Thank you to the leetcode community for being extremely helpful, now it's my turn to contribute my experience and feedback.

MYSELF / SUMMARY

Experience: 2.5 years as Full-Stack Software Engineer
Education: unrelated eng degree from top 10 eng school (self-taught dev)
Previous Role: Full-stack Software Engineer + Part-time Remote Full-stack Software Engineer (2 jobs)
Date started applying: End of January / Beginning of February 2020
Date offer accepted: Mid-March 2020
Total companies applied to: ~60
Total companies interviewed with: ~16
Leetcode: ~150 (60 easy / 80 medium / 10 hard)

PREPARATION

Overview

The bulk of my preparation was doing leetcode-style questions on leetcode and other places. I did a total of ~150 problems, comprised of ~60 easy, ~80 medium, and ~10 hard. I also completed the Udemy course "Master the Coding Interview: Data Structures + Algorithms" by Andrei Neagoie. For system design I mainly watched YouTube videos, such as: Design Twitter, Design a Parking Lot, and TechLead's video. Lastly, I also spent a good 15-20 hours of just thinking back and writing out in great detail my top 6-8 most successful/complex projects in preparation for the behavioral stages.

Video Links:
(if you can't see the embedded videos, just right-click on the hyperlink text and open in a new tab)

The Turning Point

I was initially trying to go through all of the materials above while holding both my full-time + part-time job, so I was navigating everything a bit slowly. Once I started getting interviews with big-name companies I became much more stressed because I knew that not having a CS background put me at a disadvantage, which means I needed a lot more time to prepare if I wanted to have a fighting chance. Watching this video from Nick White was the main turning point for me. He mentions that he was able to do well in interviews for Google and Uber because he studied all day every day for a month straight. The next day I put in my 2-week notice at my full-time job, which gave me me about 2.5 weeks to study full-time. Had I not done that I know that I would not have been as successful in my interviews.

Leetcode Strategy

I made a list of all of the "core" data structures and algorithms that were recommended for study in the Google and Facebook prep materials (which are consistent with the ones you'd find in a YouTube or Google search). These are, for example, strings, arrays, trees, linked lists, stacks, queues, hash tables, sorting, graphs, dynamic programming, etc. Then I simply bounced around and did problems from each area, starting with "Easy" difficulty problems and then moving on to only "Medium" difficulty. This means that I would NOT do multiple tree problems and then multiple array problems, instead I would follow something more like: array => string => dynamic programming => linked list, etc. As onsite dates approach, I used leetcode premium to filter problems by that given company and focused mainly on those problems.

In regards to my approach to individual problems, if I was completely stumped, I would still give myself up to an hour or two to try to come up with a brute-force solution. Once I solved, or didn't solve, a given problem, I would look at the solutions in the discussion and see how my solution compared in complexity and conciseness. If my answer was significantly weaker, then I would clear out my code and try again later that day or the following day to ensure that I understood how to write the more optimal solution. For problems that were super new to me, or just really challenging, I would search YouTube for detailed explanation videos, such as for n-queens, word ladder, longest palindromic substring, etc.

INTERVIEW EXPERIENCES

Dropbox [REJECTED]

• Position
  • Front-end Software Engineer
• Application
  • I applied directly on their site and got an email to schedule a call with one of their recruiters
  • This was, if I remember correctly, the first technical interview I did (during my 2020 interviewing)
• Stage 1: Recruiter Call
  • Quick and straightforward conversation about the position, the company, and my background/experience
• Stage 2: Technical Phone Screen
  • Overview
    • Two problems, one easier and one more challenging
    • I made the mistake of thinking there was only one problem, so I was very thorough in my explanation for Problem 1 and then ran out of time for Problem 2
  • Problem 1
    • Mock CSS-class matching for single class
      • Given a mock DOM tree and a single class name, return all of the DOM elements that have the class
  • Problem 2
    • Mock css-class matching for nested classes, i.e. "class1>class2>class3"
      • Given a mock DOM tree and a string of nested classes, return all of the DOM elements that match the class with the given class hierarchy
• Interview Result
  • Was rejected due to not passing all test cases in Problem 2
  • This was one of my first interviews so I was still not fully prepared, and I definitely took it as a lesson to always ask in subsequent interviews whether there are 1 or 2 problems so I can try to allocate my time appropriately
• Overall Experience
  • Very positive
  • Both the recruiter and engineer were nice, and the problems themselves were both relevant and fun/challenging
  • Dropboox was one of my top choices so it definitely hurt to be rejected, and the fact that it was one of my first interviews was demotivating

