Front End Developer Interview Questions And Answers

1. Explain What Is The Lazy Loading?
Answer: Lazy loading is a design pattern commonly used in computer programming to defer initialization of an object until the point at which it is needed.
Lazy loading is loading code only once the user needs it. For Example, there is a button on the page, which shows a different layout once the user pressed it. So there is no need to load code for that layout on the initial page load.

2. What makes up a good Front-End Developer? What are his responsibilities?
Before speaking about who a Front-End Developer is, it is important to know what front-end is all about. Front-End, for a website, concerns all the components that the user accesses as part of his interactions with the website. So, it is all about the inclusion of buttons, color schemes, images, forms, animations, and typography.

So, the job of a front-end developer basically involves the creation of user-friendly components of a website. He is a software programmer who writes codes for all the features of a website that will improve its functionality, in a number of creative ways. Making the best use of his innovative and creative web development skills, it is the prime responsibility of a front-end developer to ensure that the look and feel of a particular website are uniforms across different web browsers.

3. Why do we recommend external CSS or Javascript versus inline?
Inline CSS or Javascript has bad impact on site performance.

Your HTML code will weigh more as you use inline scripts, whereas external scripts reduce HTML file size which helps fast rendering of the webpage.

HTML code will never be cached so inline scripts. Contrary to that, external dependencies, such as CSS and JavaScript files, will be cached by the visitor’s web browser. So it reduces https requests each time users click through web pages.

It is hard to maintain Inline CSS and Javascript code. Where having code in just one centralized location is a lot more preferable than changing exactly the same kind of code snippets spread all over the files on the web site.

4. Why do we need to use the W3C standard code?
The goals of such standards are to ensure cross-platform compatibility and more compact file sizes. The focus of these standards has been to separate “content” from “formatting” by implementing CSS. It eases maintenance and development.

5. Why table-less layout is very important?
There are several reasons why web designers should stop using tables for layouts, and adopt the use of CSS for controlling HTML layouts.

1) It adheres to current W3C web standards and it improves the accessibility of the information to a wider variety of users, using a wide variety of user agents.

2) There are bandwidth savings as large numbers of semantically meaningless tags are removed from dozens of pages leaving fewer, but more meaningful headings, paragraphs, and lists.

3) Layout instructions are transferred into site-wide CSS stylesheets, which can be downloaded once and cached for reuse while each visitor navigates the site.

4) If coded well, CSS makes it easy to apply global changes to the layout

5) Web pages often have less code and are much thinner when XHTML and CSS are used

6) Sites may become more maintainable as the whole site can be restyled or re-branded in a single pass merely by altering the mark-up of the specific CSS, affecting every page which relies on that stylesheet.

7) New HTML content can be added in such a way that consistent layout rules are immediately applied to it by the existing CSS without any further effort.

6. Can you briefly explain what a Thread-Local object in Python Flask is?
Through Python Flask, front-end developers use thread local objects internally. This is a function of user-friendliness which does not require the user to pass objects around from one function to another. This paves the way to process a request alongside permitting to operate in a “threadsafe” condition. Although this approach is worthwhile, it is essential to obtain a valid request context for dependency injection or when you are trying to reuse the code which employs a value tagged to the request.

7. Explain how you would ensure that your web design is user-friendly and what kinds of steps would you use?
A front-end developer needs to talk about working alongside UX(User Experience) designers to imagine and conceptualize a web page that cultivates a user-centric experience, testing the website with users to ensure optimal design and assuring the web page is optimized for mobile phone viewing. 

8. Explain what is the difference between Class and Prototypal inheritance?
Inheritance in JavaScript is different from most other programming languages. The object system in JavaScript is prototype-based, not class-based. Objects in JavaScript are just a collection of a name (key) and value pairs. When it comes to inheritance, JavaScript only has one construct: objects. Every object has a private property that contains a link to another object called its prototype.

9. Describe what is a Thread-Local object in Python Flask?
A thread-local object is an object that is stored in a dedicated structure, attached to the current thread id. Flask utilizes thread local objects internally so that users don’t have to pass objects around from function to function within a request to stay threadsafe. Besides, Thread local storage performs your logger’s context global but only within the current thread. This approach is helpful, but it needs a valid request context for dependency injection or when attempting to reuse code which utilizes a value pegged to the request. 

