Top 50+ JavaScript Interview Questions and Answers in 2023

This article contains a list of 50 JavaScript interview questions and answers, covering a wide variety of topics such as data types, functions, objects, arrays, loops, and more. Each question is followed by a short answer explaining the concept, feature or syntax involved.

1. What is JavaScript?

JavaScript is a language interpreted programming layer commonly used to create interactive web pages and web applications. It is a scripting language mainly used for client-side web development, but can also be used for server-side development using Node.js.

2. What are the different data types in JavaScript?

JavaScript supports many data types, including strings, numbers, booleans, zeros, undefined values, objects, and symbols. Strings are used to represent text, numbers are used to represent numeric values, booleans are used to represent true/false values, null represents the intentional absence of an object value, undefined represents the absence of a defined value, objects represent complex data structures and symbols are used to create unique identifiers.

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

null represents the intentional absence of an object value, while undefined represents the absence of a defined value. In other words, null is a value explicitly defined by a programmer to indicate the absence of an object, while undefined is a value that JavaScript automatically assigns when a variable or property has no value.

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

The == operator is used to compare values ​​for equality, but performs type conversion when the operand types are different . The === operator is used to compare values ​​for equality, but it does not perform type coercion, so the operand types must be equal.

5. What is the difference between let and var in JavaScript?

The let keyword was introduced in ES6 and is used to declare variables related to blocks. Variables declared with let are accessible only within the block in which they are defined. The var keyword is used to declare function-related variables. Variables declared with var are accessible through the function in which they are defined.

6. What is JavaScript elevation?

Production is a JavaScript mechanism by which variable and function declarations are pushed to the top of their scope before code execution. This means that variables and functions can be used before declaration, although this is not recommended.

7. What is a closure in JavaScript?

A closure is a function in JavaScript that allows a function to access variables in its parent scope even after the parent function has returned. Closures are created when a function is defined inside another function and the inner function has access to the outer function’s variables.

8. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code executes sequentially, which means that each statement executes in order and the next statement doesn’t start, not until the previous statement is not completed. On the other hand, asynchronous code allows you to run multiple tasks simultaneously without blocking other tasks from running.

9. What is an event loop in JavaScript?

Event loop is a mechanism in JavaScript that allows for asynchronous processing. It continuously checks the message queue for new tasks to run, and if a task is found, it runs it on the call stack.

10. What’s the difference between “call” and “apply” in JavaScript?

Call and apply are used to invoke a function with a specific value, but they differ in how additional arguments are passed to the function. The call expects the arguments to be passed individually, while apply expects the second argument to be passed as an array of arguments.

11. What is the difference between Split and Splice in JavaScript?

The Split method is used to extract part of an array and return a new array without modifying the original array. The Splice method, on the other hand, is used to add or remove elements from an array and modify the original array.

12. What is the difference between forEach and Map in JavaScript?

ForEach and Map are used to iterate over an array. However, they differ in how they handle the return value. forEach runs a function on each element of the array, but doesn’t create a new array. Map, on the other hand, creates a new matrix by applying a function to each element of the matrix and returning the result.

13. What is the difference between async and slow in JavaScript?

Async and slow are attributes that can be used to load JavaScript files asynchronously. However, they differ in how they affect script execution. Async allows the script to run immediately after download, without waiting for the HTML document to be parsed. Defer, on the other hand, downloads the script asynchronously, but defers script execution until the HTML document has finished parsing.

14 . What is the difference between innerHTML and textContent in JavaScript?

innerHTML is a property that sets or returns the HTML content of an element, including HTML tags. textContent, on the other hand, sets or returns the text content in an element, without HTML tags.

15. What is the difference between querySelector and getElementById in JavaScript?

querySelector is a method that returns the first element in a document that matches a specific CSS selector. getElementById, on the other hand, is a method that returns the element with the given ID.

16. What is JSON in JavaScript?

JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write, and easy for machines to parse and generate. It is a text format used to transfer data between a server and a client.

17. What is a promise in JavaScript?

A promise is an object representing the eventual completion (or failure) of an asynchronous operation and the resulting value. It allows you to write asynchronous code in a synchronous style, making reasoning easier.

18. What is the difference between a function call and a promise in JavaScript?

A function call is a function that is passed as an argument to another function and executed when the function has done its job. A promise, on the other hand, is an object that represents the eventual completion of an asynchronous operation and can be used to handle success or failure in a more structured way.

19. What is the difference between a factory function and a constructor function in JavaScript?

A factory function is a function that returns a new object, while a constructor function is a function that uses the new key keyword to create a new object is used. Factory functions are more flexible and can be used to create different types of objects, while constructor functions are used to create objects of a specific type or class.

20 . What is the difference between the “this” keyword and the “bind” method in JavaScript?

The “this” keyword is used to refer to the current object, while the “bind” method is used to create a new function with a specified value. Bind can be used to create a new function with a fixed value, which is useful when passing a method as a callback function.

21 . What is the purpose of the filter() method in JavaScript?

The filter() method is used to create a new array containing all elements that pass a test specified by a function. Often used to extract a subset of an array based on a condition.

22. What is event bubbling in JavaScript?

Event bubbling is a phenomenon where an event fired on a child element “bubbles” through its parent elements, allowing different elements to handle it in the process . It can be useful to handle events in a parent that affect its children.

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

Null represents an intentionally undefined value, while undefined represents an uninitialized value. In other words, null is explicitly defined by the programmer, while undefined is the default value for uninitialized function parameters and variables.

