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Cracking the Frontend System Design Interview

On this article I am giving my very own expertise learning and passing within the FE system design interview in a number of excessive commonplace tech firms, I consider you’re conversant in most of what’s written on this article, however an important addition right here is that you’ll discover ways to prepare your ideas and to handle your expectations from this interview, merely a psychological mannequin for this sort of comparatively new interview sort in FE.

Supply: unsplash by firembee

Hey, if you’re studying this text whereas having a scheduled FE system design interview, initially “good luck”, second, it’s possible you’ll skip the intro if you do not have a lot time, third after end studying observe as a lot as you’ll be able to.


Twelve years in the past the share of Frontend work to Backend work was about ~10% to ~90% and it was clear that extra work and logic is occurring on the BE facet and FE was merely to validate a kind and ship the person interplay to the BE utilizing vanilla JS or possibly if you wish to write a barely cleaner code to make use of jQuery or one of many libs constructed on high of it.

I do know for some new folks to the net world this may be stunning a bit that again within the days there was no consumer state administration, no a lot look after caching, no dirct option to construct a single web page app, and this implies no routers, no commonplace modular answer (CommonJS, AMD, and most codes run in IIFEs), and the general JavaScript echo system was once a lot weaker than these days.

Now, the share of FE work has turned a lot larger, at the very least ~60% of the work (in common) might be on the FE facet (placing the DevOps half apart), loads of challenges are being raised a yr after a yr, and loads of enhancements within the echo system to boost the Frontend developer expertise like including a commonplace module system, a number of methods to construct a single web page app, routing, state administration, efficiency tuning, an enormous quantity of FE frameworks that confuse younger builders and far more.

In the previous few years it turned crucial for product firms with a purpose to rent nice senior FE engineers to gauge their talents to design a system in all of the FE ranges given all of the out there applied sciences on the market, and the share of codebase that has been moved to the FE facet.

On this article I’ll get you although some pillars which are crucial with a purpose to Crack the Frontend System Design Interview.

What to anticipate in that interview?

Normally firms decide a small downside that may be tackled in 40min max, because the interview is often beginning with about 10min for introductions from the interviewer’s facet then from the interviewee facet (you), and all the time they should preserve about 10min in the long run for query which you could ask or answering a comply with up query, so it is extremely vital to know the problem very nicely earlier than even beginning within the options.

Firms often tent to ask you to design some a part of a working product that they’re already having or well-known public website, so I recommendation you to test the product of the corporate that you’re interviewing for, problem your self to design some factor comparable (by design I do not imply type utilizing CSS), attempt to anticipate the difficult components that might be useful for them and give it some thought, and the way to set your psychological mannequin goes to be defined a bit later on this article.

FE system design interview questions (examples):

Design a chat app for customer support …

Design a search bar within the dwelling web page …

Design a collaboration board …

Design Twitter feeds …

Design Google Calendar …

As you observed, the issue is very concise with none particulars, that is intentional as a result of it’s a part of the interview to check your means to extract the small print your self particularly if you’re interviewing for a senior degree and above, asking the fitting questions and analyzing information are essential traits for skilled Frontend Engineers.

Steps to deal with the issue

Now as you recognize the query it’s time to understand how are you going to separate the time and make the most of your information in your finest curiosity, beneath is an inventory of the steps of dealing with a problem and the totally different components that often FE system design interview deal with. I’ve tried it a number of occasions as defined beneath and I handed each time 🎉 (Some modifications would possibly occur based mostly on interview or the corporate product)


  1. Extracting Data
  2. Settings scope (MVP)
  3. Information entities & API endpoints
  4. Element Structure
  5. Information Mannequin (State Administration)
  6. Community Communication
  7. Efficiency Optimization
  8. Accessibility (A11y)
  9. Different areas

Asking the proper questions and use the solutions in your design is a rule of thumb, you want first to take your time to know what it is advisable to obtain, it’s okay to inform your interviewers that you just want a silent second first to suppose about what is required, it may also be a good signal 🍀 that you do not simply say what involves your thoughts.

After you suppose begin by asking some good questions, however remember the fact that the questions ought to be related to the issue that you’re designing an answer for, additionally it is advisable to ask solely the questions that you just would possibly take determination in your design based mostly on it, questions that aren’t taken in consideration later is a pink flag 🚩.

Examples for good inquiries to extract info 👇:

  • The place is that this software going for use? this has 2 components if it is going to be used solely on Desktop, Cell, or Pill. That is essential with a purpose to know at the very least whether or not the app will likely be heavy used on Desktop for instance which makes you determine to construct for Desktop for the MVP and that may have an effect on different choices as nicely, additionally this query of the place would possibly embrace browsers whether or not you will help outdated browsers for instance or not (I doubt that persons are going to help IE11 any extra however you by no means know).
  • Who’s going to make use of it? their age, variety of customers for the MVP and likelihood of scaling sooner or later, so that you get an concept in regards to the willingness of scale comply with up questions and scope the MVP for small variety of customers and the sort and age of customers can be helpful for different areas later like A11y
  • The place geographically? this query is to extract data about supporting i18n or fascinated by CDN for internet hosting the belongings for the MVP
  • and extra of those sort of questions however crucial to ask questions that get you good data that can be utilized later in your design.