10. What is Event Bubbling?
Event bubbling is a type of event propagation where the event first triggers the deepest target element. It causes all events in the child nodes to be automatically passed to its parent nodes. The advantage of this method is speed because the code only requires to traverse the DOM tree once.

11. How do you ensure that your site or app design meets usability and web accessibility standards?
When working on your web product, a frontend developer should keep in mind your end-users and know how to provide a positive user experience for all of them. Accessible web resource will help you gain a broader audience and user-friendliness will help you keep users longer on your site, satisfy them and make them appreciate your website navigation, search features, color scheme, color contrast, underlined links, ALT text, HTML5 markup, mobile-friendliness, and so on.

That’s why checking how well your applicant is familiar with web accessibility standards and usability requirements are important. Ask about the best practices they know and have implemented on their previous projects, as well as how they were implemented.

What do you do to make other developers easily understand your CSS and JS code?
Front end developers usually work in a team along with other web developers, where any team member may have to read, use, review and communicate on each other’s code. This requires a web development process. In addition, as your web resource will exist for a long time, many different developers and teams may work on its maintenance and redesign.

So to insure everyone can easily understand the code and to prevent confusion and project delays, look for a frontend developer who is capable and willing to produce well-structured code. Ask prospective candidates about their experience with code organization and commenting. Make sure they understand the consequences of code commented incorrectly.

For example, a frontend developer can create sections for each site’s component on a stylesheet, where each section contains comments that accompany a code to allow other developers to make changes.

12. Explain What Is Ajax? Write An Ajax Call?
AJAX stands for asynchronous JavaScript and XML and allows applications to send and retrieve data to/from a server asynchronously (in the background) without refreshing the page. For example, your new Gmail messages appear and are marked as new even if you have not refreshed the page. 

13. Do You Know What Is The Importance Of The Html Doctype?
DOCTYPE is an instruction to the web browser about what version of the markup language the page is written. It’s written before the HTML Tag. Doctype declaration refers to a Document Type Definition (DTD).

14. Explain The Difference Between Inline, Block, Inline-block, And Box-sizing?
inline is the default. An example of an inline element is.
block displays as a block element, such as inline-block displays an element as an inline-level block container. Here’s an article on the topic.
box-sizing tells the browser sizing properties.

15. Explain What Is The Difference Between A Prototype And A Class?
Prototype-based inheritance allows you to create new objects with a single operator; class-based inheritance allows you to create new objects through instantiation. Prototypes are more concrete than classes, as they are examples of objects rather than descriptions of format and instantiation.

Prototypes are important in JavaScript because JavaScript does not have classical inheritance based on classes; all inheritances happen through prototypes. If the JavaScript runtime can’t find an object’s property, it looks to the object’s prototype and continues up the prototype chain until the property is found.

16. What are your favorite types of front end development projects to work on? What do you like about them?
This question gives you insight into the personal preferences of the front end developer. Some applicants may prefer to only work on website projects, while others enjoy being involved in web application teams more. What to look for in an answer:

A strong opinion on their favorite types of development
Enthusiasm for the project types they describe
Examples of their favorite front end development projects
Example: “I recently worked on a web application for a local hospital. It streamlined the patient intake process and I loved knowing that my work helped patients get the help they needed faster.”

17. Do You Know What Is Cors? How Does It Work?
Cross-origin resource sharing (CORS) is a mechanism that allows many resources (e.g., fonts, JavaScript, etc.) on a web page to be requested from another domain outside the domain from which the resource originated. It’s a mechanism supported in HTML5 that manages XMLHttpRequest access to a domain different.

CORS adds new HTTP headers that provide access to permitted origin domains. For HTTP methods other than getting (or POST with certain MIME types), the specification mandates that browsers first use an HTTP OPTIONS request header to solicit a list of supported (and available) methods from the server. The actual request can then be submitted. Servers can also notify clients whether “credentials” (including Cookies and HTTP Authentication data) should be sent with requests.

