E2 — ログアナライザー（簡単なフィルターとカウンター）

作成するもの

データとパーサー

// Simulated log lines (time level message)
const LINES = [
  "10:00 INFO  Start app",
  "10:01 WARN  Low memory",
  "10:02 INFO  User login /home",
  "10:03 ERROR DB timeout",
  "10:04 INFO  User login /products",
  "10:05 ERROR Network fail",
  "10:06 WARN  Retry request"
];

// Tiny parser: returns { time, level, text }
function parseLine(line) {
  // split on spaces, compact, then rebuild pieces
  const parts = line.trim().split(/\\s+/);
  if (parts.length < 3) return null;
  const time = parts[0];
  const level = parts[1];
  const text = parts.slice(2).join(" ");
  return { time: time, level: level, text: text };
}

console.log("parsed:", parseLine(LINES[2]));

フィルタリングの基礎

// Filter by level or by keyword (case-sensitive for simplicity)
function filterByLevel(lines, want) {
  const out = [];
  for (const line of lines) {
    const p = parseLine(line);
    if (p && p.level === want) out.push(p);
  }
  return out;
}

function filterByKeyword(lines, word) {
  const out = [];
  for (const line of lines) {
    const p = parseLine(line);
    if (p && p.text.indexOf(word) !== -1) out.push(p);
  }
  return out;
}

console.log("ERRORs:", filterByLevel(LINES, "ERROR").length);
console.log("login :", filterByKeyword(LINES, "login").length);

Mapによるカウント

// Count how many lines per level
function countByLevel(lines) {
  const counts = new Map();
  for (const line of lines) {
    const p = parseLine(line);
    if (!p) continue;
    const old = counts.get(p.level) || 0;
    counts.set(p.level, old + 1);
  }
  return counts;
}

const c = countByLevel(LINES);
console.log("INFO :", c.get("INFO"));
console.log("WARN :", c.get("WARN"));
console.log("ERROR:", c.get("ERROR"));

Top-Nの例

// Extract a path after "User login " and rank by frequency
function topLoginPaths(lines, k) {
  const freq = new Map();
  for (const line of lines) {
    const p = parseLine(line);
    if (!p) continue;
    const mark = "User login ";
    const idx = p.text.indexOf(mark);
    if (idx !== -1) {
      const path = p.text.slice(idx + mark.length).trim();
      const old = freq.get(path) || 0;
      freq.set(path, old + 1);
    }
  }
  // turn into array and sort descending by count
  const arr = Array.from(freq.entries());
  arr.sort(function (a, b) { return b[1] - a[1]; });
  return arr.slice(0, k);
}

console.log("top paths:", topLoginPaths(LINES, 3));

初心者向けガイド

Mapによるカウントのクイズ

まとめ