name. For example:Javascript

class Person {
constructor(name) {
this.name = name;
}
sayHello() {
console.log(Hello, my name is ${this.name});
}
}
const person = new Person(‘Alice’);
person.sayHello(); // Output: Hello, my name is Alice”

Question 6. Describe the event delegation pattern in JavaScript.

Ans. Event delegation is a design pattern where a single event handler is attached to a common ancestor element of multiple child elements that you want to handle events for. It allows you to handle events on dynamically created elements and reduces the number of event listeners in your application. The event handler checks the target of the event to determine which child element triggered the event.

Question 7. How do you loop through the properties of an object in JavaScript?

Ans. You can loop through object properties using a for…in loop or by using Object.keys(), Object.values(), and Object.entries() methods introduced in ES6. Here’s an example using for…in:

const obj = { a: 1, b: 2, c: 3 };
for (const key in obj) {
if (obj.hasOwnProperty(key)) {
console.log(${key}: ${obj[key]});
}
}

Question 8. What is the difference between a shallow copy and a deep copy of an object?

Ans. A shallow copy of an object creates a new object with its own properties, but if the properties are objects themselves, they are still references to the same objects in memory as the original object. A deep copy, on the other hand, creates a completely new object with its own properties and recursively copies nested objects, ensuring that they are also deep copies with no shared references.

Question 9. Explain what the “this” keyword refers to in different contexts in JavaScript.

Ans. The behavior of the this keyword in JavaScript depends on the context in which it is used: 1. In a method of an object, this refers to the object itself. 2. In a regular function (not in strict mode), this usually refers to the global object (e.g., window in a browser). 3. In an event handler, this often refers to the DOM element that triggered the event. 4. When using arrow functions, this retains the value of the enclosing lexical scope.

Question 10. How do you handle errors in JavaScript using try…catch?

Ans. Errors in JavaScript can be handled using a try…catch block. The try block contains code that might throw an error, and the catch block is executed if an error occurs. You can catch and handle specific types of errors using catch clauses for different error types.

try {
// Code that may throw an error
const result = someFunction();
} catch (error) {
// Handle the error
console.error(error.message);
}

Question 11. What is the purpose of the map() function in JavaScript, and how does it work?

Ans. The map() function is used to create a new array by applying a provided function to each element of an existing array. It iterates through each item in the array, applies the function, and returns a new array with the results. It doesn’t modify the original array.

const numbers = [1, 2, 3];
const squared = numbers.map((num) => num * num);
// Result: [1, 4, 9]”

Question 12. How can you check if an element exists in an array in JavaScript?

Ans. To check if an element exists in an array, you can use methods like indexOf(), includes(), or find(). For example:Javascript

const myArray = [1, 2, 3, 4];
const element = 3;
// Using indexOf()
const exists = myArray.indexOf(element) !== -1;
// Using includes()
const exists = myArray.includes(element);
// Using find()
const exists = myArray.find(item => item === element) !== undefined;

Question 13. Explain the difference between null, undefined, and undeclared variables.

Ans. null is a value in JavaScript that represents the intentional absence of any object value. It is a value that a variable can be explicitly assigned to.

undefined is a special value in JavaScript that represents the absence of a value, often the default value of uninitialized variables.

undeclared variables are variables that have not been declared using any keyword (var, let, const). Accessing an undeclared variable will result in a ReferenceError.

Question 14. How do you implement inheritance in JavaScript using prototypes?

Ans. Inheritance in JavaScript is achieved by setting an object’s prototype to another object. The prototype property of a constructor function is used to create a prototype chain. Here’s an example:Javascript

Question 15. What is the event loop in JavaScript, and how does it work?

Ans. The event loop is a core concept in JavaScript’s concurrency model. It’s responsible for managing the execution of code, handling asynchronous tasks, and maintaining responsiveness in web applications. The event loop continually checks the message queue for tasks, processes them in a non-blocking manner, and schedules callbacks for execution when the call stack is empty. This mechanism allows JavaScript to handle tasks like timers, I/O operations, and event handling in an asynchronous and non-blocking way.

Question 16. Describe the concept of promises in JavaScript and how they help with asynchronous code.

Ans. Promises are objects representing the eventual completion or failure of an asynchronous operation. They provide a structured way to work with asynchronous code, making it more readable and maintainable. A promise can be in one of three states: pending, fulfilled (resolved), or rejected. You can attach .then() and .catch() methods to a promise to handle success and error cases, respectively. Promises simplify error handling and allow you to chain multiple asynchronous operations together.