18. How To Optimize The Page Using Front End Code Or Technology?
Below is the list of best practices for front-end technology, which helps to optimize the page.

Improve server response by reducing resource usage per page
Combine all external CSS files into one file
Combine all external JS files into one file
Use responsive design instead of making device based redirects
Use asynchronous Javascript and remove block-level Javascript
Use the Minify version of stylesheet and javascript.
Optimize Image and use the correct format of Image. Use the lazy loading design pattern for a large size of images.
Use the browser-side cache with Cache-control.
Avoid plugins to drive functionality.
Configure viewport and use CSS best practices.
Prioritize visible content.
Load style-sheets in header and script in footer.

19. Explain The Difference Between Static, Fixed, Absolute And Relative Positioning?

static is the default.
fixed is positioned relative to the browser.
absolute is positioned relative to its parent or ancestor element.
relative is positioned relative to normal positioning/the item itself. Used alone it accomplishes nothing.

20. What is meant by Front-End Developer?
Front-end web development is the practice of producing HTML, CSS and usually JavaScript (while Web Assembly is a recent alternative to it) for a website or Web Application so that a user can see and interact with them directly. The challenge associated with front-end development is that the tools and techniques used to create the front end of a website change constantly and so the developer needs to constantly be aware of how the field is developing.

21. What are your favorite features of HTML5 and CSS3 and what would you change?
But rather than keeping up with new technologies, it reveals whether the front-end developer being interviewed is also up to date with new features within the core technologies.

22. Explain What Event Bubbling Is?
Event bubbling causes all events in the child nodes to be automatically passed to its parent nodes. The benefit of this method is speed because the code only needs to traverse the DOM tree once.

23. Why Do We Need To Use W3c Standard Code?
The goals of such standards are to ensure cross-platform compatibility and more compact file sizes. The focus of these standards has been to separate “content” from “formatting” by implementing CSS. It eases maintenance and development. 

24. Have You Ever Used A Css Preprocessor/precompiler? What Are The Benefits?
CSS preprocessors, such as SASS, have numerous benefits, such as variables and nesting.

25. Tell Me Where Do You Place Your Javascript On The Page?
It may depend on what you are using it for. There is some debate on this but generally a good question to ask to get an understanding of the JS knowledge.

26. What Is The Difference Between Call And Apply?
apply lets you invoke the function with arguments as an array. call requires the parameters to be listed explicitly. Also, check out this stackoverflow answer.

27. What Is A Javascript Object?
A collection of data containing both properties and methods. Each element in a document is an object. Using the DOM you can get at each of these elements/objects and do some cool sh*t. 

28. Can you describe your workflow when you create a web page?
The workflow of a modern front-end developer has changed vastly in the past four or five years. A huge array of tools are available to build organized, scalable web applications by abstracting out many of the complexities and automating repetitive tasks. Each developer will share a different workflow which will give some insight into their organizational capacity and general technical preferences

29. If you arrive at a new company that has 3 competing stylesheets, how would you best integrate them into the site?
A stylesheet is a template file consisting of font and layout settings to give a standardized look to a website or web application. To keep a consistent look and feel to a project, there should only be one stylesheet. I like to ask this question to judge problem-solving, communication and team skills.

30. Have you ever used an MVC? Which one and what do you like and dislike about it?
MVC stands for the model view controller. MVCs typically organize web apps into a well-structured pattern, making code easier to maintain. The term is well-known by developers and some famous examples of MVCs include backbone.js and angular.js. What makes this question interesting is not whether the frontend interviewee has used an MVC, but what his or her preferences and experience reveal. Candidates who are able to articulate why they use one MVC over another show that they are engaged in what they do, care about the technology and have considered different options. You want to be able to trust your frontend developer to keep up to date with which technologies are relevant and have a clear idea of when and what should be used. 

31. Have you learned anything interesting this week?
I recently got into Elm. I’ve been hearing about how great of a functional programming language it is for a while now. It may not be replacing JavaScript anytime soon but it is a very interesting alternative. I also got around to learning Regex. I’ve dabbled with it in the past but I never dove in deep. It’s a nice tool to add to my bag of tricks.