PayPal [NDA - LATE OFFER]

• Position
  • Full-stack Software Engineer
• Overview
  • Applied directly on their site
  • Went through onsite interview process
• Interview Result
  • Was initially rejected, but then got a call a couple weeks later if I was still interested in joining. By that time I had already accepted an offer so I thanked them and declined.
• Interview Experience
  • Mostly positive
  • Some of the interview stages felt a bit adversarial, i.e. felt like "why should we even be considering you?"

Microsoft [REJECTED]

• Position
  • Software Engineer (applied to several, including Front-end, Azure, and general Level 2)
• Application
  • I applied directly on their website for several positions, and never got a response
  • I then searched for Microsoft recruiters on LinkedIn and messaged them directly
  • After messaging 3-4 recruiters, one of them responded and then that ultimately led to a tech phone screen for a Level 2 SE position
• Stage 1: Tech Phone Screen
  • Learned about the engineer's xp, the position, and the company
  • Was asked detailed questions about my experience
  • Then we did a coding exercise, count total leaves in a tree
  • Interviewer then added some additional criteria to the question, which I didn't necessarily have to code, but more as a discussion
  • Interviewer then asked additional general technical questions across a range of topics
• Interview Result
  • Was ultimately ghosted
  • I think my background/xp was simply not enough for a Level 2 role
• Interview Experience
  • Very positive (other than being ghosted)

HackerRank [OFFER]

• Position
  • Front-end Software Engineer
• Application
  • Applied directly on their site
  • Got an email invite to do their initial online assessment (no call yet)
• Stage 1: Online Assessment (on HackerRank)
  • Had 2 hours to do two small React projects (completed via HackerRank's "projects") which allow you to clone a repo and push your changes via git (was a really cool experience)
  • First project was to build some components to render a list of items in a table, with the ability to sort by various fields
  • Second project was similar to the first, except it involved adding routes and being able to navigate to a route and filtering the data by the path in the route
• Stage 2: Phone call w/ Eng Manager (technical + behavioral)
  • Wasn't too sure what to expect, but the call ended up being a super positive experience which lasted something like 90-120 minutes
  • The call involved situational questions that were both behavioral and technical, as well as a bunch of specific front-end + React questions across the board
• Stage 3: Virtual Onsite Part I (the virtual onsite was split into two days)
  • In-person onsite was changed to virtual due to COVID-19
  • Interview 1: System Design
    • Design a web app where people can play tic-tac-toe together
  • Interview 2: Coding Challenge
    • Challenge 1
      • Given a string that is a grammatically-correct English sentence, return a string that is a new sentence with all of those words sorted by word-length, while making sure that capitalization + punctuation is correct
    • Challenge 2
      • leetcode #121 - best time to buy and sell stock
• Interview 3: React Project
  • Given part of HackerRank's custom component library, build a simple tabs component (i.e. you can select tab 1 to see that content, or select tab 2 to see that content)
• Stage 4: Virtual Onsite Part II
  • Interview 1: Behavioral
    • Was asked to give examples of when I personified HackerRank's core values
  • Interview 2: Technical + Business
    • Had a general conversation with an engineering SVP about the company and myself
  • Interview 3: Wrap-up w/ Recruiter
• Interview Result
  • Was told that I passed the onsite and that they'd be moving forward with an offer
  • I accepted an offer with my top choice company before they could finalize their offer, so I let them know and thanked them for a great xp
• Interview Experience
  • Very positive
  • Was super excited to interview with a YC company, and I was really impressed with their products as well as everyone I interviewed with

Amazon [OFFER]

