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Top 35 Javascript interview questions and answers for frontend developers 2022-23

JavaScript is a powerful scripting language that is widely used in web development. It is one of the most in-demand programming languages in the world, and as such, it is frequently used as a benchmark for technical interviews. In this article, we will take a look at some of the most common JavaScript interview questions and answers.

  1. What is JavaScript?

JavaScript is a scripting language that is commonly used in web development. It is a high-level, dynamic, and interpreted language that is used for both client-side and server-side programming.

  1. What are the differences between JavaScript and Java?

JavaScript and Java are two distinct programming languages. JavaScript is a scripting language that is used primarily for web development, whereas Java is an object-oriented programming language that is used for a wide range of applications. Additionally, JavaScript is interpreted, whereas Java is compiled.

  1. What are the data types in JavaScript?

JavaScript supports several data types, including strings, numbers, boolean values, null, undefined, and objects.

  1. What is the difference between null and undefined?

In JavaScript, null and undefined are both used to indicate the absence of a value. However, null is an explicitly assigned value that indicates the absence of any object value, whereas undefined indicates the absence of any value, including null.

  1. What is closure?

A closure is a function that has access to variables in its outer function, even after the outer function has completed execution.

  1. What is the difference between let, var, and const?

In JavaScript, let, var, and const are used to declare variables. The main difference between them is their scope. Var has function-level scope, whereas let and const have block-level scope. Additionally, var can be redeclared within the same scope, whereas let and const cannot be.

  1. What are callbacks?

Callbacks are functions that are passed as arguments to other functions. They are used to execute code asynchronously, such as when waiting for a response from a server.

  1. What is the event loop?

The event loop is a mechanism in JavaScript that allows asynchronous code to be executed in a non-blocking manner. It constantly monitors the call stack and the message queue and pushes tasks from the queue to the stack when the stack is empty.

  1. What is hoisting?

Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their scope. This means that variables and functions can be used before they are declared.

  1. What is the difference between a for loop and a forEach loop?

A for loop is a traditional loop that is used to iterate over an array or other iterable object. A forEach loop is a method that is available on arrays and is used to execute a callback function for each element in the array.

  1. What is a promise?

A promise is an object that represents the eventual completion or failure of an asynchronous operation. It has a state, which can be pending, fulfilled, or rejected, and can be used to chain together multiple asynchronous operations.

  1. What is the difference between a synchronous and asynchronous function?

A synchronous function is one that blocks the execution of the code until it completes. An asynchronous function,on the other hand, allows other code to continue executing while it completes in the background.

  1. What is the difference between apply, call, and bind?

Apply, call, and bind are methods that are used to set the value of this keyword in a function. Apply and call are used to invoke a function immediately, whereas bind returns a new function with this value set.

  1. What are arrow functions?

Arrow functions are a shorthand syntax for creating functions in JavaScript. They are more concise than traditional function declarations and do not have their own value.

  1. What is the difference between classical inheritance and prototypal inheritance?

Classical inheritance is a method of creating objects in which an object is created from a class definition, and that object inherits all the properties and methods of the class. This is commonly used in object-oriented programming languages such as Java and C++. Prototypal inheritance, on the other hand, is a method of creating objects in which an object is created directly from another object, which serves as its prototype. This is the method used in JavaScript.

In prototypal inheritance, each object has a hidden link to its prototype object, which serves as a template for the object’s properties and methods. When a property or method is accessed on an object, JavaScript first looks for it on the object itself, and if it is not found, it looks for it on the object’s prototype, and so on up the prototype chain until the property or method is found or the end of the chain is reached.

  1. What is the difference between == and ===?

In JavaScript, == and === are used to compare values. == compares values for equality, whereas === compares values for equality and type. For example, the expression 5 == “5” would return true because the values are equal, but the expression 5 === “5” would return false because the values are not only different types but also different values.

  1. What is the difference between null and undefined?

In JavaScript, null and undefined are both used to indicate the absence of a value. However, null is an explicitly assigned value that indicates the absence of any object value, whereas undefined indicates the absence of any value, including null.

  1. What is the difference between let, var, and const?

In JavaScript, let, var, and const are used to declare variables. The main difference between them is their scope. Var has function-level scope, whereas let and const have block-level scope. Additionally, var can be redeclared within the same scope, whereas let and const cannot be.