32. What are pseudo-elements used for?
The pseudo-element property is used to style one aspect of an element, such as the first letter of a paragraph. You can do lots of amazing things with them like wrap text around images, make several different shapes with a single element or create a body border.
Pseudo Element Roundup: a useful collection from CSS-Tricks.
Pseudo Elements: W3Schools resource on the subject.

33. Can you explain what Three.js is and quote its key features?
Three.js is an open-source JavaScript 3D library that allows you to create and display animated 3D computer graphics in a web browser. This is an API that uses WebGL to create impressive web demonstrations. And the best part of Three.js is that you can display all these graphics without depending on exclusive plug-ins.

Important features of Three.js along with their various functions are:

Renderers: – canvas, SVG, WebGL, CSS3D, DOM, Software; effects: stereo, cross-eyed.

Shaders: – You can completely access GLSL capabilities which include: lens flare, depth pass, and the all-in-one post-processing library.

Scenes: – You can add or delete objects at run-time.

Cameras: – Allows you to make the most of Orthographic and Perspective controllers like Trackball, FPS and Path.

Lights: – This feature allows you to flip between various lighting options like spot and point lights, additionally permitting you to cast and receive shadows.

Animations: – You can morph and perform Keyframe operations.

Materials: – Your website can sport a smooth-shading, with various options coming in the form of textures and sheen. Phong, depth, lambert, and face are some of the capabilities that you can employ.

Objects: – Through this feature, you can create meshes, lines, sprites, ribbons, and particles

Geometry: – Modifiers like a tube, lathe and extrude are available along with facilities to draw geometrical figures like cubes, spheres, and cylinders.

Export/Import: – With this feature, you can work with CTM, Blender, FBX, and OBJ

Debugging: – WebGL Inspector, Stats.js and Three.js inspector are the features under this head

Loaders: – This feature facilitates JSON, binary, scene, XHR and Image

Support: – You can check out the world of information that is available in the form of a public forum, API documentation, and Wiki

After answering the above questions, you may be asked a series of questions related to CoffeeScript

34. Describe Coffee Script?
CoffeeScript is a little programming language that compiles into JavaScript. It is an attempt to exhibit the good parts of JavaScript in a simple way. It also assists to write JavaScript code better by presenting you with a more constant syntax and skirting the unusual nature of JavaScript language.

35. How do you structure your source code to make it easy for leverage by your colleagues?
A front-end developer needs to discuss their use of code organization and commenting. They need to explain how they use notes in their programming process to explain the steps they have taken, ensuring the efficiency of understanding amongst collaborators.

36. Explain what is lazy loading?
Lazy loading is a design pattern generally utilized in computer programming to delay the initialization of an object until the period at which it is required. It can contribute to performance in the program’s operation if correctly and properly utilized. Besides, it is a loading code only once the user needs it. For instance, there is a button on the page, which reveals a different layout once the user pressed it. Hence, there is no necessary to load code for that layout on the initial page load. 

37. How do you organize your front-end development workflow?
From initiation to completion, the work of a front-end developer has to go through the sequence of processes. This path can be passed using various options. There are many workflow software that automates repetitive tasks to some degree to simplify and speed up work. There are also some tasks that are better to do manually.

Just as every person has their own tastes, every front-end developer has their own preferences and priorities regarding the workflow. Answers to this question for frontend developers will provide you with valuable insights into unique organizational and technical specifics of each developer.

38. What Is The Difference Between Form Get And Form Post?

Get: Getting the form data is encoded into a URL by the browser. The form data is visible in the URL allowing it to be bookmarked and stored in web history. The form of data is restricted to ASCII codes. Because URL lengths are limited there can be limitations on how many forms data can be sent.

Post: With POST all the name-value pairs are submitted in the message body of the HTTP request which has no restrictions on the length of the string. The name-value pairs cannot be seen in the web browser bar.

POST and GET correspond to different HTTP requests and they differ in how they are submitted. Since the data is encoded indifferently, different decoding may be needed.

