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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Building an in-process cache with a plain object or array that never evicts entries

// Wrong: unbounded, never garbage collected
const cache = {};
app.get('/user/:id', (req, res) => {
  cache[req.params.id] = req.body; // grows forever
});

// Correct: bounded cache with eviction
const LRU = require('lru-cache');
const cache = new LRU({ max: 500, ttl: 1000 * 60 * 5 });

The Solution //

Anything stored in a module-level object or array stays reachable from the global root for the lifetime of the process, so the Mark-and-Sweep GC can never collect it — the heap grows until you hit the ~1.4GB default limit and crash. Use a bounded structure (LRU cache with a max size) or move the cache to Redis with a TTL instead of raw JS objects.

The Error //

Registering event listeners inside a function that gets called repeatedly, instead of once

// Wrong: adds a new listener on every request
app.get('/data', (req, res) => {
  emitter.on('update', () => res.json({ ok: true }));
});

// Correct: register once outside the handler, or use .once()/removeListener
emitter.on('update', handleUpdate); // registered once at module load

The Solution //

Every call to emitter.on() attaches a new closure that V8 keeps reachable through the emitter's internal listener array — if that registration happens inside a per-request handler instead of at startup, thousands of duplicate listeners pile up and each one keeps its captured scope alive, leaking memory and eventually triggering Node's MaxListenersExceededWarning.