Learn software development, five minutes a day.

And 15 projects you finish and keep

Each project is a handful of functions written in order, every one building on the last, every one checked by tests. Projects use JavaScript and require a Full Access subscription.

• Build a worker that retries safely

  Capped backoff, due-time filtering, a retry budget that ends in a dead letter, idempotency, and one testable tick of a worker.

  5 steps · 20 tests · ≈25 min

• Build a cache that forgets

  Five steps that turn a plain object into a cache with expiry, a size cap and least-recently-used eviction.

  5 steps · 20 tests · ≈25 min

• Build a rate limiter

  Five steps from a fixed window that leaks bursts to a per-caller token bucket a service could run on.

  5 steps · 20 tests · ≈30 min

• Build a rollout controller

  Health as a share, batches that bound the blast radius, a canary judged against the running version, and a rollout that stops.

  5 steps · 20 tests · ≈25 min

• Build a text search index

  Five functions that turn a pile of documents into a search box that ranks what it finds.

  5 steps · 20 tests · ≈30 min

• Build the retrieval half of RAG

  Five steps that turn a document into a prompt: chunking, an inverted index, scoring, ranking, and a prompt that cites what it used.

  5 steps · 19 tests · ≈25 min

• Build a commit graph

  Five steps that build the data structure git actually is: commits with parents, reachability, merge-base, and the three kinds of merge.

  5 steps · 20 tests · ≈25 min

• Build a receipt printer

  Five functions that add up to a receipt you could hand someone.

  5 steps · 20 tests · ≈25 min

• Build k-nearest neighbours

  Five steps that build a working classifier from distance alone — and the scaling bug that silently ruins it.

  5 steps · 20 tests · ≈25 min

• Build a word-frequency report

  Raw text to a readable report: deciding what a word is, counting, ranking with repeatable ties, and aligning the columns.

  5 steps · 20 tests · ≈25 min

• Build a token gate

  An untrusted string to an authorisation decision: shape, a constant-time signature check, expiry, scope, and a refusal that says why.

  5 steps · 20 tests · ≈25 min

• Build a tiny query engine

  Five steps that turn arrays of objects into WHERE, ORDER BY, GROUP BY and JOIN — and show why a database applies them in that order.

  5 steps · 20 tests · ≈25 min

• Build a test runner

  Five functions that grow into the runner every testing framework is underneath.

  5 steps · 20 tests · ≈25 min

• Build a type checker

  What TypeScript does to your code: literal types, operator rules, narrowing a union, checking a call, and walking an expression.

  5 steps · 22 tests · ≈25 min

• Build an HTTP router

  A raw URL to a response: segments, patterns with parameters, the query string, honest status codes, and 405 instead of 404.

  5 steps · 20 tests · ≈25 min