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JS Memory Leak School

Closures in callbacks

Middleware and job queues often store callbacks. If each callback closes over a large request scope, memory accumulates.

Broken code

Section titled “Broken code”

examples/node/03-closure-cache-leak.js:

const jobs = [];


setInterval(() => {
  const big = new Array(80_000).fill({ v: Math.random() });
  jobs.push(() => big.length);
}, 300);

Fixed code

Section titled “Fixed code”

examples/node/03-closure-cache-fixed.js — store only needed fields, bound job queue with max size.

Run it

Section titled “Run it”
Terminal window
node examples/node/03-closure-cache-leak.js
node examples/node/03-closure-cache-fixed.js
flowchart TD
  queue[jobs array]
  fn1[callback]
  fn2[callback]
  big1[big array]
  queue --> fn1
  queue --> fn2
  fn1 --> big1