Question 17. What is the purpose of the localStorage and sessionStorage objects in JavaScript?

Ans. localStorage and sessionStorage are client-side storage mechanisms provided by web browsers for storing data in a key-value format. They are used to store data persistently (across browser sessions) or for the duration of a single session, respectively. They are often used to store user preferences, authentication tokens, or other application-specific data without the need for server-side storage.

Question 18. How can you create and manipulate HTML elements dynamically in JavaScript?

Ans. You can create and manipulate HTML elements dynamically using the Document Object Model (DOM) API in JavaScript. Here are some common methods: -document.createElement(): Create a new HTML element. -element.appendChild(): Add an element as a child to another element. -element.removeChild(): Remove a child element. -element.setAttribute(): Set an attribute of an element. -element.addEventListener(): Attach event listeners to elements. -element.innerHTML: Get or set the HTML content of an element. -element.style: Access and modify CSS styles of an element.

Question 19. Explain the concept of currying in JavaScript functions.

Ans. Currying is a functional programming technique where a function that takes multiple arguments is transformed into a series of unary (single-argument) functions. Each unary function takes one argument and returns another unary function or the final result. Currying allows you to partially apply arguments to a function, creating more specialized

functions. // Non-curried function
function add(a, b) {
return a + b;
}
// Curried function
function curriedAdd(a) {
return function (b) {
return a + b;
};
}
const addTwo = curriedAdd(2);
const result = addTwo(3); // Result: 5″

Question 20. What is a JavaScript IIFE (Immediately Invoked Function Expression)?

Ans. An IIFE is a JavaScript function that is defined and executed immediately after its declaration. It is typically wrapped in parentheses to prevent it from being treated as a regular function declaration. IIFE is often used to create private scopes for variables and to encapsulate code to avoid polluting the global scope.

(function () {

// Code here

})();

Question 21. Describe the differences between the map(), filter(), and reduce() array methods in JavaScript.

Ans. map() is used to create a new array by applying a provided function to each element of an existing array. It returns a new array with the results.

filter() is used to create a new array containing elements that satisfy a specified condition. It returns a new array with elements that pass the condition.

reduce() is used to accumulate values of an array into a single value by applying a provided function that processes each element sequentially. It returns a single result.

const numbers = [1, 2, 3, 4, 5];
// Using map()
const squared = numbers.map((num) => num * num); // Result: [1, 4, 9, 16, 25]
// Using filter()
const evenNumbers = numbers.filter((num) => num % 2 === 0); // Result: [2, 4]
// Using reduce()
const sum = numbers.reduce((accumulator, num) => accumulator + num, 0); // Result: 15″

Question 22. How do you remove duplicates from an array in JavaScript?

Ans. You can remove duplicates from an array in JavaScript using various methods. One common approach is to use a Set to store unique values, and then convert the set back to an array.

const arrayWithDuplicates = [1, 2, 2, 3, 4, 4, 5];
const uniqueArray = […new Set(arrayWithDuplicates)];
// Result: [1, 2, 3, 4, 5]

Question 23. What is the JavaScript event loop, and how does it handle asynchronous tasks?

Ans. The JavaScript event loop is a mechanism that manages the execution of code in a non-blocking and asynchronous manner. It consists of two main components: the call stack and the message queue. When a script starts, it enters the call stack. Asynchronous tasks, such as timers, I/O operations, and events, are offloaded to the browser’s APIs. When an asynchronous task completes, it places a message in the message queue. The event loop continuously checks the message queue and moves messages to the call stack when it is empty, allowing asynchronous tasks to be processed in the order they were scheduled.

Question 24. How do you use the bind() method in JavaScript to set the value of “this” in a function?

Ans. The bind() method in JavaScript is used to create a new function with a specified value for the this keyword. It allows you to explicitly set the value of this within the function when it is called. Here’s an example:Javascript

Question 25. Explain what the “hoisting” behavior is in JavaScript.

Ans. Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase. This means that you can use a variable or function before it’s declared in your code, but it’s important to understand that only the declarations are hoisted, not the initializations.

console.log(x); // Outputs “”undefined”” (declaration is hoisted)
var x = 10; // Initialization
foo(); // Outputs “”Hello, World!”” (function declaration is hoisted)
function foo() {
console.log(‘Hello, World!’);
}

Question 26. How does the “use strict” directive affect JavaScript code?

