Top JavaScript Interview Questions and Answers



Question : What is JavaScript ?

Answer : JavaScript is a client-side scripting language. JavaScript is used to develop interactive web pages as it inserts dynamic text into HTML element. JavaScript is also known as the browser’s language.

Question : Is JavaScript and Java same programming language ?

Answer : No they are completely different from each other. Java 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. “Write Once, Run Anywhere”

Question : What is === operator?

Answer : === is called as strict equality operator which returns true when the two operands are having the same value without any type conversion.

Question : What would be the result of 4+1+”6″?

Answer : Since 4 and 1 are integers, they will be added numerically. And since 6 is a string, its concatenation will be done. So the result would be 56.

Question : What is an undefined value in JavaScript?

Answer : Undefined value means the

  • Variable used in the code doesn’t exist
  • Variable is not assigned to any value
  • Property doesn’t exist

Question : What are escape characters?

Answer : Escape characters (Backslash) is used when working with special characters like single quotes, double quotes, apostrophes and ampersands. Place backslash before the characters to make it display.


document.write "I m a "good" boy"
document.write "I m a \"good\" boy"

Question : What are JavaScript Cookies?

Answer : Cookies are the small test files stored in a computer and it gets created when the user visits the websites to store information that they need. Example could be User Name details and shopping cart information from the previous visits.

Question : What is the use of isNaN function ?

Answer : The Number.isNan function in JavaScript is used to determines whether the passed value is NaN (Not a Number) and is of the type “Number”. In JavaScript, the value NaN is considered a type of number. It returns true if the argument is not a number else it returns false.

Question : What are undeclared and undefined variables ?

Answer :
Undefined: It occurs when a variable has been declared but has not been assigned with any value. Undefined is not a keyword.
Undeclared: It occurs when we try to access any variable which is not initialized or declared earlier using var or const keyword. If we use ‘typeof’ operator to get the value of an undeclared variable, we will face the runtime error with return value as “undefined”. The scope of the undeclared variables is always global.

Question : Is javascript case-sensitive?

Answer :
Yes, JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

Question : What is a prompt box ?

Answer : It is used to display 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.

Question : How can we create an object in JS?

Answer :

var object = 
         name: "obj",
         age: 10


Question : What is SetTimeout()?

Answer : When we setTimeout it becomes asynchronous and it has to wait on the stack to get everything got finished

Question : Why is javascript called Richer Interface?

Answer : We can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to our site visitors.


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