• Position
  • Software Development Engineer
• Application
  • Recruiter reached out to me via Hired.com and invited me to have an initial phone call
• Stage 1: Call with recruiter
  • Quick and straightforward conversation about the position, the company, my background/experience, and the interview process
• Stage 2: Online Assessment
  • Overview
    • Had something like 90 minutes to do two medium leetcode-style questions and then write about my solutions and their space/time complexity
  • Question 1
    • leetcode #419 - battleships on a board
  • Question 2
    • leetcode #200 - number of islands
• Stage 3: Tech Phone Screen
  • Overview
    • Had a 45-min call with a sr engineer which included some behavioral questions plus a coding challenge
  • Coding Challenge
    • Write a function that takes an array of numbers and returns the second-largest number
• Stage 4: Virtual Onsite
  • Interview 1: LP + Coding Challenge
    • Overview
      • As usual, Amazon interviews have a strong focus on the leadership principles, so I was asked 1 or 2 LP questions + the coding challenge
    • Coding Challenge
      • Assume you have something like a google analytics page tracker that logs which page a user went to and which page the user came from, i.e. user went from Page 1 to Page 2
      • Write a function that takes an array of page logs and two pages and then returns whether there exists a transitive link between those two pages, i.e. if user went from Page 1 to Page 2 to Page 3, there exists a transitive link between Page 1 and Page 3
  • Interview 2: LP + System Design
    • System Design
      • Design a system that handles the checkout payment part of an e-commerce app, i.e. when a user is in their cart and then submits their payment
  • Interview 3: LP + Coding Challenge
    • Coding Challenge
      • Write a function that takes a tree and returns the max width
  • Interview 4: LP + Coding Challenge
    • Coding Challenge
      • "Build tic-tac-toe"
• Interview Result
  • Heard back later that afternoon that I passed and that they were gonna make me an offer
  • Offer was extremely attractive, but I wanted to wait to hear from Google and FB first
  • Ultimately declined the offer due to accepting FB
• Interview Experience
  • Very positive
  • Recruiters were outstanding and super helpful
  • Interviews were challenging
  • Only non-positive comment is that I don't really feel like I "clicked" with 3 out of the 4 onsite interviewers, and in all of the interviews it was pretty much impossible to gauge whether the interviewers were happy/unhappy with my responses

Google [was in hiring committee stage]

• Position
  • Software Engineer
• Application
  • Google recruiter cold-messaged me on LinkedIn like 6 months earlier, when I wasn't interviewing
  • I followed up with that recruiter and was able to go directly to an onsite interview somehow
• Stage 1: Call with recruiter
  • Quick and straightforward conversation about the position, the company, my background/experience, and the interview process
• Stage 2: Virtual Onsite
  • Interview 1: Coding Challenge
    • Coding Challenge
      • Write a function that takes an array of words and returns their shortened letter-count version, i.e. ["google", "word"] => ["g4e", "w2d"]. If the shortened version is not unique, expand out 1 letter at a time
  • Interview 2: Coding Challenge
    • Coding Challenge
      • The question was a real-world application of weighted Dijsktra's algorithm
    • Coding Challenge Result
      • I made the mistake of not practicing Dijsktra's algorithm, so I pretty much bombed this round, but I tried to fail as gracefully as possible
      • Tried to rack up points on the parts of the problem which were more general math, and then I just kept asking questions, including questions about how Dijkstra's algorithm works
      • The interviewer was nice and answered all of my questions where possible, which helped me feel less embarassed/incompetent
      • The prep materials said to practice Dijkstra's algorithm but I ignored it because i was like, "nah i doubt they would ask that during a 45-min interview", and lo and behold. Moral of the story, study EVERYTHING on your guide (whichever study guide the company gives you)
  • Interview 3: Coding Challenge
    • Coding Challenge
      • Something about tracking when RPC calls were made and detecting when a particular call exceeds the timeout threshold
  • Interview 4: Googlyness
    • Was asked several situational behavioral questions
  • Interview 5: Coding Challenges
    • Overview
      • Was asked 1 coding challenge, and to discuss the start of a second technical challenge
    • Coding Challenge 1
      • Don't remember the exact problem, but something related to taking an array of numbers and a target number, and then rotating the array upon the target number without changing the original order
    • Technical Question 2 (no coding)
      • Given a map as a square, and a set of locations on the map, users are able to select a rectangle on the map and all of the locations on the map are returned
      • How would you preprocess the map data so that the find operation for the locations in the inner rectangle can be done in less than O(n) time?
• Interview Result
  • Feedback on my performance was mostly positive, so I made it to the hiring committe stage. At this point, the HC decided that because of my poor performance in the second interview but strong performance in the other rounds, they wanted me to do two more 45-min interviews focused on front-end and JavaScript (since JS is my strongest language)
  • While waiting to schedule the additional two interviews, I started the team-matching process and met with a team that I ended up feeling really strongly/positive about
  • Shortly before my interviews, however, FB made me an offer, and that was my top choice so I accepted FB and ended my application/candidacy with Google and thanked them for the great opportunity and experience
• Interview Experience
  • Very positive
  • Recruiters were outstanding
  • Interviews were challenging
  • I enjoyed the engaging feedback from the interviewers during the coding challenges

