メモリリークを避ける — クロージャ、タイマー、参照

リークとは何か

インターバルを解除する

// Start an interval and then clear it to avoid leaks
function startTicker() {
  // store id so we can clear later
  const id = setInterval(function () {
    // work would run every X ms; omitted for demo
  }, 1000);

  // simulate screen close / cleanup
  clearInterval(id);
  console.log("interval cleared");
}

startTicker();

クロージャを小さくする

// Avoid capturing large objects when only small data is needed
function makeReporter(bigArray) {
  // BAD: store the whole array inside the closure
  const large = bigArray;

  return function reportLength() {
    // Only the length is used
    return large.length;
  };
}

// BETTER: capture only what you need
function makeReporterBetter(bigArray) {
  const count = bigArray.length; // small number
  return function reportLength() {
    return count;
  };
}

const arr = new Array(1000).fill(0);
const bad = makeReporter(arr);
const good = makeReporterBetter(arr);
console.log("bad:", bad(), "good:", good());

バッファーに上限を設ける

// A growing buffer can leak if never trimmed or reset
const buffer = []; // module-level array

function logItem(x) {
  buffer.push(x); // grows forever if not managed
}

// Simulate usage
logItem("a");
logItem("b");

// FIX: trim or reset when size is over a limit
function trimBuffer(limit) {
  if (buffer.length > limit) {
    buffer.length = limit; // drop old items
  }
}

trimBuffer(1);
console.log("buffer size:", buffer.length);

WeakMap の使いどころ

// WeakMap holds keys weakly: when the object is gone, the entry can be collected
const meta = new WeakMap();

function attachMeta(obj, info) {
  meta.set(obj, info);
}

function readMeta(obj) {
  return meta.get(obj); // undefined if not set or collected
}

let user = { name: "Ayla" };
attachMeta(user, { seen: true });
console.log("meta now:", readMeta(user));

// Later: drop the only strong reference
user = null; // after this, the object and its metadata can be GC'd at some point
console.log("object reference dropped (cannot show GC in console)");

リークを防ぐ習慣

タイマーのクリーンアップクイズ

まとめ