Top 50 JavaScript Interview Questions You Must Prepare in 2023

JavaScript Interview Questions and Answers | Full Stack Web Development Training | Edureka

This Edureka video on “JavaScript Interview Questions” will help you to prepare yourself in advanced Java script concepts for you to clear the Java script interview on your first attempt.

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Intermediate Level JS Interview Questions and Answers

Q22. What is the difference between Attributes and Property?

Attributes-  provide more details on an element like id, type, value etc.

Property-  is the value assigned to the property like type=”text”, value=’Name’ etc.

Q23. List out the different ways an HTML element can be accessed in a JavaScript code.

Here are the list of ways an HTML element can be accessed in a Javascript code:
(i) getElementById(‘idname’): Gets an element by its ID name
(ii) getElementsByClass(‘classname’): Gets all the elements that have the given classname.
(iii) getElementsByTagName(‘tagname’): Gets all the elements that have the given tag name.
(iv) querySelector(): This function takes css style selector and returns the first selected element.

Q24. In how many ways a JavaScript code can be involved in an HTML file?

There are 3 different ways in which a JavaScript code can be involved in an HTML file:

• Inline
• Internal
• External

An inline function is a JavaScript function, which is assigned to a variable created at runtime. You can differentiate between Inline Functions and Anonymous since an inline function is assigned to a variable and can be easily reused. When you need a JavaScript for a function, you can either have the script integrated in the page you are working on, or you can have it placed in a separate file that you call, when needed. This is the difference between an internal script and an external script.

Q25. What are the ways to define a variable in JavaScript?

The three possible ways of defining a variable in JavaScript are:

• Var – The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable. Example: var a =10; Variable declarations are processed before the execution of the code.
• Const – The idea of const functions is not allow them to modify the object on which they are called. When a function is declared as const, it can be called on any type of object.
• Let – It is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in.

Q26. What is a Typed language?

Typed Language is in which the values are associated with values and not with variables. It is of two types:

• Dynamically: in this, the variable can hold multiple types; like in JS a variable can take number, chars.
• Statically: in this, the variable can hold only one type, like in Java a variable declared of string can take only set of characters and nothing else.

Q27. What is the difference between Local storage & Session storage?

Local Storage – The data is not sent back to the server for every HTTP request (HTML, images, JavaScript, CSS, etc) – reducing the amount of traffic between client and server. It will stay until it is manually cleared through settings or program.

Session Storage – It is similar to local storage; the only difference is while data stored in local storage has no expiration time, data stored in session storage gets cleared when the page session ends. Session Storage will leave when the browser is closed.

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Q28. What is the difference between the operators ‘==‘ & ‘===‘?

The main difference between “==” and “===” operator is that formerly compares variable by making type correction e.g. if you compare a number with a string with numeric literal, == allows that, but === doesn’t allow that, because it not only checks the value but also type of two variable, if two variables are not of the same type “===” return false, while “==” return true.

Q29. What is the difference between null & undefined?

Undefined means a variable has been declared but has not yet been assigned a value. On the other hand, null is an assignment value. It can be assigned to a variable as a representation of no value. Also, undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Q30. What is the difference between undeclared & undefined?

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.

Q31. Name some of the JavaScript Frameworks

A JavaScript framework is an application framework written in JavaScript. It differs from a JavaScript library in its control flow. There are many JavaScript Frameworks available but some of the most commonly used frameworks are:

• Angular
• React
• Vue

Q32. What is the difference between window & document in JavaScript?

Q33. What is the difference between innerHTML & innerText?

innerHTML – It will process an HTML tag if found in a string

innerText – It will not process an HTML tag if found in a string

Q34. What is an event bubbling in JavaScript?

Event bubbling is a way of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event. With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. The execution starts from that event and goes to its parent element. Then the execution passes to its parent element and so on till the body element.

Q35. What is NaN in JavaScript?

NaN is a short form of Not a Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number. When a string or something else is being converted into a number and that cannot be done, then we get to see NaN.

In case you are facing any challenges with these JavaScript Interview Questions, please comment on your problems in the section below.

Q36. How do JavaScript primitive/object types passed in functions?

One of the differences between the two is that Primitive Data Types are passed By Value and Objects are passed By Reference.

• By Value means creating a COPY of the original. Picture it like twins: they are born exactly the same, but the first twin doesn’t lose a leg when the second twin loses his in the war.
•  By Reference means creating an ALIAS to the original. When your Mom calls you “Pumpkin Pie” although your name is Margaret, this doesn’t suddenly give birth to a clone of yourself: you are still one, but you can be called by these two very different names.

Q37. How can you convert the string of any base to integer in JavaScript?

The parseInt() function is used to convert numbers between different bases. It takes the string to be converted as its first parameter, and the second parameter is the base of the given string.

For example-

parseInt("4F", 16)

Q38. What would be the result of 2+5+”3″?

Since 2 and 5 are integers, they will be added numerically. And since 3 is a string, its concatenation will be done. So the result would be 73. The ” ” makes all the difference here and represents 3 as a string and not a number.