Facebook [OFFER - ACCEPTED]

• Position
  • Rotational Software Engineer
• Application
  • Interviewed with FB in 2018 and got rejected. Recruiter followed up with me a year later and asked if I was interested in interviewing again. At the time I wasn't but then several months later I followed up with that recruiter and was able to schedule a call to start the interview process again
• Stage 1: Call with recruiter
  • Quick and straightforward conversation about the position, the company, my background/experience, and the interview process
• Stage 2: Tech Phone Screen
  • Coding Challenge 1
    • "Easy" level leetcode question
    • Return the sum of an array that contains either numbers or other arrays of numbers, and multiply the total of each array by its nested depth
  • Coding Challenge 2
    • "Medium" level leetcode question
    • Given a string with alphanum characters and open or close parentheses, remove the minimum number of parentheses necessary to ensure all parentheses correctly close (only "(" and ")" are used, no brackets or curly braces)
• Stage 3: Virtual Onsite
  • Interview 1: Coding Challenge
    • Challenge 1
      • Write a function that accepts an array of numbers and returns the length of the longest monotonic subsequence
    • Challenge 2
      • Write a function that accepts a tree and returns the length of the longest path between any two nodes
  • Interview 2: System Design
    • Problem
      • You're working on a music streaming app. How would you build a feature to show users their top 10 most-played songs each week?
      • Interviewer had follow-up questions, and also asked about things like how to measure the feature's success, and what other similar/related features can be built on top of this
  • Interview 3: Behavioral
    • Was asked several behavioral questions about how I would handle a certain scenario or to talk about past challenges/successes
  • Interview 4: Coding Challenge
    • something like leetcode #460 - subarray sum equals k
• Interview Result
  • Received offer for Rotational Software Engineer E4
• Interview Experience
  • Extremely Positive
  • Recruiters were outstanding
  • Interviewers were really experienced
  • All of the discussions felt like real, down-to-earth conversations, as if we were working a problem together
  • FB was my #1 choice so I accepted the offer and then declined my other offers/candidacies

CLOSING THOUGHTS

1. I think that I was probably largely carried by my communication/interpersonal skills, especially during the times that I could not fully solve a problem, or had only a brute-force solution
   • I made every effort to never let there be more than a couple seconds of silence or "nothingness"
   • If I was stuck, I would just think out loud "hmm, can we use another array here, or maybe a hash table would help..", and if I was heading in the wrong direction then the interiewer would give some useful feedback like "i don't think we need to use a hash table, what if we did the problem in reverse..." or something like that
   • So, my advice would be to use *** or practice with friends until you feel strong/confident about your communication skills
   • I'll say it again: If you don't feel confident about your communication skills: PLEASE PRACTICE
2. Related to #1: DON'T UNDERESTIMATE THE BEHAVIORAL QUESTIONS (especially for Amazon)
   • I was lucky and found someone else's post on their Amazon onsite experience here on leetcode where they talked about how they were underprepared for the leadership principles questions. Because I saw this post, I spent more than one entire day (> 14 hours) just thinking back through all of my past jobs and projects, and then typing up notes that I then referenced during interviews (following the STAR template). I can say with certainty that going through this exercise and having all of those notes readily available allowed me to strongly succeed during behavioral interviews with multiple companies.
3. You have to be thinking, breathing, and living coding challenges and design interviews for at least a month straight to really be fully prepared (or at least that was the case for me)
   • It wasn't until my ~120th leetcode question that I started to feel more confident, to the point where I could at least come up with a brute-force, if not optimal, solution for many of the medium problems in my target categories, and solve all (or most) easy problems
4. Mental health and routine
   • I used to sleep only 5-6 hours several nights a week, but during this interview phase I made sure to get the full 8 hours of sleep every night
   • For me, exercise helped me stay sane through all of the preparation and back-to-back 5+ hour-long interviews
   • Preparation/study days
     • I'm a morning person so I would normally wake up around 6am, have coffee, and start studying/practicing
     • After 3-4 hours of intense studying, I need a break because I become unproductive. This is when I would go to the gym or go outside for a run
     • After exercise I eat, have a coffee, and get another 3-4 hour power session in
     • By the time the evening rolls around, the mental barrier to work on another coding challenge is super high, so this is when I would watch videos on algorithms or system design
   • Interview days
     • During my interview with PayPal, I made the mistake of going into the interview "cold" (no warmup problems). This made me stumble through my first interview, after which I was able to do much better. From that point on I made sure to always do 1-2 practice problems before any interview (but not too many to where I felt tired or strained)
   • My routine for interview days was like:
     • wake up early
     • have a coffee
     • do a practice problem and review my solutions to problems that are commonly asked by that company
     • go exercise
     • eat, and then have a coffee
     • do another practice problem and review solutions again
     • do interview
5. During the interview
   • Virtual interviews (due to COVID-19)
     • Having a whiteboard in my office was extremely helpful during the virtual interviews. I can say with certainty that having my large whiteboard helped me draw out and solve many of the coding challenges, so I recommend getting one that you can use and show via video during your interviews.
   • Clarify ALL assumptions and get agreement on your gameplan (as we've all heard many times)
     • If someone asks "return the max width of a tree", you should ask what kind of tree is it? What is the definition of width? On multiple occasions, my assumptions were different from the interviewer, so had I not asked clarifying questions from the beginning, I probably would've started off on a very wrong path

