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Build a pub/sub system from 60 lines of JavaScript: subscribe, publish, unsubscribe, once(), namespace wildcards, error isolation, and async publish.

Strip Express and Koa down to their core: a 30-line compose() function that chains async middleware with before/after hooks, short-circuit support, and error propagation.

Build an LRU cache from scratch: doubly linked list + hashmap for O(1) get and put, eviction by access order, and a bounded memoize wrapper on top.

The naive 10-line debounce works for demos. Build the real thing: leading edges, trailing calls, cancel, flush, and a throttle that never silently drops the last event.

Lodash's curry is 10 lines. Build curry, partial, compose, and pipe from scratch in JavaScript — then use them to write clean data pipelines without a single nested callback.

An Observable is just a function. Build one from scratch in about 120 lines of JavaScript and finally understand why unsubscribing stops work, how operators chain, and what 'lazy' actually means.

Redux's core is under 60 lines of JavaScript. Build createStore, combineReducers and a middleware system with logger and thunk from scratch, then test the parts that are easy to get quietly wrong.

A virtual DOM is a data structure plus four functions, about 150 lines. Build h, createElement, diff and patch — then measure where "minimal DOM operations" is true and where it quietly isn't.

Build Node.js's EventEmitter from scratch in 100 lines. Learn why listener removal requires saved references, how once() wraps callbacks, and what that MaxListenersExceededWarning actually means.