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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Implementing an in-memory cache (Map or plain object) with no eviction policy at all

// Wrong: grows forever, effectively a memory leak
const cache = new Map();

// Correct: bounded, evicts least-recently-used entries
import { LRUCache } from "lru-cache";
const cache = new LRUCache({ max: 500 });

The Solution //

A cache with no maximum size or expiration policy grows without bound for as long as the process runs, which is functionally a slow-motion memory leak rather than a genuine cache. Use a bounded caching structure (like an LRU cache with a max size) that evicts old entries once a limit is reached.

The Error //

Registering an event listener inside a per-request handler without ever removing it

// Wrong: a new listener added on every request, never removed
app.get("/subscribe", (req, res) => {
  emitter.on("update", handler); // accumulates forever
});

// Correct: cleaned up when the connection closes
res.on("close", () => emitter.off("update", handler));

The Solution //

Each incoming request that registers a new listener without cleanup accumulates listeners indefinitely — every one of them, along with its entire captured closure, remains in memory for the life of the process, even after the associated request/connection has long since ended.