I hope this information helps some of you out there, as many previous leetcode posts helped me. Remember to never give up. There were some problems that I was able to understand after practicing them once or twice, but there were many more problems that I had to do over and over and over again, more than 6 times. Also, try to develop a routine so that you have a consistent system and schedule for practicing and interviewing.

I'm happy to answer any additional questions where possible.

========== FOLLOW-UPS FROM COMMENTS ==========

"How long should I spend on a problem before I look at the solution?"

Working on leetcode problems is about striking a balance between "i need to spend as much time working on a challenge to develop my problem-solving muscle" and "i need to get as many tools and solutions under my belt so that I have a better chance during the interview".

As I mentioned earlier, I personally tried not spend more than 1-2 hours trying to solve a particular problem. Why? Well, you usually won't get more than like 30-40 minutes in any given interview, and beyond the 1-2 hours it becomes kind of helpless. I would try to solve a problem in under 45 minutes. If I failed, I would start over and try to really use my still-developing heuristics, i.e. "what data do i have, what data do we need to get to, how should we store that data, does sorting help, does dynamic programming help, does working in reverse help, does bfs help, does dfs help", etc. Once I went through this exercise 2 or 3 times, it was like, "okay i have no idea how to proceed", so I would like at the solutions in the discussion. The more frequently you do this, the better you get at recognizing patterns or similarities between problems, but you also have to remember to build your heuristics. In other words, develop your problem-solving framework, instead of just going from problem to problem and having it be a toss-up between whether you've seen something similar or not. This means, for example, making (or writing) a list of steps that you can apply to every problem.

"Where can I find the google study guide?"

https://careers.google.com/how-we-hire/interview/#interviews-for-software-engineering-and-technical-roles

Remember that this guide is for Google interviews only. Other companies may (and usually do) have their own study guides. The point is to study everything that's on the guide that your given company gives you.

"What does a sample behavioral interview question response look like?"

QUESTION

"Tell me about a time that you were asked to build a feature, but you were given very few or very vague instructions. How did you go about building that feature, and how did you know it was correct?"

SAMPLE RESPONSE (this is a completely made-up story, where the main goal is to show the process/template)

Scenario
"Let me just take a few moments to gather my thoughts and look over my notes. Ok, I have an example from my second job where I was working as a back-end engineer at Company X. To provide a bit more context, Company X is a website that allows people to find the cheapest shipping option and then print shipping labels."

Task
"The feature that I was tasked with building was to develop an API that allowed another internal team to fetch data and see how many shipping labels were printed for a particular zipcode."

Action
"The first thing I did was come up with a list of questions that would guide the API design, such as: are we sure that only internal teams will be consuming this API (this impacts authentication/authorization)? How frequently will this API be consumed (for performance/scalability considerations)? What data is needed in the response?"