2. Settings scope (MVP)

After having all of the vital info, it’s time to set a scope on your MVP (Most Viable Product), this step is essential because the interview time may be very restricted round 40~45min, and also you already spent about 5~10min asking questions that provides you sufficient understanding, so you have got solely round 30min to design the system.

Cut up the requirement into 2 sorts ✌️:

1. Useful

These are those that the interviewer asks you to construct and the person will see and work together with, so for instance if you’re requested to construct a Twitter feed web page, it is advisable to record what will likely be useful as comply with 👇:

  • person can write a tweet
  • person can see the tweets feed in desc order
  • person can like and retweet tweets
  • and so forth …

2. Technical (Non-functional)

These ones are those that you extracted from the interviewers solutions from step one and different assumptions you would possibly make and confirm with the interviewer, a few of them as comply with 👇:

  • the app will likely be actual time
  • the app will work offline
  • the app ought to be responsive
  • and so forth …

After writing all of them down, you’ll attempt to deal with the principle components that the interviewer provides you sign that they’re going to deal with and embrace them beneath the scope of the MVP, the remainder might be listed beneath the title of past the MVP to be mentioned in case you have got time in the long run.

I do actually create two lists on the board used for the interview, one titled MVP and the opposite one is titled Past MVP and record them and get approval from the interviewer in the long run.

It is rather vital to ensure that the scope you outlined on your MVP is authorised by the interviewers earlier than begin in something.

3. Information entities & API endpoints

Now you have got the scope and a greater understanding of what’s wanted, it’s time to actually design the applying ranging from the information construction, how the information appears to be like like and the way you anticipate the API endpoint to seem like?

Somebody would possibly ask why ought to I determine that since BE engineers will do it for me? and it is a good and (barely) legitimate query to be sincere, and I will attempt to inform you why it is best to do this and in some instances it’s essential to.

As a Senior FE Engineer you may be requested to construct a new characteristic earlier than the endpoint has been created, and based mostly in your UI you’re in the very best place to determine what you anticipate from the endpoint with a purpose to present the proper and full information within the UI.

This information is essential if you find yourself constructing the sort system utilizing TypeScript for instance, the kinds of the properties and the construction is now not dictated solely by the BE as previously, so, designing the information entities within the interview might be finished via sorts like those in TypeScript that are very acquainted to any FE developer.

Right here is an instance for a solution 👇:

sort Tweet = {
  tweet_id: string;
  author_id: string;
  tweet_body: string;
  likes_count: quantity;
  retweet_count: quantity;

const getTweetUrl = '/api/v1/tweets/:tweet_id';

The objective of this half is to present your information of the code behind and that you may simply work with BE Engineers and produce a whole product and set the contract of information, this step is absolutely vital with a purpose to perceive and present your understanding of the system.

4. Element Structure

Now it’s time to ought to your creativeness of the UI, in some instances the interviewer would possibly provide you with a screenshot or ask you to design one thing very well-known or a part of their product (very helpful to take a look on their product earlier than the interview).

what is required right here is to not code, it’s about drawing a wire-frame first if there isn’t any screenshot to point out your means to show the textual content into one thing visible, then to attract a chart for the record of parts you would possibly construct in case of implementing this design, begin from the principle guardian part then break them right down to the smallest you consider.

It is vital to talk as you draw the charts, present why you would possibly construct your answer like that, go mild in addition to it isn’t actually implementation however you present the people who find themselves going to implement the system why you made it like that within the first place.

Sure, your might take into consideration the system design interview as a documentation for an answer that different folks will use to implement, so attempt to go high-level with none implementation particulars however write down your rationals.

Right here is wire-frames and parts structure for sort forward widget 👇:

component architecture

5. Information Mannequin (State Administration)

After you constructed the part structure it is advisable to clarify the best way these parts are going to speak with every others, in a easy approach as said above how you’ll design your state?

Utilizing the identical parts chart you draw within the earlier step, you may make some annotations on arrows about the place will you fetch information? and through which part? additionally which part is utilizing which piece of the state?

state management

Present your determined approach of sharing data between parts, is it props drilling or centralized retailer like Redux, MobX, or possibly a state machine like XState? vital to elucidate all of the choices that may match right here and communicate in regards to the drawbacks of every and why you select any.

