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TRACK 01

JavaScript + TypeScript

The data problems every frontend job is made of. Graded by running your code, not by reading it.

29 challenges29 free8h of build time29 graded by running your code

JavaScript + TypeScript

The data problems every frontend job is actually made of.

  1. Product CatalogFive lookups over a product list. This is map, filter, find and reduce, and nothing else.Beginner · 10 min
  2. Shopping CartA cart with rules: no duplicate lines, quantities that cannot reach zero, and a total that is always right.Beginner · 15 min
  3. The Mutation TrapThree functions that all quietly change the data they were given. Two are broken. Work out which.Beginner · 10 min
  4. From SQL to JavaScriptWrite one function that returns exactly what a GROUP BY query would. No loops.Intermediate · 20 min
  5. API Data TransformationTurn the rows an API actually returns into the shape your table wants to render.Intermediate · 12 min
  6. Search, Filter & SortThe three operations behind every table you will ever build, and the traps in each.Intermediate · 15 min
  7. PaginationOne function you will write in every job you ever have. Get the edges right.Beginner · 12 min
  8. Async States & DebounceModel a request that can fail, handle every state the compiler knows about, and stop firing one per keystroke.Advanced · 20 min

Code Someone Else Can Change

Three refactors where the output must not move while the shape does. The starter already works — that is the difficulty.

  1. The pyramid in the middle of the functionFour levels of nesting, and the one line that does the work is the hardest to find. Invert every condition, return early, and keep all four error responses exactly as they are.Beginner · 15 min
  2. The vendor’s field names, everywhereA weather provider’s field names — main_temp_k, w_cond_str_v2, precip_mm_h — have leaked into the code that displays them. Put one adapter between the two so the day the provider renames a key, one file changes.Intermediate · 20 min
  3. “Something went wrong”Two failure paths, two string messages, and nothing a caller can branch on without matching the prose. Give errors a machine-readable code and a context object — and make sure a token handed to you never survives into what gets logged.Intermediate · 20 min

The Write Side

Reads are where the syntax is. Writes are where the bugs are — and every one of them costs somebody real money.

  1. The Double ChargeThe request timed out, so the phone retried it, so the customer has two identical orders and one of them will be refunded by hand. Make the write safe to repeat.Beginner · 15 min
  2. The Wiped FieldSomeone changed an order status and the delivery note vanished. The client only sent status. Work out the difference between "not sent" and "sent as empty".Beginner · 15 min
  3. The Lost UpdateTwo people opened the same order. Both pressed save. One of those edits is gone and there is no error, no log, and nothing to find later. Refuse the second write instead.Intermediate · 20 min
  4. Orphans and TombstonesA customer was deleted. Their orders are still in the revenue report, attached to a customer id that no longer resolves. Delete properly, and be able to undo it.Intermediate · 20 min
  5. All Or NothingThe debit succeeded. The credit threw. The money is now nowhere. Build the thing that makes a sequence of writes either all happen or none of them.Advanced · 25 min

Data Structures at Work

The page is slow, the list is the wrong shape, the order is wrong. The structure is the fix.

  1. Index, Do Not ScanAn orders table shows the customer name on every row, and it takes four seconds to render. The data is fine. The lookup is not.Beginner · 15 min
  2. Group and TotalThe dashboard needs revenue per category and the top two sellers. Four small functions, and every reporting screen you ever build is made of them.Beginner · 15 min
  3. Threaded CommentsThe API returns a flat list with parentId. The UI needs it nested. This is the same problem as a category menu, an org chart and a file browser.Intermediate · 20 min
  4. An LRU CacheSearch results get refetched every keystroke. Cache them — but a cache with no eviction is a memory leak with good intentions.Intermediate · 20 min
  5. The Job SchedulerJobs declare what they need. Work out what runs, in what order, what can run in parallel — and which jobs can never run at all.Advanced · 25 min

Functions as Values

Pure functions, functions that build functions, and keeping the effects at the edge.

  1. Same answer, every timeA discount that works once. Run it twice and the basket is wrong.Beginner · 10 min
  2. Functions that build functionsonce, memoize and retry. Three functions that take a function and return one.Beginner · 15 min
  3. Small steps, joined upOne function doing four jobs, none of which can be tested alone.Intermediate · 15 min
  4. Decide, then doTo test whether a refund is owed you currently need a mailer and a ledger.Intermediate · 18 min

Objects That Behave

Objects that protect themselves, dispatch without a switch, and keep the promises their callers rely on.

  1. The object that protects itselfA basket whose rules live in every caller. Move them inside.Beginner · 12 min
  2. One caller, many shapesA switch that must be edited for every new type. Let the objects answer instead.Intermediate · 15 min
  3. Has-a beats is-aA hierarchy with no room for a Contractor who approves leave but is not a Manager.Intermediate · 15 min
  4. The subclass that broke its parentA square is a rectangle. The code disagrees, and only a shared test shows why.Advanced · 18 min