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Concepts that don't fit one technology

Curated explainers for the concepts that come up in interviews and real work: the event loop, idempotency, caching, CORS, indexing. Diagrams, examples, and interview questions.

Notes by stack Quizzes

Web Performance

HTTP CachingHow browsers and CDNs decide to reuse a response: freshness with Cache-Control, validation with ETag, cache keys with Vary, and the recipes that actually work.Intermediate·12 min·interview favorite

Concurrency & Async

The Event Loop: Microtasks vs MacrotasksHow JavaScript runs async code on a single thread, why promise callbacks beat setTimeout, and how to predict the output of any ordering puzzle.Intermediate·11 min·interview favorite

System Design

Idempotency in APIsWhy retries create duplicate orders and double charges, which HTTP methods are safe to repeat, and how idempotency keys make any request safe to retry.Intermediate·10 min·interview favorite

Data & Storage

Database IndexingWhat a B-tree index actually is, how the planner decides to use it, why column order in a composite index matters, and when an index makes things worse.Intermediate·13 min·interview favorite

Security

CORS: Cross-Origin Resource SharingWhy the browser blocks a response that curl can read, what a preflight is, how credentials change the rules, and how to configure a server without opening a hole.Intermediate·10 min·interview favorite

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