You can build different online apps using JavaScript, a high-level server-side programming language. JavaScript is a popular programming language with millions of employment opportunities for web developers.
One of the stumbling blocks many web developers face is failure to pass a JavaScript interview or test. However, if you’re preparing for a JavaScript interview, read on to learn about the top JavaScript interview questions and their answers.
JavaScript Interview Questions and Answers
Q1. What is JavaScript?
Answer: JavaScript is an object-oriented, interpreted programming language that enables you to add interactivity to HTML pages that would otherwise be static. It allows you to handle multimedia, animate graphics, and generate dynamically changing content.
Q2. Is JavaScript a Case-sensitive language?
Answer: Yes. As a “basic” language that adheres to a naming convention for the defined objects, JavaScript is case-sensitive.
Q3. What Exactly Does Javascript’s Term “NULL” Imply?
Answer: The NULL value is used to denote the absence of any object or value. It suggests that there is no object, string, a valid boolean value, number, or array object.
Example:
function hiObject(nitro) {
if (!nitro) {
return null;
}
return { message: `Hello dear, ${nitro}!` };
}
hiObject('khan');
hiObject();
//output 1
'Hello dear, khan!'
//output 2
null
Q4. Is Javascript Client or Server Side Language?
Answer: JavaScript is a significant client-side scripting language that is frequently used in dynamic web pages. The script can be included in the HTML or kept in a separate file.
Q5. What are the Three Possible Ways JavaScript Script Files Can Be Used with HTML?
Answer:
- Inline
- Internal
- External
Q6. How do you Define a Variable in JavaScript?
Answer: To define a Variable in JavaScript, we use;
- Var
- Const
- Let
The VAR keyword variable has a function- or global scope. It implies that global access is possible to variables defined outside of the function.
When a const variable is declared, it cannot be modified because the CONST keyword has block scope.
The VAR keyword has been upgraded to the LET keyword. It is not available outside of the specific block (“block”); it is block-scoped.
Q7. List the Scopes of a Variable in JavaScript?
Answer: JavaScript has 3 types of scope:
- Block scope
- Function scope
- Global scope
Q8. What are the Two Fundamental Categories of Data Types in JavaScript?
Answer:
- Primitive: Primitive data types, also known as built-in data types, are the sorts of data that are already specified and offered by the JavaScript language.
- Reference types: In contrast to primitive data types, reference data types are dynamic. Since most of them are regarded as objects and do not have a defined size, they have methods.
Q9. What is a Callback Function in Javascript?
Answer: A simple JavaScript function sent as an argument or option to a method is known as a callback. You can use a function to call another function using this method. A callback function may execute following the conclusion of another function. Not in the order they are stated, but rather in the order they should be executed.
Q10. Give a Sample Showing a JavaScript Form Submission?
Answer:
<!DOCTYPE html>
<html>
<body>
<h2 style="color:green">NitroLeX.com</h2>
<b>Submit form details</b>
<form id="form_sub" action="form.php" method="post">
<label>NAME: </label><br />
<input type="text" name="name" /><br />
<label>AGE: </label><br />
<input type="number" name="age" /><br />
<label>SEX: </label><br />
<input type="text" name="sex" /><br /><br />
<a href="#" onclick="submitForm()">Submit Here</a>
</form>
<script>
function submitForm() {
let form = document.getElementById("form_sub");
form.submit();
}
</script>
</body>
</html>
Q11. How Do You Create an Object in JavaScript?
Answer: There are two ways to create an object in JavaScript: utilizing the Object Literal/Initializer Syntax. 2) using the new keyword in conjunction with the Object() Constructor function.
var p2 = new Object(); // Object() constructor function
p2.name = "Steve"; // property
**The creation of objects using object literal syntax is demonstrated in the example below**.
var emptyObject = {}; // object with no properties or methods
var person = { firstName: "John" }; // object with single property
// object with single method
var message = {
showMessage: function (val) {
alert(val);
}
};
// object with properties & method
var person = {
firstName: "James",
lastName: "Bond",
age: 15,
getFullName: function () {
return this.firstName + ' ' + this.lastName
}
};
Q12. How Can You Create an Array in JavaScript?