  1. What are arrow functions?

Arrow functions are a shorthand syntax for creating functions in JavaScript. They are more concise than traditional function declarations and do not have their own value.

  1. What is the difference between synchronous and asynchronous code?

Synchronous code executes in a sequential manner, meaning that each line of code is executed one after the other in the order in which it is written. Asynchronous code, on the other hand, does not block the execution of other code while it is waiting for a task to complete. Instead, it allows other code to execute while it is waiting and then executes a callback function once the task is complete.

  1. What is a callback function?

A callback function is a function that is passed as an argument to another function and is executed once the main function has completed its task. It is commonly used in asynchronous programming to handle the result of an asynchronous operation.

  1. What is the difference between a synchronous and asynchronous function?

A synchronous function is one that blocks the execution of the code until it completes. An asynchronous function, on the other hand, allows other code to continue executing while it completes in the background.

  1. What is the event loop?

The event loop is a mechanism in JavaScript that allows asynchronous code to be executed in a non-blocking manner. It constantly monitors the call stack and the message queue and pushes tasks from the queue to the stack when the stack is empty.

  1. What are promises?

Promises are a way to handle asynchronous operations in JavaScript. A promise represents a value that may not be available yet but will be at some point in the future. Promises have a state, which can be pending, fulfilled, or rejected, and can be used to chain together multiple asynchronous operations.

  1. What is hoisting?

Hoisting is a behavior in JavaScript where variable and function declarations are moved.to the top of their respective scopes before the code is executed. This means that even if a variable or function is declared later in the code, it can be used before its declaration without causing an error. However, it is important to note that only the declaration is hoisted, not the initialization or assignment.

to the top of their respective scopes before the code is executed. This means that even if a variable or function is declared later in the code, it can be used before its declaration without causing an error. However, it is important to note that only the declaration is hoisted, not the initialization or assignment.

  1. What is the difference between a reference type and a value type?

In JavaScript, a reference type is a type that is stored as a reference to its value in memory, whereas a value type is stored directly in memory. Examples of reference types include objects and arrays, whereas examples of value types include numbers, strings, and booleans.

  1. What is the difference between a for loop and a for…in the loop?

A for loop is used to iterate over a block of code a specified number of times, whereas a for…in the loop is used to iterate over the properties of an object. In a for…in the loop, the loop variable represents the property name, whereas, in a for loop, the loop variable represents the index.

  1. What is the difference between a for loop and a while loop?

A for loop is used to iterate over a block of code a specified number of times, whereas a while loop is used to iterate over a block of code until a specified condition is met.

  1. What is the difference between the call and apply methods?

The call and apply methods are used to call a function with a specified value and arguments. The difference between them is in how the arguments are passed to the function. In the call method, the arguments are passed as a comma-separated list, whereas in the apply method, the arguments are passed as an array.

  1. What is the difference between an object and an array?

In JavaScript, an object is a collection of key-value pairs, where each key is a string and each value can be of any type, whereas an array is a collection of ordered elements, where each element can be of any type. Arrays have a length property that represents the number of elements in the array, whereas objects do not.

  1. What is the difference between the slice and splice methods?

The slice method is used to create a new array that contains a portion of an existing array, without modifying the original array. The splice method, on the other hand, is used to modify an array by adding or removing elements from it.

  1. What is the difference between a constructor function and a class?

In JavaScript, a constructor function is used to create objects, whereas a class is a more recent addition to the language that provides a more formal way of creating objects. Both constructor functions and classes can be used to create objects with properties and methods, but classes provide a more structured approach to object-oriented programming.

  1. What is the difference between an instance method and a static method?

An instance method is a method that is called on an instance of a class or constructor function, whereas a static method is a method that is called on the class or constructor function itself, rather than on an instance of it.

  1. What is closure?

A closure is a function that has access to its parent scope, even after the parent function has returned. This allows the closure to maintain access to variables and other resources that were in scope when it was created.

  1. What is the difference between this keyword and the bind method?

This keyword is a reference to the object that a function is called on, whereas the bind method is used to create a new function with a specified value. The bind method can be used to create a new function with a fixed value, which is useful in certain programming patterns.

Conclusion

In conclusion, JavaScript is a powerful and versatile programming language that is used extensively in web development. In order to become a proficient JavaScript developer

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