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EJS, Handlebars, Pug and Nunjucks all do the same three things: tokenize, parse, compile. Build a real one — loops, conditionals, partials, escaping by default — and see exactly where HTML escaping stops protecting you.

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.

Call fib(45) naively and JavaScript will make over a billion recursive calls. Call it twice and you double that. Your CPU fan kicks on, the browser tab freezes, and the function you thought was fast becomes your performance bottleneck.

Picture this: a live dashboard that polls for new notifications every 3 seconds. The count increments in state, the UI updates, everything looks right — except the interval callback is silently reading count = 0 on every tick, because it.

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.

Open a modal 10 times without cleanup and you've got 10 stacked timers silently draining memory. Here's what this looks like in a PR and how senior devs catch it.

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.

Floating promises — async calls nobody awaits — don't crash immediately. They fail silently, corrupt state, and get cut off by serverless runtimes. Here's what to look for in code review.