Take-Home: Playlist Track-Position

Welcome, and thanks for taking the time to work on this. This is a small, self-contained Node + TypeScript backend service (Express, backed by an in-memory SQLite database). The assignment is designed to take around 2 hours, with a maximum recommended time of 3 hours, so please avoid spending significantly more time than that.

The product

We track music playlists. Each playlist has an ordered list of tracks, and the product shows a track's position in two places:

1. A playlist page that lists every track with its position (#1, #2, #3, …). This is the numbering the customer sees in the product.
2. A search box that lets a customer search for a track inside a playlist and tells them which position that track sits at.

What customers are reporting

"When I look at my playlist, a song shows at one position — but when I search for that same song inside the playlist, it tells me it's at a different position. And the number it gives me doesn't even match where the song actually is in the playlist right now."

There are two separate problems hiding in that report. The work is split into two parts, and the test suite is grouped to match. Reproduce things first:

npm install
npm run dev   # starts the service on http://localhost:3000

# In another terminal:
curl "http://localhost:3000/playlist/101"
curl "http://localhost:3000/playlist/101/search?q=Calm%20Down"

NOTE: A track's position in the product is the source of truth for what "correct" looks like.

Part 1 — the two views disagree

Look at what the playlist page says about a track's position, then search for that same track inside the playlist. The two report different positions for the same track. They should always agree.

Your job: make search and the playlist page report the same position for a track.

Part 2 — the position is out of date

Once Part 1 is done and the two views agree, there's still a problem: the position they report is wrong. A customer tells us:

"Calm Down is the 3rd song in my playlist right now, but your service keeps telling me it's #5."

They're right — on the live playlist on Spotify, Calm Down currently sits at #3.

Here's how the data flows: we ingest each playlist from Spotify on a recurring basis and store every capture in our database. That table is the source of truth for where a track sits today. You can inspect it the way you would in real life — by querying the database: open data/seed.sql to read the schema and rows directly and write a SQL query to get the data that you need from the table. Then to visualize the return of your query you can run:

npm run data   # runs your query and prints the result as a table, straight from the DB

Work out what each track's current position should be from that data, then make the service report it.

Your job: make the service report each track's correct, current position.

Your task, in short

1. Get the service running and reproduce both problems.
2. Investigate and fix Part 1, then Part 2 (see above).
3. Make the full test suite pass (npm test).

We care more about how you reason about the data than about lines of code. Use the pr_description_template.md file to explain your solution. The template already includes sections for user-facing and team-facing messages, which will be used to evaluate your communication skills.

Getting started

npm install
npm test          # the suite currently fails — that's expected
npm run dev       # starts the service on http://localhost:3000

Try it by hand:

curl "http://localhost:3000/playlist/101"
curl "http://localhost:3000/playlist/101/search?q=Calm%20Down"

What we're evaluating

• Correctly identifying each root cause, rather than patching the symptom or tweaking values until the tests go green.
• Choosing the right source of truth and keeping the customer-facing behavior correct.
• Clean, well-placed fixes. Bonus points if you make this class of bug hard to reintroduce.
• Clear communication about what was wrong and what you changed.

Project layout

data/
  seed.sql     # schema + seed data, loaded into an in-memory SQLite DB
src/
  db.ts        # boots the SQLite DB and exposes a query helper
  queries.ts   # the SQL that powers each endpoint
  server.ts    # builds the express app with the two endpoints
  index.ts     # entry point that starts the server
  printSnapshots.ts  # `npm run data` — write a query here to inspect the table
test/
  position.test.ts   # the acceptance tests

You should only need to touch src/. You shouldn't need to edit the tests or the seed data, but read them carefully.

Handy commands

npm run data   # run your inspection query and print the result
npm test       # run the acceptance suite
npm run dev    # start the service with reload

Submission Instructions

1. Fork this repository.

2. Complete the assignment in your own fork.

3. Open a pull request with your completed solution.

4. Use the pr_description_template.md file for your PR description.

   • Explain the root cause and your solution.
   • Include the customer-facing follow-up message requested in the template.
   • Include the team-facing message requested in the template.

5. Email the GitHub link to your pull request to hiring@chartmetric.com with the subject line:

   [Your Name - Customer Success Engineer] Take-Home Assignment