Q39. What are Exports & Imports?

Imports and exports help us to write modular JavaScript code. Using Imports and exports we can split our code into multiple files. For example-

//------ lib.js ------</span>
export const sqrt = Math.sqrt;</span>
export function square(x) {</span>
return x * x;</span>
}
export function diag(x, y) {
return sqrt(square(x) + square(y));
}

//------ main.js ------</span>
 { square, diag } from 'lib';
console.log(square(5)); // 25
console.log(diag(4, 3)); // 5

Now with this, we have reached the final section of JS Interview Questions.

Advanced Level JavaScript Interview Questions and Answers for Experienced Professionals

Q40. What is the ‘Strict’ mode in JavaScript and how can it be enabled?

Strict mode is a way to introduce better error-checking into your code.

• When you use strict mode, you cannot use implicitly declared variables, or assign a value to a read-only property, or add a property to an object that is not extensible.
• You can enable strict mode by adding “use strict” at the beginning of a file, a program, or a function.

Q41. What is a prompt box in JavaScript?

A prompt box is a box which allows the user to enter input by providing a text box. The prompt() method displays a dialog box that prompts the visitor for input. A prompt box is often used if you want the user to input a value before entering a page. When a prompt box pops up, the user will have to click either “OK” or “Cancel” to proceed after entering an input value.

Q42. What will be the output of the code below?

var Y = 1;
if (function F(){})
{
y += Typeof F;</span>
}
console.log(y);

The output would be 1undefined. The if condition statement evaluates using eval, so eval(function f(){}) returns function f(){} (which is true). Therefore, inside the if statement, executing typeof f returns undefined because the if statement code executes at run time, and the statement inside the if condition is evaluated during run time.

Q43. What is the difference between Call & Apply?

The call() method calls a function with a given this value and arguments provided individually.

Syntax-

fun.call(thisArg[, arg1[, arg2[, ...]]])

The apply() method calls a function with a given this value, and arguments provided as an array.

Syntax-

fun.apply(thisArg, [argsArray])

Q44. How to empty an Array in JavaScript?

There are a number of methods you can use to empty an array:

Method 1 –

arrayList = []

Above code will set the variable arrayList to a new empty array. This is recommended if you don’t have references to the original array arrayList anywhere else, because it will actually create a new, empty array. You should be careful with this method of emptying the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged.

Method 2 –

arrayList.length = 0;

The code above will clear the existing array by setting its length to 0. This way of emptying the array also updates all the reference variables that point to the original array. Therefore, this method is useful when you want to update all reference variables pointing to arrayList.

Method 3 –

arrayList.splice(0, arrayList.length);

The implementation above will also work perfectly. This way of emptying the array will also update all the references to the original array.

Method 4 –

while(arrayList.length)
{
arrayList.pop();
}

The implementation above can also empty arrays, but it is usually not recommended to use this method often.

Q45. What will be the output of the following code?

var Output = (function(x)
{
Delete X;
return X;
}
)(0);
console.log(output);

The output would be 0. The delete operator is used to delete properties from an object. Here x is not an object but a local variable. delete operators don’t affect local variables.

In case you are facing any challenges with these JavaScript Interview Questions, please comment on your problems in the section below.

Q46. What will be the output of the following code?

var X = { Foo : 1}; 
var Output = (function() 
{ 
delete X.foo; 
return X.foo; 
} 
)(); 
console.log(output);

The output would be undefined. The delete operator is used to delete the property of an object. Here, x is an object which has the property foo, and as it is a self-invoking function, we will delete the foo property from object x. After doing so, when we try to reference a deleted property foo, the result is undefined.

Q47. What will be the output of the following code?

var Employee =
{
company: 'xyz'
}
var Emp1 = Object.create(employee);
delete Emp1.company Console.log(emp1.company);

The output would be xyz. Here, emp1 object has company as its prototype property. The delete operator doesn’t delete prototype property. emp1 object doesn’t have company as its own property. However, we can delete the company property directly from the Employee object using delete Employee.company.

Q48. What will be the output of the code below? 

//nfe (named function expression)
var Foo = Function Bar()
{
return 7;
};
typeof Bar();

The output would be Reference Error. A function definition can have only one reference variable as its function name.

Q49. What is the reason for wrapping the entire content of a JavaScript source file in a function book?

This is an increasingly common practice, employed by many popular JavaScript libraries. This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries.
Another feature of this technique is to allow for an easy alias for a global variable. This is often used in jQuery plugins.

Q50. What are escape characters in JavaScript?

JavaScript escape characters enable you to write special characters without breaking your application. 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.

For example-

document.write "I am a "good" boy"
document.write "I am a "good" boy"

With this, we have come to the end of the JavaScript interview questions blog. I Hope these JS Interview Questions will help you in your interviews. In case you have attended interviews in the recent past, do paste those interview questions in the comments section and we’ll answer them. You can also comment below if you have any questions in your mind, which you might face in your JavaScript interview.

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