JavaScript Interview Questions and Answers for Fresher - AsgarTech

 JavaScript Interview Questions and Answers

Q: What is JavaScript?

A: JavaScript is a programming language that is used to create dynamic, interactive web pages. It is a client-side scripting language that runs on the web browser and is used to add interactivity to web pages.

Q: What is the typeof operator used for in JavaScript?

A: The typeof operator is used to determine the data type of a value or variable in JavaScript.

Q: What is scope in JavaScript?

A: Scope refers to the area of the program where a variable or function is accessible. In JavaScript, there are two types of scope: global scope and local scope.

Q: What is event delegation in JavaScript?

A: Event delegation is a technique where a single event listener is attached to a parent element, which then handles events triggered by its child elements.

Q: What is the difference between a for loop and a forEach loop?

A: A for loop is a traditional loop that iterates over an array or object, while a forEach loop is a method of an array that iterates over its elements.

Q: What is a callback function?

A: A callback function is a function that is passed as an argument to another function and is called when the first function is finished.

Q: What is a closure in JavaScript?

A: A closure is a function that has access to variables in its outer scope, even after the outer function has returned.

Q: What is the difference between a let and a const declaration?

A: A let declaration allows the variable to be reassigned, while a const declaration does not allow reassignment after it has been assigned a value.

Q: What is JSON?

A: JSON stands for JavaScript Object Notation and is a lightweight data interchange format that is easy to read and write for humans and machines.

Q: What is a promise in JavaScript?

A: A promise is an object that represents a value that may not be available yet, and allows you to attach callbacks to handle success or failure.

Q: What is the purpose of the keyword "this" in JavaScript?

A: The this keyword refers to the current object that the code is being executed in. Its value depends on the context in which it is used.

What is the difference between a named function expression and an anonymous function expression?

Answer: A named function expression has a name identifier that can be used to refer to the function, while an anonymous function expression does not have a name identifier.

What is the difference between an arrow function and a regular function?

Answer: An arrow function is a shorter syntax for writing a function, and does not have its own this value.

What is the difference between call and apply methods in JavaScript?

Answer: Both call and apply methods are used to invoke a function with a specific this value, but the apply method allows you to pass arguments to the function as an array.

What is the difference between a for loop and a forEach loop in JavaScript?

Answer: A for loop is a general-purpose loop that can be used to iterate over an array or object, while a forEach loop is a specific method on an array that iterates over each element of the array.

What is hoisting in JavaScript?

Answer: Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their respective scopes during runtime.

What is the difference between a stack and a queue in JavaScript?

Answer: A stack is a data structure that operates on a last-in-first-out (LIFO) basis, while a queue operates on a first-in-first-out (FIFO) basis.

What is the difference between JSON and JavaScript objects?

Answer: JSON is a data format that is used to transmit data between a server and a client, while JavaScript objects are used to store and manipulate data within a JavaScript program.

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

Answer: let and const declarations are block-scoped, meaning they are only accessible within the block they are declared in, while var declarations are function-scoped and are accessible within the entire function they are declared in. const declarations cannot be reassigned, while let and var can be.

What is event delegation in JavaScript?

Answer: Event delegation is a technique where a single event listener is added to a parent element, which handles events triggered by its child elements.

What is the difference between a synchronous and asynchronous function in JavaScript?

Answer: A synchronous function is executed in a single thread and blocks further code execution until it has completed, while an asynchronous function allows for concurrent execution of multiple functions without blocking the execution of other code.

What is a higher-order function in JavaScript?

Answer: A higher-order function is a function that takes one or more functions as arguments or returns a function as its result.

What is 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 first function has completed its operation.

What is the difference between mutable and immutable values in JavaScript?

Answer: Mutable values can be changed after they are created, while immutable values cannot be changed after they are created.

What is the difference between prototypal and classical inheritance in JavaScript?

Answer: Classical inheritance uses classes and class-based inheritance to create new objects, while prototypal inheritance uses prototypes and object-based inheritance to create new objects.