24. What is the difference between let and var in JavaScript?

Let and var are used to declare variables but they differ in their scope rules. let has block scope, which means it is only visible within the block in which it is declared, while var has function scope, which means it is visible in every function in which it is declared.

25 . 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 when the function completes its job. Often used with asynchronous functions to process the result of the asynchronous operation.

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

For and forEach loops are used to iterate over arrays, but they differ in their syntax and functionality. A for loop allows for more flexibility in iterating over the array, while a forEach loop is simpler and easier to read.

27. What is the purpose of the reduce() method in JavaScript?

The red() method is used to apply a function to each element of an array and reduce the array to a single value. The result of each function call is passed as an argument to the next function call until there is only one value left.

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

A class is a template for creating objects, while an object is an instance of a class. A class defines the properties and methods an object will have, while an object has its own state and behavior.

29. What is the difference between a static and a non-static method in JavaScript?

In JavaScript, all methods are non-static by default. A static method is a method that belongs to the class itself and not to a specific instance of the class. It can be called without creating an instance of the class.

30. What is the difference between a function declaration and a function expression in JavaScript?

A function declaration is a declaration that defines a function and makes it available in the current scope, while an expression of function function d is a function assigned to a variable or passed as an argument to another function. Function declarations reach the top of the current scope, but function expressions do not.

31. What is the try…catch statement used for in JavaScript?

The try…catch statement is used for error handling in JavaScript. It allows you to run a block of code and catch errors that may occur, allowing you to properly handle the error and prevent your code from failing.

32 . What is a promise in JavaScript?

A promise is an object representing the eventual completion (or failure) of an asynchronous operation and the resulting value. It allows you to write asynchronous code that is easier to read and reason with by using the then() and Catch() methods to process the result of the operation.

33 . What is the purpose of the async/await keywords in JavaScript?

The async/await keywords are used to write asynchronous code that looks and behaves more like synchronous code. The async keyword is used to mark a function as asynchronous, while the await keyword is used to wait for a promise to complete.

34 is elevation in JavaScript?

Elevation is a JavaScript behavior where the declarations of variables and functions are moved up in their respective scopes (global scope or function scope) during compile time. This means that you can use a variable or function before it is declared, but it may not have the expected value.

35. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code executes in sequence, one statement at a time, and blocks execution of subsequent statements until the end of the chain. Asynchronous code, on the other hand, allows for the simultaneous execution of multiple operations without blocking the execution of subsequent instructions. Use callbacks, promises or async/await to process the results of asynchronous operations.

36 What is the purpose of the Object.keys() method in JavaScript?

The Object.keys() method is used to return an array of enumerable property names specific to a particular object. It is often used to iterate over the keys of an object or to check if an object has a certain property.

37. What is the purpose of the “Strict Use” directive in JavaScript?

The “Strict Use” directive is used to enable strict mode in JavaScript, which enforces stricter syntax rules and prevents common mistakes. It also prohibits the use of undeclared variables and other dangerous behavior.

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

A shallow copy creates a new object that has the same properties as the original object, but the values of those properties are still references to the values of the original object. A deep copy creates a new object with new values that are not references to the original object’s values.

39. What is the typeof operator used for in JavaScript?

The typeof operator is used to determine the data type of a variable or expression in JavaScript. Returns a string indicating, for example, the type of operand. For example, “number”, “string”, “object”, “function”, “undefined”, or “boolean”.

40. What is the difference between a map and an array in JavaScript?

A map is a collection of key-value pairs, where each key can only appear once. A set is a collection of unique values, where each value can only appear once. A map is typically used to store data that can be accessed with a single key, while an array is typically used to store a collection of individual values.

41 What is the expansion operator used in JavaScript?

The expansion operator (…) is used to expand an iterable object, such as an array or string, into individual elements. It is commonly used to concatenate arrays or make a copy of an existing array.

42. What is event bubbling in JavaScript?

Event bubbling is a behavior whereby an event that occurs on a child element is propagated to its parent and ancestor elements in the DOM tree . This means that when you click a button on a div, both the button and the div receive the click event.

43. What is the difference between an arrow function and a regular function in JavaScript?

An arrow function is a shorthand way of writing a function expression in JavaScript. It has a concise syntax and lexically relates that value to the surrounding context. A regular function, on the other hand, has a more verbose syntax and associates that value with the function itself.

44. What is a JavaScript widget used for?

A widget is a global object in the browser that represents the current window or tab. Provides a number of properties and methods for manipulating the browser window, such as opening a new window, resizing the current window, or switching to a new page.

45 . What is the this keyword in JavaScript for?

The this keyword in JavaScript refers to the object whose current function is a method, or to the global object if the function is not a method of any object. It is commonly used to access properties and methods of the current object.

46. What is the difference between the let and var keywords in JavaScript?

The let keyword is used to declare a variable with block scope in JavaScript, which means that the variable is only visible in block scope . block to declare in The var keyword, on the other hand, declares a variable with functional scope or global scope, which means that the variable is visible throughout the function or program.

47. 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 when the main function is called. It is often used to handle asynchronous operations, such as B. to read data from a file or send an HTTP request.

48. What is the Array.prototype.map() method used for in JavaScript?

The Array.prototype.map() method creates a new array by applying a function to each element applied to an existing array. . The resulting values are collected in a new array which the method returns.

49. What is the difference between break and continue statements in JavaScript?

The break statement is used to exit a loop or switch, while the continue statement is used to continue the loop. skip a cycle. Repeat and go to the next iteration.

50. What is the purpose of the isNaN() function in JavaScript?

The isNaN() function is used to determine whether a value is NaN (not a number) or not. Returns true if the value is NaN and false if it is a valid number.