The most widely used scripting, interpreted, lightweight programming language is called JavaScript (JS). Web sites, mobile apps, web servers, and numerous other platforms are all familiar with JavaScript as a programming language. Since numerous job types need fluency in JavaScript, both front-end and back-end engineers must have a solid command of the language.
We will offer more than 25 JavaScript interview questions and answers in this post, suitable for both novice and seasoned developers. Everything that will help you ace your next JavaScript interview will be covered here, including ES6+ capabilities, DOM manipulation, asynchronous JavaScript, error handling, JavaScript frameworks and libraries, and Core JavaScript Concepts.
Table of Content
- JavaScript Interview Questions for Freshers
- JavaScript Intermediate Interview Questions
JavaScript Interview Questions for Freshers
Let’s discuss some common questions that you should prepare for the interviews. These questions will be helpful in clearing the interviews specially for the frontend development role.
1. What are the differences between Java and JavaScript?
Java is an object Oriented Programming language while JavaScript is a client-side scripting language. Both of them are totally different from each other.
- Java: It is one of the most popular programming languages. It is an object-oriented programming language and has a virtual machine platform that allows you to create compiled programs that run on nearly every platform. Java promised, “Write Once, Run Anywhere”.
- JavaScript: It is a light-weighted programming language (“scripting language”) for developing interactive web pages. It can insert dynamic text into the HTML elements. JavaScript is also known as the browser’s language.
2. What are Data Types in JavaScript?
JavaScript data types are categorized into two parts i.e. primitive and non-primitive types.
- Primitive Data Type: The predefined data types provided by JavaScript language are known as primitive data type. Primitive data types are also known as in-built data types.
- Numbers
- Strings
- Boolean
- Symbol
- Undefined
- Null
- BigInt
- Non-Premitive Data Type: The data types that are derived from primitive data types are known as non-primitive data types. It is also known as derived data types or reference data types.
- Objects
- Functions
- Arrays
3. Which symbol is used for comments in JavaScript?
Statements cannot be executed because of comments. While the code is being executed by the compiler, comments are ignored. In JavaScript, comments are represented by one of two types of symbols:
Double slash: It is known as a single-line comment.
// Single line comment
Slash with Asterisk: It is known as a multi-line comment.
/*
Multi-line comments
. . .
*/
4. What would be the result of 3+2+”7″?
Here, 3 and 2 behave like an integer, and “7” behaves like a string. So 3 plus 2 will be 5. Then the output will be 5+”7″ = 57.
5. What is the use of the isNaN function?
The number isNan function checks if the supplied value is of the type “Number” and is NaN (Not a number). The value NaN is regarded as a sort of number in JavaScript. If the argument is not a number, it returns true; if it is, it returns false.
6. Which is faster in JavaScript and ASP script?
JavaScript is faster compared to ASP Script. JavaScript is a client-side scripting language and does not depend on the server to execute. The ASP script is a server-side scripting language always dependable on the server.
7. What is negative infinity?
The negative infinity is a constant value represents the lowest available value. It means that no other number is lesser than this value. It can be generate using a self-made function or by an arithmetic operation. JavaScript shows the NEGATIVE_INFINITY value as -Infinity.
8. Is it possible to break JavaScript Code into several lines?
Yes, it is possible to break the JavaScript code into several lines in a string statement. It can be broken by using the ‘\n’ (backslash n).
Example:
console.log("A Online Computer Science Portal\n for Geeks")The code-breaking line is avoid by JavaScript which is not preferable.
let gfg= 10, GFG = 5,
Geeks =
gfg + GFG;
9. Which company developed JavaScript?
Netscape developed JavaScript and was created by Brenden Eich in the year of 1995.
10. What are undeclared and undefined variables?
- Undefined: This happens when a variable is declared but has no value assigned to it. A keyword is not “undefined.”
- Undeclared: This happens when we attempt to use the var or const keyword to access any variable that hasn’t been initialized or declared previously. We will encounter a runtime error with the return result “undefined” if we use the “typeof” operator to retrieve the value of an undeclare variable. The undeclared variables’ scope is always worldwide.
11. Write a JavaScript code for adding new elements dynamically.
<html> <head> </head> <body> <button onclick="create()"> Click Here! </button> <script> function create() { let geeks = document.createElement('geeks'); geeks.textContent = "Geeksforgeeks"; geeks.setAttribute('class', 'note'); document.body.appendChild(geeks); } </script> </body> </html>
12. What are global variables? How are these variables declared, and what are the problems associated with them?
In contrast, global variables are the variables that define outside of functions. These variables have a global scope, so they can be used by any function without passing them to the function as parameters.
Example: let petName = "Rocky"; // Global Variable myFunction(); function myFunction() { console.log("Inside myFunction - Type of petName:", typeof petName); console.log("Inside myFunction - petName:", petName); } console.log("Outside myFunction - Type of petName:", typeof petName); console.log("Outside myFunction - petName:", petName);
Output
Inside myFunction - Type of petName: string Inside myFunction - petName: Rocky Outside myFunction - Type of petName: string Outside myFunction - petName: Rocky
It is difficult to debug and test the code that relies on global variables.
13. What do you mean by NULL in JavaScript?
The NULL value represents that no value or no object. It is known as empty value/object.
14. How to delete property-specific values?
The delete keyword deletes the whole property and all the values at once like
let gfg={Course: "DSA", Duration:30};
delete gfg.Course;15. What is the difference between null and undefined in JavaScript?
A deeper comparison between the two special values, null (intentional absence of value) and undefined (uninitialized variables).
16. What is a prompt box?
The prompt box is a dialog box with an optional message prompting the user to input some text. It is often used if the user wants to input a value before entering a page. It returns a string containing the text entered by the user, or null.
