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JavaScript is one of the most commonly tested subjects in tech interviews. Employers use it to assess both your coding fundamentals and how well you understand advanced programming concepts. In Part 3 of our JavaScript interview questions and answers guide, we go beyond the basics and explore more technical questions you are likely to face in real job interviews. This includes important topics like scope, callbacks, the difference between var, let, and const, and how asynchronous code works with promises or async/await.
These are the types of questions that often trip up candidates, especially if they’re only prepared at the beginner level. But don’t worry—we’ve broken them down in a simple and clear way so you can understand not just what the answer is, but why it’s correct. Whether you’re switching careers, upgrading your skills, or actively interviewing, this blog will help you build confidence. Read through the examples, take notes, and practice answering these questions out loud to improve your clarity and speed during interviews.
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border_sizes_right=”10px” border_sizes_bottom=”10px” border_sizes_left=”10px” border_color=”#007dc6″ border_style=”solid” box_shadow=”yes” box_shadow_vertical=”4″ box_shadow_horizontal=”4″ box_shadow_blur=”30″ box_shadow_spread=”0″ box_shadow_color=”rgba(0,0,0,0.32)” box_shadow_style=”” z_index=”” overflow=”” gradient_start_color=”” gradient_end_color=”” gradient_start_position=”0″ gradient_end_position=”100″ gradient_type=”linear” radial_direction=”center center” linear_angle=”180″ background_color=”” background_image=”” background_position=”center center” background_repeat=”no-repeat” fade=”no” background_parallax=”none” enable_mobile=”no” parallax_speed=”0.3″ background_blend_mode=”none” video_mp4=”” video_webm=”” video_ogv=”” video_url=”” video_aspect_ratio=”16:9″ video_loop=”yes” video_mute=”yes” video_preview_image=”” render_logics=”” absolute=”off” absolute_devices=”small,medium,large” sticky=”off” sticky_devices=”small-visibility,medium-visibility,large-visibility” 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order_small=”0″ spacing_left_medium=”” spacing_right_medium=”” spacing_left_small=”” spacing_right_small=”” spacing_left=”” spacing_right=”” margin_top_medium=”” margin_bottom_medium=”” margin_top_small=”” margin_bottom_small=”” margin_top=”” margin_bottom=”” padding_top_medium=”” padding_right_medium=”” padding_bottom_medium=”” padding_left_medium=”” padding_top_small=”” padding_right_small=”” padding_bottom_small=”” padding_left_small=”” padding_top=”20px” padding_right=”20px” padding_bottom=”20px” padding_left=”20px” hover_type=”none” border_sizes_top=”” border_sizes_right=”” border_sizes_bottom=”” border_sizes_left=”” border_color=”” border_style=”solid” border_radius_top_left=”” border_radius_top_right=”” border_radius_bottom_right=”” border_radius_bottom_left=”” box_shadow=”no” box_shadow_vertical=”” box_shadow_horizontal=”” box_shadow_blur=”0″ box_shadow_spread=”0″ box_shadow_color=”” box_shadow_style=”” background_type=”single” gradient_start_color=”” gradient_end_color=”” gradient_start_position=”0″ gradient_end_position=”100″ gradient_type=”linear” radial_direction=”center center” linear_angle=”180″ background_color=”” background_image=”” background_position=”left top” background_repeat=”no-repeat” background_blend_mode=”none” render_logics=”” filter_type=”regular” filter_hue=”0″ filter_saturation=”100″ filter_brightness=”100″ filter_contrast=”100″ filter_invert=”0″ filter_sepia=”0″ filter_opacity=”100″ filter_blur=”0″ filter_hue_hover=”0″ filter_saturation_hover=”100″ filter_brightness_hover=”100″ filter_contrast_hover=”100″ filter_invert_hover=”0″ filter_sepia_hover=”0″ filter_opacity_hover=”100″ filter_blur_hover=”0″ animation_type=”” animation_direction=”left” animation_speed=”0.3″ animation_offset=”” last=”true” border_position=”all” first=”true”][fusion_accordion type=”accordions” boxed_mode=”no” border_size=”1″ border_color=”” background_color=”” hover_color=”” divider_line=”” title_font_size=”” icon_size=”” icon_color=”” icon_boxed_mode=”” icon_box_color=”#4f4f4f” icon_alignment=”right” toggle_hover_accent_color=”#3ed9df” hide_on_mobile=”small-visibility,medium-visibility,large-visibility” class=”interview_list” id=””][fusion_toggle title=”Question 41: What are the arrow functions in JavaScript?” open=”no” class=”” id=””]
Answer:
Arrow functions are a feature in JavaScript introduced with ES6 that provide a more concise syntax for writing anonymous functions. They are sometimes referred to as “fat arrow” functions because of the syntax used to define them.
