Memory Leak

A memory leak happens when a program allocates memory and never releases it, or keeps references to data it will never use again, so memory consumption grows steadily over time.

Why it matters

Long-running processes such as web servers, browsers and background workers are the most exposed. A small leak per request becomes gigabytes after days, ending in slowdowns, swapping or an out-of-memory kill by the operating system. Restarting the process hides the symptom, which is why a service that "needs a restart every few days" often has a leak. In languages with manual memory management a leak means forgetting to free; in garbage-collected languages it usually means an unintended reference keeps objects alive.

Example

const listeners = [];
function onRequest(req) {
  listeners.push(() => console.log(req.body)); // never removed
}

Each request adds a closure that holds its body forever. The array grows without limit and the garbage collector cannot free any of it.

Common mistakes

• Adding event listeners, timers or cache entries without ever removing them.
• Using an unbounded in-memory cache with no size limit or expiry.
• Blaming the garbage collector instead of looking for what still references the objects.

Related

• Garbage collection
• Pointer
• Closure
• Uptime

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