Free original lessons and guided browser activities. No sign-in required. Progress stays on this device and is not synced with paid courses.
Choose counting, parsing or statistics; run the example, change the data and inspect boundary cases.
Python · Intermediate · Exercise · 30 minutes
Outcomes: Choose a library by its data contract; Validate parsed data before using it; Distinguish mean and median
Before you start: Basic programming and the introductory activities
Edit three tiny vectors and inspect dot products, normalized weights and the weighted output. No model downloads or API calls.
AI · Intermediate · Simulation · 30 minutes
Outcomes: Compute scaled dot-product attention; Explain causal masking; Observe temperature and normalization
Before you start: Vectors, dot products and weighted averages
Compare bursty arrivals, processing capacity and dropped jobs in a bounded, deterministic queue.
System design · Intermediate · Simulation · 30 minutes
Outcomes: Distinguish throughput from queue size; Locate overload in a trace; Explain a backpressure trade-off
Before you start: Basic programming and the introductory activities
Implement add and complete operations without mutating prior state. Use the checks as project milestones.
Projects · Intermediate · Project · 30 minutes
Outcomes: Define a small state transition contract; Reject empty titles; Preserve previous state while updating one task
Before you start: Basic programming and the introductory activities
Build a small data pipeline with map, filter and reduce. Handle empty input instead of guessing the happy path.
Coding · Beginner · Exercise · 20 minutes
Outcomes: Transform values without mutating input; Explain the empty-list result; Use checks to find edge cases
Before you start: JavaScript arrays and functions
Normalize text and build a frequency table using real Python in an isolated browser runtime.
Python · Beginner · Exercise · 25 minutes
Outcomes: Write a dictionary accumulator; Normalize whitespace and case; Compare a standard-library alternative
Before you start: Python loops and dictionaries
Change the sorted input, choose a target and inspect every comparison. No animation hides the invariant.
Algorithms · Intermediate · Simulation · 20 minutes
Outcomes: Trace low, high and midpoint; Explain logarithmic search; Recognize the sorted-input prerequisite
Before you start: Ordered numbers and indexes
Create a responsive HTML/CSS result with a real preview and explicit project milestones.
Projects · Beginner · Project · 35 minutes
Outcomes: Use semantic HTML; Build a readable responsive card; Inspect an isolated preview
Before you start: Basic HTML and CSS
Choose subjects and daily time, include prerequisites, and turn the catalog into a dated plan saved on this device.
Computer science · Beginner · Tool · 15 minutes
Outcomes: Schedule a sustainable practice habit; Place prerequisites before advanced topics; Track completed activities
Before you start: No prerequisites
Answer original questions by topic, reveal explained answers and review a session score without a timer penalty.
Interviews · Beginner · Practice · 25 minutes
Outcomes: Connect claims to reasons; Explain tradeoffs; Review why alternatives fail
Before you start: Basic programming
Send a request sequence through a least-recently-used cache and compare hits, misses and memory limits.
System design · Intermediate · Simulation · 25 minutes
Outcomes: Explain eviction; Calculate a hit rate; Identify the limits of a cache model
Before you start: Arrays and key-value lookup
Search original deduplication, grouping and average recipes, then edit and run each one.
Coding · Beginner · Exercise · 20 minutes
Outcomes: Preserve order while deduplicating; Group records by key; Define an empty-average result
Before you start: JavaScript arrays
An original on-site essay about observations, hypotheses and minimal reproductions, with a checkpoint and bookmark.
Reading · Beginner · Reading · 18 minutes
Outcomes: Separate observation from explanation; Design a discriminating experiment; Keep useful debugging evidence
Before you start: No prerequisites
Compare SQL concatenation with parameter binding using controlled strings. No real target or database is contacted.
Security · Beginner · Exercise · 25 minutes
Outcomes: Explain injection as a boundary failure; Construct a parameterized request; Separate validation from authorization
Before you start: Strings and simple SQL concepts
Rank controlled documents by token overlap and measure retrieval accuracy. A real local experiment, not an LLM call.
AI · Intermediate · Simulation · 30 minutes
Outcomes: Trace tokenization and ranking; Measure hit-at-one; Recognize lexical retrieval failure
Before you start: Python or JavaScript basics
Follow the executable and distinguish a missing package from the wrong interpreter.
Python · Beginner · Guide · 30 minutes
Outcomes: Read interpreter evidence; Match installer to interpreter; Avoid destroying a working environment
Before you start: Basic terminal familiarity
Separate transport errors, HTTP failures and invalid response bodies with a reproducible fixture.
Coding · Beginner · Guide · 30 minutes
Outcomes: Check response.ok; Preserve useful failure context; Await work at the right boundary
Before you start: JavaScript promises
Reproduce allow and deny decisions without sending requests to someone else’s server.
Computer science · Intermediate · Guide · 30 minutes
Outcomes: Explain origins and preflight; Separate CORS from authorization; Model an explicit allowlist
Before you start: HTTP request basics
Implement a pure request handler, validate data and check status codes before connecting a real server.
Projects · Intermediate · Guide · 35 minutes
Outcomes: Separate routing from domain logic; Validate untrusted input; Test accepted and rejected requests
Before you start: JavaScript objects
Inspect calibration, smoothing and thresholds with controlled ADC samples, without claiming a hardware test.
Projects · Beginner · Guide · 30 minutes
Outcomes: Convert a raw count to a voltage estimate; Smooth noise; Separate simulation from measurement
Before you start: Arithmetic and Python lists
Test a small validator and distinguish a successful build from a working deployment.
Projects · Beginner · Guide · 35 minutes
Outcomes: Choose checks by risk; Verify routes and static files; Plan a reversible release
Before you start: HTML and JavaScript basics