[/fusion_toggle][fusion_toggle title=”Question 42: What is the difference between Window and Document in JavaScript?” open=”no” class=”” id=””]
Answer:
In JavaScript, “Window” and “Document” are two important objects, but they serve different purposes within the context of web development:
- Window Object:
- The window object is the top-level object in the browser’s Document Object Model (DOM).
- It represents the entire browser window or tab and provides access to various properties and methods related to the browser itself.
- Document Object:
- The document object is a property of the window object and represents the HTML document loaded in the current browser window or tab.
- It provides access to the structure and content of the web page, allowing you to manipulate the DOM elements, such as modifying HTML content, changing styles, or adding/removing elements.
The key difference is that the window object represents the entire browser window or tab and deals with browser-related functionality, while the document object represents the HTML document loaded within that window and allows you to interact with and manipulate the content of the web page.
[/fusion_toggle][fusion_toggle title=”Question 43: What is the use of a constructor function?” open=”no” class=”” id=””]
Answer:
A constructor function is used to create and initialize objects. It’s essentially a blueprint for creating multiple instances of objects with similar properties and methods. Constructor functions are commonly used in object-oriented programming to define and create objects that share a common structure or behavior.
[/fusion_toggle][fusion_toggle title=”Question 44: What do you mean by prototype design pattern?” open=”no” class=”” id=””]
Answer:
The Prototype Design Pattern is a creational design pattern in software engineering. It’s used to create new objects by copying an existing object, known as the prototype, rather than creating them from scratch. This pattern is particularly useful when the cost of creating an object is more expensive or complex than copying an existing one.
[/fusion_toggle][fusion_toggle title=”Question 45: What are generator functions?” open=”no” class=”” id=””]
Answer:
Generator functions are a special type of function that allow you to pause and resume their execution, potentially yielding multiple values in the process. They are defined using the function* syntax, and they use the yield keyword to produce a sequence of values over time, rather than returning a single value like regular functions.
[/fusion_toggle][fusion_toggle title=”Question 46: What is the use of callbacks?” open=”no” class=”” id=””]
Answer:
Callbacks are a fundamental concept used to control the flow of asynchronous operations. They are functions that are passed as arguments to other functions and are executed at a later time, often when an asynchronous operation is completed. Callbacks are crucial for handling events, making HTTP requests, and working with timers.
[/fusion_toggle][fusion_toggle title=”Question 47: What is WeakSet?” open=”no” class=”” id=””]
Answer:
A WeakSet is an ordered collection of distinct elements that exclusively stores objects with weak references.
[/fusion_toggle][fusion_toggle title=”Question 48: What is a WeakMap?” open=”no” class=”” id=””]
Answer:
A WeakMap is a data structure in JavaScript that provides a way to create a collection of key-value pairs where the keys are weakly referenced. This means that if there are no other references to a key stored in a WeakMap, it can be automatically garbage collected by the JavaScript engine.
[/fusion_toggle][fusion_toggle title=”Question 49: Explain is NaN function?” open=”no” class=”” id=””]
Answer:
The isNaN function in JavaScript is used to determine whether a given value is NaN (Not-a-Number) or not. NaN is a special value in JavaScript that represents the result of an invalid mathematical operation, such as dividing zero by zero.
[/fusion_toggle][fusion_toggle title=”Question 50: State the difference between ViewState and SessionState?” open=”no” class=”” id=””]
Answer:
‘ViewState’ is specific to a page in the session whereas ‘SessionState’ is specific to user-specific data accessed across each web application pages.
[/fusion_toggle][fusion_toggle title=”Question 51: How can you read and write a file through JavaScript?” open=”no” class=”” id=””]
Answer:
There are mainly two ways to read and write a file through JavaScript which are as follows:
- Start by using JavaScript extensions.
- And then by using a web page and Active X objects.
