JavaScript, commonly known as JS, is a server-side programming language with a high level. Because JavaScript is widely utilized to create diverse online apps around the world, there are several possibilities for JavaScript development.

Candidates must pass the interview in order to get a job in JavaScript programming. Various JavaScript interview questions and responses are posed to them.

The list of JavaScript interview questions and answers that are most likely to be asked during the interview follows. Depending on their experience and other circumstances, candidates may be asked anything from simple JavaScript interview questions to advanced JS interview questions.

Let's Begin 

1. What is JavaScript?

JavaScript is a client-side and server-side scripting language inserted into HTML pages and is understood by web browsers. JavaScript is also an Object-based Programming language.

2. What are JavaScript Data Types?

Following are the JavaScript Data types:

• Number
• String
• Boolean
• Object
• Undefined

3. Difference between “ == “ and “ === “ operators.

Both are comparison operators. The difference between both the operators is that,“==” is used to compare values whereas, “ === “ is used to compare both value and types. 

4. Which is faster between JavaScript and an ASP script?

JavaScript is faster. JavaScript is a client-side language,, and thus it does not need the assistance of the webserver to execute. On the other hand, ASP is a server-side language and hence is always slower than JavaScript. Javascript now is also a server-side language (nodejs).

5. What is NaN property in JavaScript?

NaN property represents “Not-a-Number” value. It indicates a value which is not a legal number.

typeof of a NaN will return a Number .

To check if a value is NaN, we use the isNaN() function,

**Note- isNaN() function converts the given value to a Number type, and then equates to NaN.

6. Is it possible to break JavaScript Code into several lines?

Breaking within a string statement can be done by using a backslash, ‘\,’ at the end of the first line.

7. Which company developed JavaScript?

Netscape is the software company that developed JavaScript.

8. What are undeclared and undefined variables?

Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.

Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.

9. What is === operator?

=== is called a strict equality operator, which returns true when the two operands have the same value without conversion. 

10. Can an anonymous function be assigned to a variable?

Yes, you can assign an anonymous function to a variable.

11. What do you mean by NULL in Javascript?

The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number, and no array object.

12. What is the function of the delete operator?

The delete keyword is used to delete the property as well as its value.

13. What are escape characters?

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.

14. What are JavaScript Cookies?

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

15. What is the ‘Strict Mode in JavaScript, and how can it be enabled?

Strict Mode adds certain compulsions to JavaScript. Under the strict Mode, JavaScript shows errors for a piece of code, which did not show an error before, but might be problematic and potentially unsafe. Strict Mode also solves some mistakes that hamper the JavaScript engines from working efficiently.

Strict mode can be enabled by adding the string literal “use strict” above the file. 

16. What is for-in loop in Javascript?

The for-in loop is used to loop through the properties of an object.

The syntax for the for-in loop is –

for (variable name in object){
    statement or block to execute
}

17. Is JavaScript case sensitive? Give its example.

Yes, JavaScript is case-sensitive. For example, a function parseInt is not the same as the function Parseint. 

18. What boolean operators can be used in JavaScript?

The ‘And’ Operator (&&), ‘Or’ Operator (||), and the ‘Not’ Operator (!) can be used in JavaScript.

*Operators are without the parenthesis.

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