"So what I did next was send a slack message to one of the engineers on that particular internal team and I scheduled a quick 30-min meeting to go over these questions. The meeting ended up being super helpful because I learned exactly how our customer success team will be using our internal web tools to see this data, which answered questions 1 and 3. And then I also learned that this feature will mainly be used for monthly reporting, so this API won't be called nearly as much as some of our public-facing APIs.

Result
"The result of me taking these actions to outline questions and then schedule a meeting was that I was able to get all of the missing pieces for this feature, as well as get a much better understanding of some of the security and performance constraints, and I was able to also learn and expand my knowledge about the company's internal tools. This enabled me to build the feature successfully and on time for the appropriate use case."

The interviewer will likely step in throughout your responses and ask for further elaboration, and might ask how you would do things differently given some other criteria.

"What's your educational background?"

Something like mechanical engineering or aerospace engineering

"Were you allowed to refer to notes during your interview"

Yes, I referred to my notes in almost all of my interviews. Keep in mind that it was just quick references to bullet points, not reading an entire essay line-by-line.

"Why did you choose FB over Google? I heard work life balance is non existent in FB while Google has the best work life balance. Is it about Total Compensation?"

It's not about total compensation. It's based on a number of factors:

• Being this early in my career, and being self-taught, i AM looking for less work-life balance so that I can grow as fast as possible during the next several years
• My virtual onsite experience was extremely positive, and I felt a stronger connection with the interviewers at FB
• You can argue that both Google and FB have some of the most ubiquitous products in areas like video, chat, networks, and images, but I have a personal preference for Facebook's because they're more geared towards building community (which is one of my own personal values)
• This may be completely due to poor sampling, but I tend to see more people going from Google to FB than FB to Google
• I get more of a startup vibe from FB (which is what I'm looking for)
• I use FB products daily to keep in touch with a lot of my friends and extended family (so it feels more fun as a product, versus what I use for professional reasons, such as Gmail and Hangouts)

To be honest it's extremely difficult to pick one over the other, so it likely comes down to personal preference rather than 100% objective reasons. But I do feel like I can grow faster at FB, and I'm really interested in working on completely b2c products and social networks/community.

"How did you change careers and get your first developer job?"

My career progression went something like:

• Month 0 - start reading and learning about HTML, CSS, and JavaScript using books and online courses to build up my portfolio
• Month 3 - apply and get 3-month full-time contractor job as a jr developer for a wordpress site
• Month 4 - while working as a jr developer, get a part-time non-coding job in web accessibility
• Month 5 - while working 2 jobs, get third job as a TA for coding bootcamp. This one somehow happened by making the most out of a really lucky opportunity. This made me both a TA and also secretly a student.
• Month 7 - Get first position as full-stack developer. By this point my 3-month contractor job ended, and then I left my two part-time positions, so now I just had my new full-time role.
• Month 9 - I started working on a full-fledged personal project w/ my friend using the same tech stack as my full-time role, putting in at least 40+ hours a week on my personal project in addition to working my full-time job. This allowed me learn a lot, go out of my comfort zone, and grow much faster.
• Up to now - From that point on I worked at a few other companies as a full-stack software engineer and had the opportunitiy to get experience in a somewhat broad range of areas, such as: front-end, back-end, APIs, testing, databases, IDM, devOps, data engineering, software architecture, Azure, AWS, serverless, payments

My biggest pieces of advice:

1. Do online courses and tutorials (we need the knowledge, and this is where the knowledge is)
2. Have at least one real, live, full-fledged personal project in a tech stack that is closely related to your target dev job (this shows passion, and forces you to quickly grow way out of your comfort zone)
3. Apply yourself at work, ask questions, and be eager to learn (even a small task can be a huge learning opportunity if you make it so)
4. Pay it forward (take your lessons and share them with others who are also trying to learn and better themselves)
5. There is no silver bullet. Both getting started and getting ahead is simply a matter of staying heads-down and putting in many many hours of work.
6. Try to surround yourself with people who are like-minded (i.e. people who also want to learn and grow)

If you're looking for some recommendations on online courses:

1. Udemy React Course - I've done this course and highly recommend it for learning React front-end
2. Udemy Full-stack Course - Full-stack course from the same instructor as above
3. Udemy Nodejs / Back-end Course