What is the difference between synchronous and asynchronous XMLHttpRequests in JavaScript?

Answer: Synchronous XMLHttpRequests block further code execution until they are completed, while asynchronous XMLHttpRequests allow for concurrent execution of other code while waiting for a response.

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

Answer: A shallow

Q: What is the difference between an arrow function and a regular function?

A: An arrow function is a shorter syntax for writing a function, and does not have its own this value.

Q: What is the purpose of the "use strict" directive in JavaScript?

A: The "use strict" directive enables strict mode in JavaScript, which enforces stricter rules for code execution and can help prevent errors.

Q: What is the difference between a map and a set in JavaScript?

A: A map is an object that stores key-value pairs, while a set is an object that stores unique values.

Q: What is the difference between the window and document objects in JavaScript?

A: The window object represents the browser window, while the document object represents the web page loaded in the window.

Q: What is a generator function in JavaScript?

A: A generator function is a special type of function that can be paused and resumed, allowing you to create iterators that generate a series of values over time.

Q: What is a template literal in JavaScript?

A: A template literal is a string that allows you to embed expressions and variables using interpolation, using the backtick (`) character instead of quotes.

Q: What is the difference between a shallow copy and a deep copy?

A: A shallow copy of an object or array creates a new reference to the original object, while a deep copy creates a new object with new references to all nested objects and arrays.

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

A: let and const are both block-scoped declarations introduced in ES6, while var is function-scoped. let can be reassigned, while const cannot be reassigned once it is assigned a value. var allows you to declare the same variable multiple times, while let and const do not.

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Q: What is hoisting?

A: Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their scope before execution. This means that you can use a variable or function before it is declared, but it will have a value of undefined until it is assigned a value.

Q: What are closures?

A: Closures are functions that have access to variables in their outer scope, even after the outer function has returned. They are created whenever a function is defined inside another function and can be used to create private variables and methods.

Q: What is the difference between synchronous and asynchronous programming?

A: Synchronous programming is when code is executed in a single thread, one statement at a time, while asynchronous programming is when code is executed in multiple threads, allowing statements to be executed concurrently. Asynchronous programming is often used for I/O operations or long-running tasks, while synchronous programming is used for simple, short-running tasks.

Q: What is the difference between callbacks, promises, and async/await?

A: Callbacks are functions that are passed as arguments to other functions and are called when the operation is complete. Promises are objects that represent a value that may not be available yet, and allow you to attach callbacks to handle success or failure. Async/await is a syntax for working with promises that allows you to write asynchronous code in a synchronous style, using the await keyword to pause execution until a promise is resolved.

Q: What is event bubbling?

A: Event bubbling is a behavior in the DOM where events are propagated up the DOM tree from the target element to its parent elements. This means that if you click on a button inside a div, the click event will be triggered on the button first, then on the div.

Q: What is the difference between the == and === operators?

A: The == operator compares the values of two operands, while the === operator compares both the values and the types of the operands. This means that === is more strict than == and is often preferred for comparisons. For example, 1 == "1" is true, but 1 === "1" is false.

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Q: What is the difference between a constructor function and a class in JavaScript?

A: A constructor function is a function that is used to create and initialize objects, while a class is a syntactic sugar for creating constructor functions and prototypes. Classes provide a more object-oriented style of programming and allow for inheritance and encapsulation.

Q: What is the difference between null and undefined in JavaScript?

A: null is a value that represents intentional absence of any object value, while undefined represents a variable that has been declared but not assigned a value, or a property that does not exist.

Q: What is a try-catch block in JavaScript?

A: A try-catch block is used for error handling in JavaScript. Code inside the try block is executed, and if an error is thrown, it is caught by the catch block, which can then handle the error.

Q: What is the difference between synchronous and asynchronous code?

A: Synchronous code is executed in a single thread, where each line of code is executed one at a time, while asynchronous code is executed in multiple threads, allowing multiple lines of code to execute at the same time.

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