プロファイリング入門（Node/DevTools） — 小さな計測習慣

プロファイリングが必要な理由

console.time の例

// Measure a small loop with console.time/timeEnd
console.time("sum");
let s = 0;
for (let i = 0; i < 100000; i = i + 1) {
  s = s + i;
}
console.timeEnd("sum");
console.log("sum value:", s);

手動計測ヘルパー

// A tiny now() helper that works in many runtimes
const now = (typeof performance !== "undefined" && performance.now)
  ? function () { return performance.now(); }
  : function () { return Date.now(); };

// Time a function manually
function timeFn(label, fn) {
  const t1 = now();
  const out = fn();
  const t2 = now();
  console.log(label + " ms:", (t2 - t1));
  return out;
}

timeFn("small work", function () {
  let x = 0;
  for (let i = 0; i < 50000; i = i + 1) { x = x + i; }
  return x;
});

ホットコードを見つける

// Count calls and total time to find hot spots
const metrics = { calls: 0, totalMs: 0 };

function hotWork(n) {
  const t1 = now();
  let sum = 0;
  for (let i = 0; i < n; i = i + 1) {
    sum = sum + (i % 3);
  }
  const t2 = now();
  metrics.calls = metrics.calls + 1;
  metrics.totalMs = metrics.totalMs + (t2 - t1);
  return sum;
}

// Simulate usage
for (let k = 0; k < 5; k = k + 1) {
  hotWork(80000);
}
console.log("calls:", metrics.calls, "avg ms:", metrics.totalMs / metrics.calls);

変更前と変更後の計測

// Compare two approaches with the same input
const hay = [];
for (let i = 0; i < 2000; i = i + 1) { hay.push(String(i)); }

const needles = [];
for (let i = 0; i < 1000; i = i + 1) { needles.push(String(i * 2)); }

function slowIncludes() {
  let count = 0;
  for (const n of needles) {
    if (hay.includes(n)) { count = count + 1; }
  }
  return count;
}

function fastSet() {
  const set = new Set(hay);
  let count = 0;
  for (const n of needles) {
    if (set.has(n)) { count = count + 1; }
  }
  return count;
}

const c1 = timeFn("includes run", slowIncludes);
const c2 = timeFn("set.has run", fastSet);
console.log("counts:", c1, c2);

よい計測習慣

console.time の基本クイズ

まとめ