Ans. use strict is a pragma that enforces stricter parsing and error handling in JavaScript code. When used at the beginning of a script or function, it enables a set of rules and restrictions that help catch common coding mistakes and promote better code quality. Some of the effects of strict mode include disallowing the use of undeclared variables, preventing the use of reserved words, and disabling potentially unsafe features.

Question 27. What are JavaScript promises, and how do they work with async/await?

Ans. JavaScript promises are objects representing the eventual completion (or failure) of an asynchronous operation. Promises have three states: pending, fulfilled (resolved), and rejected. Promises work seamlessly with async/await, which is a syntax introduced in ES2017 for working with asynchronous code in a more synchronous-like manner. You can use await within an async function to pause the execution until a promise is resolved or rejected. This makes asynchronous code more readable and easier to reason about.

Question 28. How can you check if an object is an array in JavaScript?

Ans. You can use the Array.isArray(obj) method to check if an object is an array. It returns true if the object is an array and false otherwise.

const myArray = [1, 2, 3];
console.log(Array.isArray(myArray)); // Output: true
const myObject = { name: ‘Alice’ };
console.log(Array.isArray(myObject)); // Output: false”

Question 29. Explain the concept of prototypal inheritance in JavaScript.

Ans. In prototypal inheritance, objects inherit properties and methods from a prototype object. Every object in JavaScript has a prototype, which is another object. When you access a property or method on an object, JavaScript first looks for it on the object itself. If it’s not found, it searches for it in the prototype chain by following the __proto__ link to the prototype object. This chain continues until the property or method is found or until it reaches the end of the prototype chain (the Object.prototype).

Question 30. What is the difference between a shallow clone and a deep clone of an object in JavaScript?

Ans. A shallow clone of an object creates a new object with its own properties, but if the properties are objects themselves, they still reference the same objects in memory as the original object. In contrast, a deep clone creates a completely new object with its own properties and recursively clones nested objects, ensuring that there are no shared references between the original and the cloned object. Deep clones create independent copies of the entire object hierarchy.

Question 31. How do you create and manipulate DOM elements using JavaScript?

Ans. You can create and manipulate DOM elements in JavaScript using various methods and properties. Here are some common operations:

-Creating new elements: Use document.createElement(‘elementName’) to create a new HTML element.

-Modifying element properties: Use properties like element.textContent, element.innerHTML, or element.setAttribute() to set content and attributes.

-Appending elements: Use parentElement.appendChild(newElement) to add a new element as a child of another element.

-Removing elements: Use parentElement.removeChild(elementToRemove) to remove an element.

-Modifying CSS styles: Use element.style.property to change the style of an element.

-Adding event listeners: Use element.addEventListener(‘event’, callback) to handle events like clicks.”

Question 32. Describe the purpose of the “debounce” and “throttle” functions in JavaScript.

Ans. Debounce: Debouncing is a technique used to control the rate at which a function is executed. It ensures that a function is called only after a specified time has passed since the last time it was invoked. It’s useful for scenarios like handling user input to avoid excessive function calls.

Throttle: Throttling limits the number of times a function can be called within a specific time interval. It ensures that a function can be executed at most once per interval. Throttling is helpful for optimizing resource-intensive operations like scrolling or resizing event handlers.”

Question 33. What is event delegation, and why is it useful in JavaScript?

Ans. Event delegation is a technique in JavaScript where a single event handler is attached to a common ancestor element of multiple child elements. Instead of attaching event handlers to each child element individually, the ancestor element listens for events bubbling up from its descendants. This approach is useful for:

-Reducing the number of event handlers, improving performance.

-Dynamically handling events for elements created or removed at runtime.

-Simplifying event management for large or complex DOM structures.

Question 34. How do you handle asynchronous operations using promises in JavaScript?

Ans. Promises are used to manage asynchronous operations in JavaScript. You can create a promise using the Promise constructor, and it can be in one of three states: pending, resolved (fulfilled), or rejected. Promises are typically used with the .then() and .catch() methods to handle successful results and errors, respectively. Here’s an example:

Question 35. Explain the concept of a closure in JavaScript and provide an example.

Ans. A closure is a JavaScript feature that allows a function to remember and access its outer (enclosing) function’s variables and parameters even after the outer function has finished executing. Closures are created whenever a function is defined within another function. Here’s an example:

function outer() {
const outerVar = ‘I am from outer function’;
function inner() {
console.log(outerVar); // Accesses outerVar from the outer function
}
return inner;
}
const closureFunc = outer();
closureFunc(); // Outputs: “”I am from outer function””

Question 36. How can you iterate over the properties of an object in JavaScript?

