JavaScript Interview Questions JavaScript interview questions and answers for provides a list of top 20 interview questions. The frequently asked JavaScript interview questions with answers for beginners and professionals are given below. 1) What is JavaScript? JavaScript is a scripting language. It is different from Java language. It is object-based, lightweight and cross platform. It is widely used for client side validation. More details... 2) What is the difference between JavaScript and jscript? Netscape provided the JavaScript language. Microsoft changed the name and called it JScript to avoid the trademark issue.In other words, you can say JScript is same as JavaScript, but it is provided by Microsoft. 3) How to write a hello world example of JavaScript? A simple example of JavaScript hello world is given below. You need to place it inside the body tag of html. <script type="text/javascript">   document.write("JavaScript Hello World!");   </script>   4) How to use external JavaScript file? I am assuming that js file name is message.js, place the following script tag inside the head tag. <script type="text/javascript" src="message.js"></script>   5) Is JavaScript case sensitive language? Yes. 6) What is BOM? BOM stands for Browser Object Model. It provides interaction with the browser. The default object of browser is window. 7) What is DOM? What is the use of document object? DOM stands for Document Object Model. A document object represent the html document. It can be used to access and change the content of html. 8) What is the use of window object? The window object is automatically created by the browser that represents a window of a browser. It is used to display the popup dialog box such as alert dialog box, confirm dialog box, input dialog box etc. 9) What is the use of history object? The history object of browser can be used to switch to history pages such as back and forward from current page or another page. There are three methods of history object. history.back() history.forward() history.go(number): number may be positive for forward, negative for backward. 10) How to write comment in JavaScript? There are two types of comments in JavaScript. Single Line Comment: It is represented by // (double forward slash) Multi Line Comment: It is represented by slash with asterisk symbol as /* write comment here */ 11) How to create function in JavaScript? To create function in JavaScript, follow the following syntax. function function_name(){   //function body   }   12) What are the JavaScript data types? There are two types of data types in JavaScript: Primitive Data Types Non-primitive Data Types 13) What is the difference between == and ===? The == operator checks equality only whereas === checks equality and data type i.e. value must be of same type. 14) How to write html code dynamically using JavaScript? The innerHTML property is used to write the HTML code using JavaScript dynamically. Let's see a simple example: document.getElementById('mylocation').innerHTML="<h2>This is heading using JavaScript</h2>";    15) How to write normal text code using JavaScript dynamically? The innerText property is used to write the simple text using JavaScript dynamically. Let's see a simple example: document.getElementById('mylocation').innerText="This is text using JavaScript";    16) How to create objects in JavaScript? There are 3 ways to create object in JavaScript. By object literal By creating instance of Object By Object Constructor Let's see a simple code to create object using object literal. emp={id:102,name:"Rahul Kumar",salary:50000}    More details... 17) How to create array in JavaScript? There are 3 ways to create array in JavaScript. By array literal By creating instance of Array By using an Array constructor Let's see a simple code to create array using object literal. var emp=["Shyam","Vimal","Ratan"];     18) What does the isNaN() function? The isNan() function returns true if the variable value is not a number. 19) What is the output of 10+20+"30" in JavaScript? 3030 because 10+20 will be 30. If there is numeric value before and after +, it is treated is binary + (arithmetic operator). 20) What is the output of "10"+20+30 in JavaScript? 102030 because after a string all the + will be treated as string concatenation operator (not binary +).