Answer: The values in an array must all be of the same type, whether they be ints, doubles, or any other type. You must first declare a variable with an array type before you can really generate the array.
var x = [];
var stringArray = ["one", "two", "three"];
var numericArray = [1, 2, 3, 4];
var decimalArray = [1.1, 1.2, 1.3];
var booleanArray = [true, false, false, true];
var mixedArray = [1, "two", "three", 4];
Q13. Which Keywords are Used to Handle Exceptions in JavaScript?
Answer: A program or piece of code that manages exceptions uses a try-catch-finally statement. The code that creates exceptions is executed by the try clause. Exceptions are caught using the catch clause. Finally clauses are always carried out. Throw statements cause exceptions to be produced.
Try{
Code
}
Catch(exp){
Code to throw an exception.
}
Finally{
Code runs either it finishes successfully or after catch
}
Q14. What Does the if Statement Do?
Answer: An instruction that directs a program to make judgments depending on predetermined criteria is the IF statement. If a certain condition is satisfied (TRUE), the IF statement executes one piece of code; otherwise, it executes another set of code if the other code evaluates to FALSE.
Q15. Use if Statement, Make a “Good Morning ” Greeting if the Hour is Greater Than 18:00:
if (hour > 18) {
greeting = "Good a.m";
}
Q16. What are the Different Types of Errors in JavaScript?
Answer:
Load time errors: Known as load time errors, these errors are generated dynamically and appear as a web page is loading.
Runtime errors: These are errors that result from the misuse of a command in the HTML language.
Logical Errors:These errors result from poor argument made to a function that performs a different operation.
Q17. List Some JavaScript Frameworks You Know
Answer: A JavaScript framework is a JavaScript-based application framework. Its control flow makes it different from a JavaScript library. Some popular examples are:
- Node.js
- Vue.js
- Angular.js
- Ember.js
- React.js
Q18. What Are JavaScript Libraries?
Answer: JavaScript libraries contain a variety of functions, methods, or objects you can use to carry out useful operations on a website or JS-based application. There are about 83 of them, each designed to fulfill a specific purpose,
Q19. List the Methods JavaScript Uses to Interact With HTML Elements
getElementById('idname')
getElementsByClass('classname')
getElementsByTagName('tagname')
querySelector()
Q20. Is it Possible to Divide Lines of JavaScript Code?
Answer: Yes. And you can do this with a backslash, ‘\,’ at the end of a JavaScript line of code.
Example:
document. Write ("This is \a program,");
Q21. Show How You Can Convert the String of Any Base to an Integer in JavaScript
Answer:
<!DOCTYPE html>
<html>
<head>
<title>
Conversion using parseInt
</title>
</head>
<body>
<h2 style="color:yellow">
Nitrolex.com
</h2>
</body>
<script>
let StringConversion = (string_value, base) => {
console.log(
"String value is : " + string_value +
" and base value is: " + base
);
let Integer_value = parseInt(string_value, base);
console.log("Integer value is: " + Integer_value);
};
StringConversion("1010", 2);
StringConversion("111", 10);
</script>
</html>
Q22. Write the JavaScript Code for Dynamically Adding New Items to HTML
Answer:
<html>
<head>
<title>javascript</title>
<script type="text/javascript">
function add_new_node () { var addP = document. createElement("p");
var textNode = document.createTextNode(" This is a new text node");
addP.appendChild(textNode); document.getElementById("cellOne").appendChild(addP); }
</script> </head>
<body> <p id="cellOne">cellOne<p> </body>
</html>
Q23. What is the Unshift Method in JavaScript?
Answer: The Unshift method pushes values intl of the array. Using this technique, one or more elements can be prepended to the beginning of the array.
Q24. What is for-in loop in Javascript?
Answer: The for-in loop is used to loop through the properties of an object.