Ans. You can iterate over the properties of an object in JavaScript using various methods, such as for…in loops or methods like Object.keys(), Object.values(), and Object.entries(). Here’s an example using for…in:Javascript

const person = {
name: ‘Alice’,
age: 30,
city: ‘New York’
};
for (const key in person) {
if (person.hasOwnProperty(key)) {
console.log(${key}: ${person[key]});
}
}

Question 37. What is memoization, and how can it be implemented in JavaScript?

Ans. Memoization is an optimization technique used to cache the results of expensive function calls and return the cached result when the same inputs occur again. It can improve the performance of functions that are called with the same arguments multiple times. You can implement memoization in JavaScript using an object to store the results. Here’s a simple example:Javascript

Question 38. Describe the purpose of the “arguments” object in JavaScript functions.

Ans. The “arguments” object is a special object available within all JavaScript functions. It contains an array-like list of the arguments passed to the function, regardless of the number of named parameters defined in the function’s signature. It allows you to work with a variable number of arguments. However, it is not a true array but an “array-like” object and lacks array methods. Modern JavaScript encourages using the rest parameters syntax (…args) instead of the “arguments” object for better flexibility and compatibility with array methods.

Question 39. How can you compare two JavaScript objects for equality?

Ans. To compare two JavaScript objects for equality (i.e., having the same properties and values), you can’t use == or === directly because they compare references, not object contents. You need to implement a custom comparison function that recursively checks the properties and values of the objects. Here’s a simple example:Javascript

Question 40. What is a JavaScript generator, and how does it work?

Ans. A JavaScript generator is a special type of function that allows you to pause and resume its execution. It’s defined using the function* syntax and contains one or more yield statements. Generators are used to generate a sequence of values lazily and iteratively. When you call a generator function, it returns an iterator object that can be used to control the execution of the generator. You can pause the generator using yield and resume it using the iterator’s .next() method. Here’s a simple example:Javascript

function* countToThree() {
yield 1;
yield 2;
yield 3;
}
const iterator = countToThree();
console.log(iterator.next().value); // Output: 1
console.log(iterator.next().value); // Output: 2
console.log(iterator.next().value); // Output: 3

Hard

Question 1. What is the difference between the “call,” “apply,” and “bind” methods in JavaScript?

Ans. 1.call() and apply() are methods that allow you to invoke a function with a specific this value and arguments. The main difference is how you pass arguments:

-call(thisArg, arg1, arg2, …) accepts arguments individual

-apply(thisArg, [arg1, arg2, …]) accepts arguments as an array.

2.bind() creates a new function with a fixed this value and, optionally, pre-specified arguments. It doesn’t invoke the function immediately but returns a new function that can be called later.

Question 2. How does JavaScript handle memory management, and what is garbage collection?

Ans. JavaScript uses automatic memory management, which includes garbage collection. Here’s how it works:

-When variables go out of scope or are no longer referenced, they become eligible for garbage collection.

-The garbage collector identifies these unreferenced objects and frees up memory by deallocating them.

-JavaScript engines use various garbage collection algorithms to optimize memory usage.

-Developers can help by being mindful of memory usage, avoiding memory leaks, and using tools for memory profiling.

Question 3. Explain the concepts of “thunk” and “saga” in the context of JavaScript Redux.

Ans. -Thunk: A thunk is a function that encapsulates an action to be dispatched in Redux. It delays the action’s execution until a later time or condition is met. Thunks are often used for asynchronous actions, enabling side effects and API calls.

-Saga: A saga is a middleware library for Redux that provides a way to manage side effects, such as handling asynchronous actions, in a more structured and testable manner. Sagas use generator functions to describe the flow of actions and can listen for specific actions to trigger side effects.”

Question 4. Describe the differences between the ES6 “class” and “constructor function” approaches for creating objects.

Ans. -Class: In ES6, the class syntax provides a more structured and readable way to create constructor functions and define methods. It supports constructor functions, inheritance, and getters/setters.

-Constructor Function: Traditional constructor functions use the function keyword and require defining the constructor and adding methods to the prototype manually. They have been used prior to ES6 and are less verbose.

Question 5. What is the Event Emitters pattern in JavaScript, and how is it used?

