JavaScript Interview Questions

1.      What is JavaScript and what is its role in web development?

JavaScript is a programming language that is primarily used for creating interactive front-end web applications. It allows for dynamic updates on web pages, and can be used to create things like image sliders, form validation, and interactive maps.

 

2.      Can you explain the difference between let, var and const in JavaScript?

var is function scoped, let and const are block scoped. var variables can be reassigned and re-declared within their scope, while let and const cannot. let can be reassigned, but const cannot be reassigned.

 

3.      Can you explain the difference between == and === in JavaScript?

== (loose equality) compares two values for equality after converting them to the same type, whereas === (strict equality) compares two values for equality without any type conversion.

 

4.      What is closure in JavaScript and how is it used?

A closure is a function that remembers the variables in its lexical scope, even when it is invoked outside of that scope. Closures are often used to create private variables and methods, and to create closures that keep track of their own state.

 

5.      Can you explain event bubbling and event capturing in JavaScript?

Event bubbling is when an event propagates from the innermost element, and bubbles up to the outer elements. Event capturing is the opposite, where the event propagates from the outermost element to the inner elements.

 

6.      Can you explain the difference between synchronous and asynchronous code in JavaScript?

Synchronous code is executed in the order it is written, whereas asynchronous code can be executed at a later time. Asynchronous code allows for non-blocking operations and prevents the execution of the program from being blocked.

 

7.      Can you explain the prototype chain in JavaScript?

The prototype chain is a mechanism in JavaScript that allows objects to inherit properties and methods from their prototype. Every object has a prototype, and the prototype can have its own prototype, creating a chain of prototypes that can be searched to find a property or method.

 

8.      What is hoisting in JavaScript and how does it work?

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 in the code.

 

9.      Can you explain the difference between a forEach and a map function in JavaScript?

forEach is used to iterate over an array and execute a function on each element, whereas map creates a new array with the results of a function on each element.

 

10.   Can you explain the difference between null and undefined in JavaScript?

undefined means that a variable has been declared but has not been assigned a value, whereas null is a value that represents no value or no object.

 

 

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

A for loop is used to iterate over an array by using a counter, whereas forEach is a method that is directly called on an array and takes a callback function as an argument.

 

12.   What is a callback function in JavaScript?

A callback function is a function that is passed as an argument to another function and is executed after the first function has completed.

 

13.   What is a promise in JavaScript?

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.

 

14.   What is the difference between a class and an object in JavaScript?

A class is a blueprint for creating objects, and an object is an instance of a class.

 

15.   What is a template literal in JavaScript?

A template literal is a special type of string that allows for the embedding of expressions, and can be defined using backticks (`) instead of quotes.

 

16.   What is the difference between a set and an array in JavaScript?

A set is an unordered collection of unique values, whereas an array is an ordered collection of values.

 

17.   What is a spread operator in JavaScript?

The spread operator allows for the expansion of an array or an object into its individual elements.

 

18.   What is a destructuring assignment in JavaScript?

Destructuring assignment is a way to extract values from arrays or objects and assign them to variables.

 

19.   What is the difference between a while loop and a do-while loop in JavaScript?

A while loop only executes its block of code if the condition is true, whereas a do-while loop will execute its block of code at least once, and will then continue to execute as long as the condition is true.

 

20.   What is the difference between a shallow copy and a deep copy in JavaScript?

A shallow copy creates a new object with references to the same properties as the original object, whereas a deep copy creates a new object with new copies of all properties of the original object.

 

21.   How can you prevent a form from submitting in JavaScript?

You can prevent a form from submitting by calling the preventDefault() method on the event object, or by returning false from the event handler function.

 

22.   What is a Regular Expression in JavaScript?

A regular expression is a pattern that describes a set of strings. It can be used to match, search and replace text.

 

23.   What is a try-catch block in JavaScript?

A try-catch block is used to handle runtime errors by trying a block of code and catching any errors that are thrown.

 

24.   What is a generator function in JavaScript?

A generator function is a special type of function that can be paused and resumed, and can return multiple values.

 

25.   What is an arrow function in JavaScript?

An arrow function is a shorthand for creating anonymous functions. They have a shorter syntax and do not have their own this, arguments, super, or new.target bindings.

 

26.   What is a WeakMap in JavaScript?

A WeakMap is a collection of key/value pairs in which the keys are objects and the values can be arbitrary values. The keys are held weakly, and the garbage collector can clear them when there are no other references to them.

 

27.   What is a WeakSet in JavaScript?

A WeakSet is a collection of objects where the objects are held weakly, and the garbage collector can clear them when there are no other references to them.

 

 

 

28.   What is a Prototype in JavaScript?

A prototype is an object that serves as a template for creating other objects. Every JavaScript object has a prototype, and the properties and methods of the prototype are inherited by the objects that are created from it.

 

29.   What is a Module in JavaScript?

A module is a self-contained piece of code that exports certain values, and can be imported by other parts of the code. Modules are used to organize and reuse code.

 

30.   What is the difference between a static method and an instance method in JavaScript?

A static method is a method that is called on a class, whereas an instance method is a method that is called on an instance of a class.

 

31.   What is a Symbol in JavaScript?

A Symbol is a new primitive type in JavaScript that is used to create unique identifiers.

 

32.   What is a Map in JavaScript?

A Map is a collection of key/value pairs that can be iterated over in insertion order.

 

33.   What is a Set in JavaScript?

A Set is an unordered collection of unique values.

 

34.   What is a WeakReference in JavaScript?

A WeakReference is an object that holds a weak reference to another object, and does not prevent the garbage collector from clearing the referenced object.

 

35.   What is a Decorator in JavaScript?

A Decorator is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class.

 

36.   What is a Higher-Order Function in JavaScript?

A Higher-Order Function is a function that takes one or more functions as arguments and/or returns a function.

 

37.   What is a Currying in JavaScript?

Currying is a technique of evaluating a function with multiple arguments into a sequence of functions, each with a single argument.

 

38.   What is a Partial Application in JavaScript?

Partial Application is a technique of creating a new function by providing some of the arguments to an existing function.

 

39.   What is a Transpiling in JavaScript?

Transpiling is the process of converting code written in one programming language into another programming language.

 

40.   What is a Transpiler in JavaScript?

A Transpiler is a tool that converts code written in one programming language into another programming language.

 

41.   What is a Lazy Evaluation in JavaScript?

Lazy Evaluation is a technique of delaying the evaluation of an expression until its value is needed.

 

42.   What is a Async/Await in JavaScript?

Async/Await is a way to write asynchronous code that makes it look more like synchronous code.

 

43.   What is a Async Iteration in JavaScript?

Async Iteration is a way to iterate over asynchronous data sources.

 

44.   What is a Async Generator in JavaScript?

An Async Generator is a way to generate asynchronous data.

 

45.   What is a Concurrent Mode in JavaScript?

Concurrent Mode is a feature of JavaScript that allows multiple tasks to be executed at the same time.

 

46.   What is a Suspense in JavaScript?

Suspense is a feature of JavaScript that allows components to wait for the data they need before rendering.

 

47.   What is a Hook in JavaScript?

A Hook is a way to use state and other React features in functional components.