17. What is the ‘this’ keyword in JavaScript?
JavaScript functions are fundamental objects. It can be returned from functions, passed to other functions, and assigned to variables just like objects. They also have their own qualities, much like things do. The JavaScript program’s current execution context is stored in “this.” The value of “this” will therefore vary when it is used inside a function based on the function’s definition, how it is called, and the default execution context.
18. Explain the working of timers in JavaScript. Also explain the drawbacks of using the timer, if any.
The timer uses the functions setTimout, setInterval, and clearInterval to run a certain code at a specified time or any little bit of code repeatedly. If the JavaScript code sets the timer to two minutes, the page will display the alert message “times up” when the allotted time has passed. After a predetermined number of milliseconds, the setTimeout() method evaluates an expression or calls a function.
19. What is the difference between ViewState and SessionState?
- ViewState: It is specific to a single page in a session.
- SessionState: It is user specific that can access all the data on the web pages.
20. How to submit a form using JavaScript?
You can use document.form[0].submit()method to submit the form in JavaScript.
21. Does JavaScript support automatic type conversion?
Yes, JavaScript supports automatic type conversion.
22. What is a template literal in JavaScript?
A template literal in JavaScript is a way to define strings that allow embedded expressions and multi-line formatting. It uses backticks (`) instead of quotes and supports ${} for embedding variables or expressions inside the string.
23. What is a higher-order function in JavaScript?
In JavaScript, a higher-order function is one that either returns a function as its result or accepts one or more functions as arguments. Functional programming patterns are made possible by these functions, which make code more abstract and reusable.
For instance, because they accept callback functions as arguments, map() and filter() are higher-order functions.
JavaScript Intermediate Interview Questions
24. What are all the looping structures in JavaScript?
- while loop: This type of control flow statement enables code to run repeatedly in response to a specified Boolean condition. You can think of the while loop as a repeated if statement.
- for loop: The loop structure can be written succinctly using a for loop. The for statement provides a shorter, easier-to-debug looping structure than a while loop since it consumes the initialization, condition, and increment/decrement on a single line.
- do while: An example of an exit control loop, a do-while loop is comparable to a while loop except that it checks the condition after executing the statements.
25. What is lexical scope in JavaScript?
In JavaScript, lexical scope describes how variables are resolved according to where they appear in the source code. The location of the code where a variable is defined determines its scope, and any nested functions or blocks can access it. This indicates that functions can access variables in the outside (lexical) scopes and their own scope, but not those in the inner scopes. let outer be equal to “I am outside!” console.log(outer); } function inner() { inner();
In this example, inner() can access the outer variable because of lexical scoping.
26. How does lexical scoping work with the this keyword in JavaScript?
Lexical scoping in JavaScript is mainly used for variable resolution, whereas this keyword’s action depends on how a function is called rather than where it appears in the code. Depending on the context of the function (e.g., whether it is called as a method, in a global context, or using call, apply, or bind), the value of this is dynamically decided at runtime. // “JavaScript”; const obj = { name: “JavaScript”, greet: function () { console.log(this.name); } }; obj.greet(); //?
Since the function is invoked as an object method, this here refers to obj. Variable lookups are impacted by lexical scoping, but this behavior is unaffected.
27. Explain how to read and write a file using JavaScript?
The readFile() functions is used for reading operation.
readFile( Path, Options, Callback)
The writeFile() functions is used for writing operation.
writeFile( Path, Data, Callback)
28. What is called Variable typing in JavaScript?
The variable typing is the type of variable used to store a number and using that same variable to assign a “string”.
Geeks = 42;
Geeks = "GeeksforGeeks";
29. What is hoisting in JavaScript?
In JavaScript, hoisting is the process by which function and variable declarations are positioned to the top of their containing scope prior to code execution during compilation. As a result, variables and functions can be referenced before they are specified in the code. However, initializations are not raised; only declarations are. console.log(a); // var a = 5; undefined
The declaration of an is raised in this instance, but the code execution does not assign its value (5) until it reaches that line. For var, let, const, and function declarations, hoisting is applied differently.
30. How to convert the string of any base to integer in JavaScript?
In JavaScript, parseInt() function is used to convert the string to an integer. This function returns an integer of base which is specified in second argument of parseInt() function. The parseInt() function returns Nan (not a number) when the string doesn’t contain number.
31. Explain how to detect the operating system on the client machine?
To detect the operating system on the client machine, one can simply use navigator.appVersion or navigator.userAgent property. The Navigator appVersion property is a read-only property and it returns the string that represents the version information of the browser.
32. What are the types of Pop up boxes available in JavaScript?
There are three types of pop boxes available in JavaScript.
- Alert
- Confirm
- Prompt
33. What is the use of void(0) ?
The void(0) is used to call another method without refreshing the page during the calling time parameter “zero” will be passed.
34. What are JavaScript modules, and how do you import/export them?
JavaScript modules allow you to split your code into smaller, reusable pieces. They enable the export of variables, functions, or objects from one file and the import of them into another. To export an element, you use export (either named or default). To import it, you use import.// In file1.js export const greet = () => "Hello"; // In file2.js import { greet } from './file1'; console.log(greet());
Modules help organize code and avoid global namespace pollution. They are natively supported in modern JavaScript through import and export statements.
35. What are WeakMap and WeakSet, and how are they different from Map and Set?
A WeakMap is a set of key-value pairs using objects as keys and any kind of data as values. A WeakMap’s key-value pairs are “weakly” kept, which means that an entry may be garbage collected if there are no additional references to it. Similar to WeakMap, a WeakSet is a collection of distinct objects that are weakly held.
In WeakMap and WeakSet, entries can be garbage collected if there are no other references to the objects, whereas in Map and Set, entries are firmly held, preventing garbage collection. This is the primary distinction between the two sets.
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