Example:
for (variable name in object){
statement or block to execute
}
Q25. Describe With an Example How Closures Work in JavaScript?
Answer: A closure in JavaScript is a function that uses references from its inner scope to variables in the outer scope. Within its inner scope, the closure maintains the outside scope. When a function is closed, it indicates that even after the outer function has returned, the inner function can always access the variables and parameters of the outer function.
For example:
function Hello(message) {
console.log(message);
}
function greet(name, age) {
return name + " is saying that!! He is " + age + " years old";
}
// Generate the message
var message = greet("paul", 93);
// Pass it explicitly to greet
Hello(message);
#Using closures to rewrite the code
function greet(name, age) {
var message = name + " says howdy!! He is " + age + " years old";
return function greet() {
console.log(message);
};
}
// Generate the closure
var JamesGreet = greet("paul", 93);
// Use the closure
JamesGreet();
Q26. What is the Role of Break and Continue Statements?
Answer: The break statement terminates a switch or loop. It escapes the switch block in a switch. Continue Statements have slightly different functions. Contrarily, the continue command adds a new recursion to the current loop.
Q.27 What is OOPS Concept in JavaScript?
Answer: A computer programming paradigm known as object-oriented programming (OOP) arranges the architecture of software around data or objects rather than functions and logic. While object-oriented programming involves constructing objects that include both data and methods, procedural programming involves developing procedures or methods that perform actions on the data.
Q28. How are DOM Utilized in JavaScript?
Answer: How various components inside a document communicate with one another is governed by the Document Object Model or DOM. DOM is required for the development of web pages, which include objects like paragraphs, links, etc. It is also required to add more functionality to a web page. Using APIs has an advantage over other modern models as well.
Q29. How are event handlers utilized in JavaScript?
Answer: Events are the outcomes of activities, like the user clicking a link or completing a form. To ensure that each of these events is carried out correctly, an event handler is needed. Event handlers are an additional object attribute. The event’s name and the course of action, should the event occur, are included in this attribute.
Q30. What are JavaScript Cookies?
Answer: A cookie is an amount of information that persists between a server-side and a client-side. When a user accesses a website to store information they need, small test files called cookies are created and kept in the user’s computer.
Q31. What a pop()method in JavaScript is?
Answer: The pop() method removes the final element from the supplied array and returns it. The array on which it was called is then changed. The shift() method and pop() methods are analogous, except the shift method operates at the beginning of the array.
Example:
var colors = ["red", "pink", "blue "];
colors.pop();
<script>
function func() {
var arr = [4, 34, 67, 40];
// Popping the last element from the array
var popped = arr.pop();
document.write(popped);
document.write("<br>");
document.write(arr);
}
func();
</script>
Q32. Where are JavaScript Cookies Stored?
Answer: Cookies are little text files on your computer that contain data. The connection is cut off once a web server sends a web page to a browser, and the server forgets all about the user.
Q33. How do you add a new element to an array at the beginning?
Answer:
var myArray = ['a', 'b', 'c', 'd'];
myArray.push('end');
myArray.unshift('start');
console.log(myArray); // ["start", "a", "b", "c", "d", "end"]
With ES6, one can use the spread operator:
myArray = ['start', ...myArray];
myArray = [...myArray, 'end'];
Q34. What Result will the Following Code Produce?
for (var i = 0; i < 5; i++) {
setTimeout(function() { console.log(i); }, i * 1000 );
}
Answer: The displayed code sample displays 5, 5, 5, 5, and 5, rather than the expected values of 0, 1, 2, 3, and 4.
Q35. What is NaN?
Answer: The value “not a number” is represented by the NaN attribute. This particular value is the output of an operation that either failed because the operand was not a number (such as “category” / 7) or the outcome was not a number.
Q36. What is a QuickSort Algorithm in JavaScript?
Answer: Quick Sort uses a Divide and Conquer strategy. It achieves this by breaking the elements into smaller pieces based on certain criteria and then conducting various operations on those smaller pieces.