Ans. The Event Emitters pattern is a design pattern in JavaScript used for implementing custom event handling. It allows objects to subscribe to and emit custom events, facilitating communication between different parts of an application. Node.js’s EventEmitter is a built-in example of this pattern. To use it, you create an event emitter object, define custom events, and emit events when specific actions or conditions occur. Subscribers (listeners) can then react to these events by attaching event handlers.

Question 6. How can you implement a singleton pattern in JavaScript?

Ans. The singleton pattern ensures that a class has only one instance, which can be accessed globally. You can implement it in JavaScript using a module pattern or by defining a constructor function with closure. Here’s an example using a constructor function:Javascript

function Singleton() {
if (!Singleton.instance) {
Singleton.instance = this;
}
return Singleton.instance;
}
const instance1 = new Singleton();
const instance2 = new Singleton();
console.log(instance1 === instance2); // Output: true (both instances are the same)

Question 7. Explain the concept of memoization and provide a use case for it.

Ans. Memoization is an optimization technique where the results of expensive function calls are cached based on their input arguments. When the same inputs are provided again, the cached result is returned instead of re-computing it. A common use case is optimizing recursive functions or functions with expensive computations, such as Fibonacci calculations.

Question 8. What are web workers in JavaScript, and how can they be used to improve performance?

Ans. Web workers are a feature in JavaScript that allows you to run JavaScript code in the background, separate from the main browser thread. They can be used to perform tasks concurrently, improving performance by offloading CPU-intensive work. Web workers are especially useful for tasks like data processing, image manipulation, or other operations that can be parallelized.

Question 9. What is the “prototype chain” in JavaScript, and how does it relate to inheritance?

Ans. The prototype chain is a fundamental concept in JavaScript’s object-oriented programming model. It represents the way objects inherit properties and methods from their prototypes. When you access a property or method on an object, JavaScript searches for it on the object itself and, if not found, looks up the prototype chain. This chain allows objects to inherit from other objects, enabling prototype-based inheritance.

Question 10. Describe the differences between “shallow copy” and “deep copy” in JavaScript objects.

Ans. -Shallow Copy: A shallow copy of an object creates a new object with a new reference but copies only the top-level properties. Nested objects or arrays within the original object are still referenced, not duplicated. Shallow copies are created using methods like Object.assign() or the spread operator.

-Deep Copy: A deep copy creates a new object and recursively duplicates all properties and nested objects, ensuring that the entire structure is independent. It’s a true clone of the original object. Achieving a deep copy often requires custom recursive functions or libraries like Lodash’s _.cloneDeep().

Question 11. How can you implement a publish-subscribe pattern in JavaScript?

Ans. The publish-subscribe pattern (pub-sub) allows objects to subscribe to events (or messages) and receive notifications when those events occur. You can implement it using custom code or libraries like EventEmitter. Subscribers register their interest in specific events, and publishers emit those events when appropriate. It’s a way to decouple components and enable communication between them without direct dependencies.

Question 12. Explain the concept of a “promise” and its states in JavaScript.

Ans. A promise is a JavaScript object representing the eventual completion or failure of an asynchronous operation and its resulting value. Promises have three states:

-Pending: The initial state, representing an ongoing operation.

-Fulfilled (Resolved): The state when the operation is successful, and the promise holds a resolved value.

-Rejected: The state when the operation fails, and the promise holds a rejection reason.

Question 13. What is the purpose of the JavaScript async and await keywords in asynchronous code?

Ans. The async and await keywords are used in JavaScript to simplify working with asynchronous code. Functions marked as async return promises implicitly, and await is used within async functions to pause execution until a promise is resolved. This helps write asynchronous code that resembles synchronous code, making it more readable and easier to reason about.

Question 14. Describe the role of the “WeakMap” and “WeakSet” data structures in JavaScript.

Ans. WeakMap: A WeakMap is a collection of key-value pairs where keys are objects, and values can be any data type. Unlike regular maps, WeakMap keys are weakly held, meaning they don’t prevent objects from being garbage collected when they’re no longer in use. WeakMaps are often used for private data storage or for associating metadata with objects.

WeakSet: A WeakSet is a collection of unique objects, and like WeakMaps, it doesn’t prevent objects from being garbage collected. It’s typically used to store a set of objects where the reference to the objects is weak.

Question 15. How do you implement a linked list in JavaScript?

Ans. A linked list in JavaScript can be implemented using objects to represent nodes, where each node has a value and a reference (usually next) to the next node in the list. You also typically keep a reference to the head (the first node) of the list. Here’s a basic example of a singly linked list:

class Node {
constructor(value) {
this.value = value;
this.next = null;
}
}
class LinkedList {
constructor() {
this.head = null;
}
// Add methods like append, prepend, delete, search, etc.
}

Question 16. What is the purpose of the “Generator” object in JavaScript, and how can it be used?