It may be very helpful as nicely in the event you designed the state construction, present what will likely be there and the information circulation or Information Mannequin, this exhibits your understanding of the way to handle state within the frontend and the way you cut up it based mostly on software utilization, it is a crucial pillar within the abilities in FE.

It’s also possible to write some parts APIs just like the props, what most important parts are exposing for different parts to make use of, instance:

onChange: (question: string) => void;
onBlur: () => void;

In order that can provide an concept about every part and what’s uncovered in case of wrapping it right into a public npm bundle for instance.

6. Community Communication

In any case of what you defined to this point it is advisable to begin speaking about your communication with the server, it is extremely vital to know the other ways to speak with the server and the best way you’ll be able to fetch the information and why one in every of them would possibly match to the answer finest and NEVER to say as a result of that is the one I like or the one I do know or I really feel that this might work 🚩 you’ll be able to’t make structure determination based mostly in your intestine emotions

For instance if you’re requested to construct a chat widget, you want for positive a realtime communication, and you’ve got a number of choices right here together with Lengthy Polling, Server Despatched-Occasions, or Internet Sockets, you shouldn’t say for instance I will use Internet Socket and proceed, it is advisable to say as nicely why not Lengthy Polling? and why not Server Despatched-Occasions, even why not utilizing a standard http requests? these are legitimate questions that want solutions, and interviewers anticipate you to elucidate.

network communications

As you’ll be able to see within the instance above for a chat app, there’s a comparability in execs and cons for every and in the long run you would possibly change the colour of the chosen one and inform why you selected it over the others.

7. Efficiency Optimization

Right here comes the tough half (and essentially the most attention-grabbing for me), after you designed the information entities, constructed the part construction, designed the information circulation, and talked about your selection of community communication, it’s time to discuss in regards to the optimizations you see in all of those steps.

Efficiency is a large matter and it is advisable to be concise given the time you have got, based mostly on the query of the applying you’re designing it is going to be totally different, for instance when you’ve got an enormous record you would possibly want to say record virtualization however that does not match for all options, for instance when you’ve got solely 100 information virtualizing them wouldn’t provide you with a giant distinction.

Here’s a record of some widespread components that you just would possibly want to speak about 👇:

  • Core net vitals (LCP, CLS, FID, …and many others)
  • pictures (dimension and CDN)
  • compress (GZip or Brotli)
  • Code cut up and lazy loading
  • Server Facet Rendering
  • Service staff (make the app work offline)
  • loaders and skeleton design
  • and extra…

Mainly, it is advisable to study extra about particular efficiency points that may occur for the applying you’re designing and write all of them with a suggestion for it, it does not should be a full record however speaking about picture optimization in a chat app for instance is vital, speaking about core net vitals in an e-commerce app is essential and so forth.

I like to recommend to you checking the efficiency sequence of articles on my weblog right here to power your information in efficiency space normally.

8. Accessibility (A11y)

On this half, I’d say often it’s a bonus one if the time allow speak about it, nonetheless A11y is essential and would possibly provide you with additional factors if you’re conversant in it and may discuss and point out it concerning your design.

Right here is a few examples of issues which you could point out 👇:

  • all pictures to to have Alt attribute
  • all components to have aria attributes
  • use semantic HTML
  • be certain that to have excessive distinction colours
  • possibly utilizing OG tags for public content material
  • and so forth …

9. Different areas

In any case these stuff talked about right here you have got virtually a whole designed answer for many of the issues that may be launched to you in an interview, nonetheless based mostly on some instances it is advisable to deal with different stuff as nicely to get some additional credit score as comply with:

Another areas that you just would possibly want to say based mostly on query 👇:

  • Unit + e2e testing
  • Safety (CORS, XSS, HTTPS, …and many others)
  • search engine marketing for public content material
  • i18n and l10n
  • monitoring observability
  • AB testing

Every of them would possibly want a publish in its personal, however I’ll depart it so that you can examine every and put together forward, once more in the event you checked the corporate product that gives you a giant image of what to anticipate and what they may be specializing in.

Additionally looking out glassdoor and leetcode for the corporate you’re interviewing for may be very useful to know the kind of interview query within the FE system design, which is comparatively a brand new sort of interviews within the frontend world.


On this article we went via an inventory of subjects that I consider most of them as I discussed earlier may be acquainted to you however most vital is that now we have now sort of a framework (psychological mannequin) to reply frontend system design interview questions with, and most vital factor in any case is working towards this framework lots to get use to it and be prepared on your interview, the extra working towards the higher.

To summarize steps every in a single phrase: for any frontend system deign interview you set your psychological mannequin to: Data, MVP, Entities, Elements, State, Community, Efficiency, A11y, and Others

Good luck and if you need to debate this publish or have a remark about it, an edit, or if you’re enthusiastic about taking an actual case in an article or a video, do not hesitate to contact me on Twitter

Tot ziens 👋

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