JavaScript Interview Questions & Answers

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JavaScript Interview Questions & Answers

JavaScript Interview Questions & Answers for Freshers

What are the differences between Java and JavaScript?

JavaScript is a client-side scripting language and Java is object Oriented Programming language. Both of them are totally different from each other.

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.
Java: Java 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”.

What are JavaScript Data Types?

There are three major Data types in JavaScript.

Primitive
- Numbers
- Strings
- Boolean
- Symbol
Trivial
- Undefined
- Null
Composite
- Objects
- Functions
- Arrays

Which symbol is used for comments in JavaScript?

Comments prevent the execution of statements. Comments are ignored while the compiler executes the code. There are two type of symbols to represent comments in JavaScript:

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
            ....
            */

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.

What is the use of the isNaN function?

The number isNan function 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.

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.

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.

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 backslash ‘\’. For example:

        
            document.write("A Online Computer Science Portal\ for Geeks")

The code-breaking line is avoid by JavaScript which is not preferable.

        
            let gfg= 10, GFG = 5,
            Geeks =
            gfg + GFG;

Which company developed JavaScript?

Netscape developed JavaScript and was created by Brenden Eich in the year of 1995.

What are undeclared and undefined variables?

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

Write a JavaScript code for adding new elements dynamically.

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.

It is difficult to debug and test the code that relies on global variables.

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.

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;

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.

What is the ‘this’ keyword in JavaScript?

Functions in JavaScript are essential objects. Like objects, it can be assign to variables, pass to other functions, and return from functions. And much like objects, they have their own properties. ‘this’ stores the current execution context of the JavaScript program. Thus, when it use inside a function, the value of ‘this’ will change depending on how the function is defined, how it is invoked, and the default execution context.

Explain the working of timers in JavaScript. Also elucidate the drawbacks of using the timer, if any.

The timer executes some specific code at a specific time or any small amount of code in repetition to do that you need to use the functions setTimout, setInterval, and clearInterval. If the JavaScript code sets the timer to 2 minutes and when the times are up then the page displays an alert message “times up”. The setTimeout() method calls a function or evaluates an expression after a specified number of milliseconds.

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.

How to submit a form using JavaScript?

You can use document.form[0].submit() method to submit the form in JavaScript.

Does JavaScript support automatic type conversion?

Yes, JavaScript supports automatic type conversion.

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