Ans. A Generator is a special type of function in JavaScript that can be paused and resumed. It’s defined using the function* syntax and contains one or more yield statements. Generators are often used for lazy and efficient generation of values, especially in scenarios like iterating over large datasets or implementing custom iterators.

Question 17. Explain the concept of the “event loop” in JavaScript and how it handles asynchronous tasks.

Ans. The event loop is a central part of JavaScript’s concurrency model. It’s responsible for handling asynchronous tasks and callbacks. The event loop continuously checks the message queue for pending tasks. When a task is found, it’s executed, and the loop continues. This allows JavaScript to perform non-blocking I/O operations and execute callbacks when events occur, ensuring responsive and efficient code execution.

Question 18. How can you implement a deep copy of an object that contains nested objects in JavaScript?

Ans. You can implement a deep copy of an object containing nested objects using a recursive function. Here’s a basic example:

Question 19. What is the “Proxy” object in JavaScript, and how can it be used for intercepting operations on objects?

Ans. The Proxy object is a built-in feature in JavaScript that allows you to intercept and customize operations on objects. It can be used to add custom behavior, validation, or access control to objects. A Proxy is created with a target object and a handler object containing methods for intercepting various operations like property access, assignment, function invocation, etc.

Question 20. Describe the differences between JavaScript “callbacks,” “promises,” and “async/await” for handling asynchronous operations.

Ans. Callbacks: Callbacks are functions passed as arguments to other functions, typically used for asynchronous operations. They can lead to callback hell (nested callbacks) and can be challenging to read and maintain.

Promises: Promises are objects representing the eventual completion or failure of an asynchronous operation. They provide a cleaner way to handle async code with .then() and .catch(), improving readability.

async/await: async functions return promises implicitly and allow you to write async code that looks more like synchronous code. await pauses execution until a promise is resolved or rejected, simplifying error handling and making code more readable.

Question 21. Explain the concept of “composition over inheritance” in JavaScript.

Ans. “Composition over inheritance” is a design principle that suggests favoring object composition and building complex objects by combining simpler objects, rather than relying heavily on class inheritance hierarchies. This approach promotes flexibility, code reuse, and avoids some of the issues associated with deep inheritance hierarchies.

Question 22. What are “maps” and “sets” in JavaScript, and how are they used?

Ans. Maps: Maps are collections of key-value pairs, where keys can be of any data type (including objects and functions). They allow you to associate values with keys and are particularly useful for data storage and retrieval. Maps maintain the insertion order of keys, which makes them suitable for scenarios where order matters.

Sets: Sets are collections of unique values, where each value can occur only once. Sets are often used to store a collection of unique items or to eliminate duplicate values from an array.

Question 23.How can you implement a debounce function in JavaScript to limit the frequency of a function’s execution?

Ans. A debounce function is used to delay the execution of a function until a certain amount of time has passed without further function calls. Here’s a simple implementation:Javascript

Question 24. Describe the differences between “stack” and “queue” data structures in JavaScript.

Ans. Stack: A stack is a linear data structure where elements are added and removed from the same end called the “”top.”” It follows the Last-In-First-Out (LIFO) principle, meaning the last element added is the first one to be removed. Stacks are often used for function call management and undo/redo functionality.

Queue: A queue is another linear data structure where elements are added at one end (the “”rear””) and removed from the other end (the “”front””). It follows the First-In-First-Out (FIFO) principle, meaning the first element added is the first one to be removed. Queues are used for tasks like managing tasks in a print queue or handling asynchronous requests.

Question 25. What is the “Immutable.js” library in JavaScript, and why might you use it?

Ans. Immutable.js is a JavaScript library that provides immutable data structures, such as lists, maps, and sets. Immutable data structures cannot be changed after they are created, which helps prevent unintentional data mutations. You might use Immutable.js in scenarios where you want to ensure data integrity, enable efficient change detection in React applications, or work with persistent data structures efficiently.

Question 26. How can you implement a “throttle” function in JavaScript to control the rate of function calls?

Ans. A throttle function limits the rate at which a function can be called, ensuring it’s not called too frequently. Here’s a basic implementation:

Question 27. What are the differences between “prototypal inheritance” and “classical inheritance” in JavaScript?

