Technical interview questions and answers are extremely important for JavaScript Interviews because JavaScript powers most modern web applications. Companies evaluate your understanding of functions, scope, closures, events, DOM manipulation, ES6 features, promises, async/await, and real-time coding skills. JavaScript is one of the most commonly tested subjects in frontend developer interviews, full-stack roles, and placement assessments. Companies like TCS, Infosys, Wipro, Cognizant, and Accenture frequently include JavaScript questions in their technical rounds. This guide covers the most frequently asked JavaScript interview questions with detailed explanations to help freshers and job seekers prepare effectively. Learning these questions will give you confidence during coding tests, practical tasks, and web development interviews.
Frontend developers should enhance their skills by mastering CSS styling techniques and HTML structure fundamentals
Showing 10 of 78 questions
51. How do you assign object properties?
obj["age"] = 17 or obj.age = 17.
52. whats a way to append a value to an array?
arr[arr.length] = value;
53. What does the term sticky session mean in a web-farm scenario? Why would you use a sticky session? What is the potential disadvantage of using a sticky session?
Sticky session refers to the feature of many commercial load balancing solutions for web-farms to route the requests for a particular session to the same physical machine that serviced the first request for that session. This is mainly used to ensure that a in-proc session is not lost as a result of requests for a session being routed to different servers. Since requests for a user are always routed to the same machine that first served the request for that session, sticky sessions can cause une
54. You have an ASP.NET web application running on a web-farm that does not use sticky sessions - so the requests for a session are not guaranteed to be served the same machine. Occasionally, the users get error message Validation of viewstate MAC failed. What could be one reason that is causing this error?
The most common reason for this error is that the machinekey value in machine.config is different for each server. As a result, viewstate encoded by one machine cannot be decoded by another. To rectify this, edit the machine.config file on each server in the web-farm to have the same value for machinekey.
55. What does undefined value mean in javascript?
Undefined value means the variable used in the code doesn't exist or is not assigned any value or the property doesn't exist.
56. What is the difference between undefined value and null value?
(i)Undefined value cannot be explicitly stated that is there is no keyword called undefined whereas null value has keyword called null
(ii)typeof undefined variable or property returns undefined whereas typeof null value returns object
57. What is variable typing in javascript?
It is perfectly legal to assign a number to a variable and then assign a string to the same variable as follows
example
i = 10;
i = "string";
This is called variable typing
58. Does javascript have the concept level scope?
No. JavaScript does not have block level scope, all the variables declared inside a function possess the same level of scope unlike c,c++,java.
59. What are undefined and undeclared variables?
Undeclared variables are those that are not declared in the program (do not exist at all),trying to read their values gives runtime error.But if undeclared variables are assigned then implicit declaration is done .
Undefined variables are those that are not assigned any value but are declared in the program.Trying to read such variables gives special value called undefined value.
60. What is === operator ?
==== is strict equality operator ,it returns true only when the two operands are having the same value without any type conversion.