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What is caching, and why is it commonly used to improve the performance of a Node.js application? What is the difference between res.set('Cache-Control', 'no-cache') and 'no-store' in an Express response? What is the difference between in-memory caching (like a Map or node-cache) and a distributed cache (like Redis)? What HTTP caching headers (Cache-Control, ETag) can a Node.js/Express API use to let clients and CDNs cache responses? What is memoization, and how does it differ from a general-purpose external cache like Redis? How would you cache the result of a database query in Node.js while ensuring the cache is properly invalidated when the underlying data changes? What is the risk of caching sensitive or user-specific data with a shared cache key, and how do you avoid it? What is negative caching, and why might you deliberately cache the fact that a lookup returned 'not found'? What are the main cache invalidation strategies (TTL, write-through, cache-aside), and when is each appropriate? What is cache stampede (thundering herd), and how do you prevent it in a high-traffic Node.js service?
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How would you implement a Least Recently Used (LRU) cache in Node.js, and why is eviction necessary for an in-memory cache?

Intermediate

Answer

An LRU cache has a fixed maximum size and, once full, evicts the entry that hasn't been accessed for the longest time to make room for a new one -- this bounds memory usage while keeping the most actively used data available, based on the assumption that recently accessed data is more likely to be accessed again soon.
const { LRUCache } = require('lru-cache');

const cache = new LRUCache({ max: 1000, ttl: 1000 * 60 * 10 });

cache.set('user:123', userData);
const cached = cache.get('user:123'); // moves this entry to 'most recently used'
Real-world example A service caching computed search-result rankings for popular queries uses an LRU cache capped at 5,000 entries, so memory usage stays bounded regardless of how many unique queries are searched, automatically evicting rankings that have fallen out of recent popularity.

Common follow-ups: How does LRU eviction compare to other eviction policies like LFU (Least Frequently Used)?;What's the time complexity of get() and set() operations in a properly implemented LRU cache, and why does that matter at scale?

Memory Management & Garbage Collection;Performance Optimization & Profiling
All 15 Caching questions