Ans. Prototypal Inheritance: In JavaScript, objects can inherit properties and methods directly from other objects. Prototypal inheritance is based on the prototype chain, where objects have a prototype reference and can delegate property lookups to their prototypes. It’s more flexible than classical inheritance and allows objects to inherit from multiple prototypes.

Classical Inheritance: Classical inheritance is a concept from class-based languages like Java or C++. It uses class hierarchies and extends classes to create new ones. In JavaScript, classical inheritance can be emulated using constructor functions and the prototype object, but it’s less flexible than prototypal inheritance.

Question 28. Describe the “constructor pattern” and the “module pattern” in JavaScript.

Ans. Constructor Pattern: The constructor pattern is a way to create objects in JavaScript using constructor functions. Constructor functions are called with the new keyword and create instances with shared methods and properties defined in their prototypes. It’s a common pattern for defining classes-like structures in JavaScript.

Module Pattern: The module pattern is a design pattern that encapsulates code into self-contained modules. It often involves using immediately-invoked function expressions (IIFE) to create private and public members, providing a way to achieve data encapsulation and avoid polluting the global scope. Modules promote organization and reusability of code.

Question 29. How can you handle memory leaks in JavaScript, and what tools can help with memory profiling?

Ans. To handle memory leaks in JavaScript:

Ensure that you remove event listeners when they are no longer needed to prevent circular references.

Avoid global variables and use variable scope wisely.

Use tools like the Chrome DevTools Memory panel, Heap Snapshots, and Leak Detection to profile and identify memory issues.

Regularly test your application with memory profiling tools to catch and fix potential leaks early.

Question 30. What is the purpose of the JavaScript Symbol data type, and how can it be used?

Ans. Symbols are a unique and immutable data type introduced in ES6. Their main purpose is to create property keys that are guaranteed to be unique, preventing naming collisions in objects. Symbols can be used as keys in objects and are often used for creating private or hidden object properties. Example:Javascript

const mySymbol = Symbol(‘description’);
const obj = {
[mySymbol]: ‘This is a symbol property’,
};

Question 31. Explain the differences between “WebSockets” and “HTTP polling” for real-time communication in web applications.

Ans. WebSockets: WebSockets provide full-duplex communication over a single, long-lived connection. They are efficient for real-time applications, such as chat or gaming, as they allow instant data exchange in both directions. WebSockets are event-driven and have low latency.

HTTP Polling: HTTP polling involves repeatedly sending HTTP requests to a server at predefined intervals to check for updates. It’s less efficient and has higher latency compared to WebSockets, as it requires frequent requests and responses.

Question 32. What is “closure memory leak,” and how can it be avoided in JavaScript?

Ans. A closure memory leak occurs when a function retains references to variables or objects that are no longer needed, preventing them from being garbage collected. To avoid closure memory leaks:

Be mindful of what variables/functions are captured in closures.

Use functions like setTimeout() with care, as they can unintentionally capture variables.

Explicitly remove event listeners and references when they are no longer needed.

Question 33. Describe the “fluent interface” design pattern in JavaScript, and provide an example.

Ans. The fluent interface is a design pattern that allows method chaining to create expressive and readable code. Each method returns an object or value that has more methods that can be called in sequence. An example using a fictional StringBuilder class:

class StringBuilder {
constructor() {
this.value = ”;
}
append(text) {
this.value += text;
return this; // Return the instance for chaining
}
toString() {
return this.value;
}
}
const result = new StringBuilder()
.append(‘Hello’)
.append(‘ ‘)
.append(‘World’)
.toString(); // “”Hello World””

Question 34. How can you implement a “curry” function in JavaScript?

Ans. A curry function in JavaScript transforms a function that takes multiple arguments into a series of unary (single-argument) functions. Here’s a basic example of a curry function:Javascript

Question 35. What is “event-driven programming,” and how is it used in JavaScript?

Ans. Event-driven programming is a paradigm where the flow of the program is determined by events and event handlers. In JavaScript, it’s commonly used for building interactive web applications. Events can be user interactions (clicks, keypresses), HTTP requests, or custom events. JavaScript libraries and frameworks like Node.js and the browser’s DOM provide APIs for handling events using event listeners and callbacks.

Question 36. What would be the output? True or False ?

Ans.Javascript

const x = 0.1 ;
const y = 0.2 ;
console.log(x + y == 0.3) ;

FALSE because when you console it will give 0.300004 that’s why

Question 37. What are two-way data binding and one-way data flow, and how are they different?

