Forgotten timers

setInterval registers a repeating handle — a GC root — until clearInterval runs. The callback’s closure captures every variable it references.

sequenceDiagram
  participant Timer as setInterval handle
  participant CB as Callback closure
  participant Data as Captured arrays
  Timer->>CB: invoke every N ms
  CB->>Data: push more data
  Note over Data: Grows forever if never cleared

Broken code

function startPolling(fetchUser) {
  setInterval(async () => {
    const profile = await fetchUser(); // large object
    history.push(profile);             // outer scope array grows
  }, 5000);
  // interval id lost — cannot clear
}

Fixed code

let pollId = null;
const history = [];
const MAX = 50;

function startPolling(fetchUser) {
  pollId = setInterval(async () => {
    const profile = await fetchUser();
    history.push(profile);
    if (history.length > MAX) history.shift();
  }, 5000);
}

function stopPolling() {
  if (pollId !== null) {
    clearInterval(pollId);
    pollId = null;
  }
}

In frameworks, return cleanup from useEffect (React) or onDestroy (Svelte).

Live demo

Live demo: forgotten setInterval

Idle — each interval tick retains a growing closure scope.

— MBPeak: — MB