[/fusion_toggle][fusion_toggle title=”Question 52: What are the different types of Pop-up boxes available in JavaScript?” open=”no” class=”” id=””]
Answer:
Following Pop-up boxes are available in JavaScript:
- Alert Box
- Confirm Box
- Prompt Box
[/fusion_toggle][fusion_toggle title=”Question 53: Difference between an alert box and a confirmation box?” open=”no” class=”” id=””]
Answer:
An alert box and a confirmation box are two types of dialog boxes used in web development to interact with users. Here’s the key difference between them:
- Alert Box:
- Alert boxes are used to display a message to the user, typically to provide information or notify them about something important.
- They have only one button, usually labeled “OK” or “Accept.” When the user clicks the “OK” button, the alert box disappears.
- Alert boxes are often used for displaying error messages, warnings, or informational messages.
- Confirmation Box:
- Confirmation boxes are used to ask the user for confirmation before performing an action that could have significant consequences, such as deleting data or submitting a form.
- They typically have two buttons, usually labeled “OK” and “Cancel.” The user can choose to confirm or cancel the action. The result of the user’s choice can be used to control the flow of the application.
- Confirmation boxes are commonly used for actions that require user consent or to prevent accidental actions that could be irreversible.
The main difference between an alert box and a confirmation box is their purpose and interaction. Alert boxes are for displaying information to the user, while confirmation boxes are for obtaining user consent or confirmation before proceeding with an action.
[/fusion_toggle][fusion_toggle title=”Question 54: What are JavaScript Cookies?” open=”no” class=”” id=””]
Answer:
JavaScript Cookies are small pieces of data that websites can store on a user’s computer or device. They are often used to track and store information about a user’s interactions with a website. Cookies are created by websites and stored in a user’s web browser.
[/fusion_toggle][fusion_toggle title=”Question 55: What is the use of a type of operator?” open=”no” class=”” id=””]
Answer:
The “typeof” operator is used to determine the data type of a given value or variable. It is a unary operator that can be used to check whether a value is a specific data type, such as a number, string, boolean, object, function, or undefined.
[/fusion_toggle][fusion_toggle title=”Question 56: What is blur function?” open=”no” class=”” id=””]
Answer:
In JavaScript, the term “blur” typically refers to an event or a method related to handling user interface elements like HTML form fields.
[/fusion_toggle][fusion_toggle title=”Question 57: How to detect the Operating System of the client machine?” open=”no” class=”” id=””]
Answer:
You can use the navigator. appVersion string to detect the operating system on the client machine.
[/fusion_toggle][fusion_toggle title=”Question 58: What is the difference between .call() and .apply()?” open=”no” class=”” id=””]
Answer:
.call() and .apply() are two methods in JavaScript that are used to invoke functions. They are similar in that they both allow you to call a function, but they differ in how they pass arguments to the function.
- .call():
- The call() method is used to call a function with a given this value and individual arguments.
- You provide the function’s this value as the first argument, followed by the arguments you want to pass to the function one by one.
- .apply():
- The apply() method is similar to call(), but it accepts an array-like or iterable object as its second argument. It then passes the elements of the array as individual arguments to the function.
- You provide the function’s this value as the first argument, and as the second argument, you provide an array or array-like object containing the arguments you want to pass to the function.
The main difference between call() and apply() is in how they accept and pass arguments to the function. call() expects individual arguments, while apply() accepts an array of arguments.
[/fusion_toggle][fusion_toggle title=”Question 59: What is the role of break and continue statements?” open=”no” class=”” id=””]
Answer:
In JavaScript, the break and continue statements are used to control the flow of execution in loops. Here’s what each of them does:
- break statement:
- The break statement is used to exit or terminate a loop prematurely when a certain condition is met.
- It is commonly used with for, while, and switch statements.
- When a break statement is encountered within a loop, the loop is immediately terminated, and the program continues executing the code after the loop.
- It is often used when you want to exit a loop early based on some condition, avoiding unnecessary iterations.
- continue statement:
- The continue statement is used to skip the current iteration of a loop and move to the next iteration.
- It is also commonly used with for and while loops.
- When a continue statement is encountered, the rest of the code within the current iteration is skipped, and the loop proceeds to the next iteration.
- It is often used when you want to skip specific iterations based on a condition without terminating the entire loop.
[/fusion_toggle][fusion_toggle title=”Question 60: What is the difference between a web garden and a web farm?” open=”no” class=”” id=””]
Answer:
The key difference between a web garden and a web farm is the scale and distribution of server resources. A web garden operates within a single server, while a web farm involves multiple servers working together to handle web traffic. The choice between these configurations depends on factors like the level of scalability, fault tolerance, and performance requirements of a web application.
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