Ans. Two-way data binding means that UI fields are bound to model data dynamically such that when a UI field changes, the model data changes with it and vice-versa.

One way data flow means that the model is the single source of truth. Changes in the UI trigger messages that signal user intent to the model (or “store” in React). Only the model has access to change the app’s state. The effect is that data always flows in a single direction, which makes it easier to understand.

One way data flows are deterministic, whereas two-way binding can cause side-effects which are harder to follow and understand.

Question 38. Write a function that generates all permutations of a given string.

Ans.

Question 39. What will be the output of the following code?Javascript

var x = { foo : 1};
var output = (function() {
delete x.foo;
return x.foo;
})();
console.log(output);”

Ans. The result of the above code will be the output.

output. delete operator is used to delete a property from an object. In this example, x is the object with foo as a property. We are deleting foo property from a self-invoking function and in the next call (return x.foo;) trying to reference the deleted property “foo”, this will result undefined.

Question 40. What will be the output of the following code?

x = 0.1;
y = 0.3;
z = x + y;
console.log(z === 0.4);

Ans. Now, here’s where the issue arises:

JavaScript uses the IEEE 754 standard for representing floating-point numbers, which may introduce small rounding errors in calculations involving decimal fractions. In this case, the value of z might not be exactly 0.4 due to the way 0.1 and 0.3 are represented in binary floating-point.

So, when you run the code, you might find that z is very close to 0.4 but not exactly equal to it. Therefore, console.log(z === 0.4); may log false to the console because of this small floating-point precision issue.

To compare floating-point numbers for approximate equality, you can use a tolerance threshold, like so:

const tolerance = 0.000001; // Define a small tolerance threshold

console.log(Math.abs(z – 0.4) < tolerance); // Use Math.abs to handle negative differences

This code checks if the absolute difference between z and 0.4 is smaller than a defined tolerance threshold. If the difference is within the tolerance, it will be considered equal.”

Question 41. What will be the output of the following code?

const promise = new Promise((resolve, reject) => {
resolve(‘Resolved!’);
reject(‘Rejected!’);
});
promise
.then(data => console.log(‘Success:’, data))
.catch(error => console.error(‘Error:’, error));”

Ans. Expected Output: ‘Success: Resolved!’

Explanation: The resolve function is called, so the then block is executed, and the catch block is not executed.”

Question 42. What will be the output of the following code?

console.log([] == []);
console.log([] === []);

Ans. Expected Output: False False

Explanation: Arrays are objects, and objects are compared by reference. The two empty arrays are different objects in memory, so both equality checks return false.

Question 43. What will be the output of the following code?

function outer() {
let x = 10;
function inner() {
console.log(x);
}
return inner;
}
const closure = outer();
closure();

Ans. Expected Output: 10

Explanation: The inner function forms a closure over the x variable declared in the outer function, allowing it to access x even after outer has returned.”

Question 44. What will be the output of the following code?

const a = {};
const b = { key: ‘value’ };
const c = { key: ‘value’ };
a[b] = 123;
a[c] = 456;
console.log(a[b]);

Ans. Expected Output: 456

Explanation: In JavaScript, object keys are automatically converted to strings. So, a[b] and a[c] are equivalent to a[‘[object Object]’]. Therefore, the value 456 overwrites the value 123 for the key ‘[object Object]’.”

Question 45. What will be the output of the following code?

const obj = {
Symbol.toPrimitive {
if (hint === ‘number’) return 42;
if (hint === ‘string’) return ‘Hello, World!’;
return null;
}
};
console.log(obj + 1);
console.log(obj + ‘ JavaScript’);
console.log(obj.toString());

Ans. Expected Output:43

‘Hello, World! JavaScript’

‘[object Object]’

Explanation: The object obj has a special method [Symbol.toPrimitive] which is used for type coercion. When using the + operator, JavaScript first tries to convert the object into a primitive value based on the provided hint (‘number’ or ‘string’). If neither hint is provided, it defaults to null.”

Question 46. What will be the output of the following code?

const func = () => console.log(‘First’);
(async () => {
await func();
console.log(‘Second’);
})();

Ans. Expected Output: First

Second

Explanation: Even though await is used inside the async function, it doesn’t affect the execution of func(), which is still executed synchronously.

Question 47. What will be the output of the following code?

Ans. Expected Output: Getting value

Setting value to 100

Explanation: The get and set methods are defined using Object.defineProperty, so accessing obj.value triggers the get method, and setting obj.value triggers the set method.