39. Do You Know What Is A Closure?
Closures are expressions, usually functions, which can work with variables set within a certain context. Or, to try and make it easier, inner functions referring to local variables of its outer function create closures

40. Tell Me Are You Familiar With Jasmine Or Quit?
Jasmine and QUnit are JavaScript testing frameworks. I would familiarize myself with the basics.

41. How you can optimize the page using front end code or technology?

Below is the list of best practices for front-end technology, which helps to optimize the page.

  • Improve server response by reducing resource usage per page
  • Combine all external CSS files into one file
  • Combine all external JS files into one file
  • Use responsive design instead of making device based redirects
  • Use asynchronous Javascript and remove block-level Javascript
  • Use the Minify version of stylesheet and javascript.
  • Optimize Image and use the correct format of Image. Use the lazy loading design pattern for a large size of images.
  • Use browser-side cache with Cache-control
  • Avoid plugins to drive functionality
  • Configure viewport and use CSS best practices
  • Prioritize visible content
  • Load style-sheets in header and script in footer.

42. What is CoffeeScript? What are the Ways in which CoffeeScript is Superior to JavaScript?
CoffeeScript is a small programming language that helps you fine-tune JavaScript code. This language which compiles into JavaScript is a perfect alternative to the irregular syntax of JavaScript. Consistency in syntax is what makes CoffeeScript superior to JavaScript. Here are the basic rules for CoffeeScript:

Absence of curly braces
Functions that take arguments do not need parentheses
CoffeeScript is a better option compared to JavaScript on account of the following inherent advantages.

CoffeeScript simplifies your daily programming chores in contrast to JavaScript.
CoffeeScript cuts down on coding requirements and permits you to express your program with small codes when compared to JavaScript.
Through CoffeeScript, you can make the most of the lightweight add-ons like Python style list comprehension and Ruby string interpolation.

43. What are the applications of clone-function in CoffeeScript?
If you wish to create a completely new object in CoffeeScript, you can rely on the Clone function. You can do this in the following ways:

You need to copy all the attributes from the source object to the new object
You then need to repeat all the steps of copying attributes from the source object for all the sub-objects. To perform this function, you need to call the Clone function
The last step allows you to create a new object similar to the source object.

44. How do floats work?
The CSS float property is a box that can be shifted left or right horizontally. The float property has four values: left, right, inherit, and none. If you apply { float: right; } to an element, it will move to the furthest right of its parent element. If you apply { float: left;} to an element, it will move to the furthest left. { float: inherit; } causes the element to inherit the float value of its parent.
Floats: more information from
CSS Floats 101: a thorough guide on CSS floats.

45. How are the variables of CoffeeScript different from those of JavaScript?
In JavaScript, variables need to end with a semi-colon for them to be executed. However, with CoffeeScript, there is no necessity to add a semi-colon at the end of every statement. Hence, CoffeeScript simplifies the process of adding a semi-colon to variables. 

46. Explain the importance of the HTML DOCTYPE?
DOCTYPE is an instruction to the web browser regarding what version of the markup language the page is written. The DOCTYPE declaration needs to be the pretty first thing in your HTML document, before the tag. Doctype declaration points to a Document Type Definition (DTD). It provides markup language rules, so a browser can interpret the content correctly.

47. Explain the difference between classes and IDs?
Classes and ID selectors, both are utilized as hooks for CSS styles. The ID’s are commonly used to style elements that only look once on a page, such as one instance of a navigational menu. The Classes are utilized to style different elements in the same fashion, such as the presence of links.

48. Explain what is Event Delegation?
Event delegation points to the process of using event propagation to handle events at a higher level in the DOM than the element on which the event originated. It enables you to avoid adding event listeners to particular nodes; instead, you can add a single event listener to a parent element.

49. How can you increase page performance?

I can increase the page performance by the following methods.

Clean up the HTML document.
Reduce external HTTP requests.
Sprites, compressed images, smaller images.
Incorporate JavaScript at the bottom of the page.
Minify CSS, JavaScript, HTML.
CDN and Caching.

50. Explain how do you deal with browser-specific style incompatibility?
There are multiple ways to operate this. According, the simple way to proceed would be to utilize a conditional statement in the head tag of your HTML. In this way, you can recognize the browser and load an external stylesheet.

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