e-Gov法令API v2をTypeScriptで使う：依存なしfetchで法令検索からJSON保存まで

// e-Gov 法令API v2: 法令名検索 → 法令ID確定 → 本文取得 → JSON保存
// 実行例:
//   node --experimental-strip-types egov-fetch.ts "労働基準法"            (Node.js 22.6 以降)
//   node egov-fetch.ts "労働基準法" 2025-06-01                            (Node.js 24 以降は型ストリップが既定)
import { mkdir, writeFile } from 'node:fs/promises';
import path from 'node:path';

const BASE = 'https://laws.e-gov.go.jp/api/2';

type LawInfo = {
  law_id: string;
  law_num: string;
  law_type: string;
  promulgation_date: string;
};

type RevisionInfo = {
  law_revision_id: string;
  law_title: string;
  abbrev: string | null;
  amendment_enforcement_date: string;
  current_revision_status: 'CurrentEnforced' | 'UnEnforced' | 'PreviousEnforced' | 'Repeal';
};

type LawsResponse = {
  total_count: number;
  count: number;
  next_offset: number | null;
  laws: { law_info: LawInfo; revision_info: RevisionInfo }[];
};

type LawDataResponse = {
  law_info: LawInfo;
  revision_info: RevisionInfo;
  law_full_text: unknown;
};

type ErrorInfo = { code: string; message: string };

async function egovGet<T>(endpoint: string, params: Record<string, string> = {}): Promise<T> {
  const url = new URL(`${BASE}/${endpoint}`);
  for (const [key, value] of Object.entries(params)) url.searchParams.set(key, value);

  const res = await fetch(url, {
    headers: { Accept: 'application/json' },
    signal: AbortSignal.timeout(30_000),
  });
  if (!res.ok) {
    const body = await res.text();
    let detail = body.slice(0, 200);
    try {
      const err = JSON.parse(body) as ErrorInfo;
      detail = `${err.code} ${err.message}`;
    } catch {
      // JSON 以外(HTML の 400 など)はそのまま
    }
    throw new Error(`HTTP ${res.status} ${url.pathname}: ${detail}`);
  }
  return (await res.json()) as T;
}

async function resolveLawId(title: string): Promise<LawInfo & RevisionInfo> {
  const data = await egovGet<LawsResponse>('laws', { law_title: title, law_type: 'Act', limit: '10' });
  if (data.count === 0) throw new Error(`「${title}」に一致する法律がありません`);

  const exact = data.laws.filter((l) => l.revision_info.law_title === title);
  if (exact.length === 1) return { ...exact[0].law_info, ...exact[0].revision_info };

  const lines = data.laws.map((l) => `  ${l.law_info.law_id}  ${l.revision_info.law_title}`);
  throw new Error(`完全一致が1件に絞れません(${data.total_count}件)。ID を指定してください:\n${lines.join('\n')}`);
}

async function main(): Promise<void> {
  const [title, asof] = process.argv.slice(2);
  if (!title) {
    console.error('usage: node egov-fetch.ts <法令名> [asof=YYYY-MM-DD]');
    process.exitCode = 2;
    return;
  }

  const law = await resolveLawId(title);
  console.log(`law_id=${law.law_id}  ${law.law_title}  (${law.law_num})`);

  const params: Record<string, string> = { json_format: 'light' };
  if (asof) params.asof = asof;
  const data = await egovGet<LawDataResponse>(`law_data/${law.law_id}`, params);

  const rev = data.revision_info;
  console.log(`law_revision_id=${rev.law_revision_id}  enforced=${rev.amendment_enforcement_date}  status=${rev.current_revision_status}`);

  const outDir = path.join(process.cwd(), 'out');
  await mkdir(outDir, { recursive: true });
  const file = path.join(outDir, `${rev.law_revision_id}.json`);
  await writeFile(
    file,
    JSON.stringify(
      {
        fetched_at: new Date().toISOString(),
        source: `${BASE}/law_data/${law.law_id}${asof ? `?asof=${asof}` : ''}`,
        asof: asof ?? null,
        law_info: data.law_info,
        revision_info: rev,
        law_full_text: data.law_full_text,
      },
      null,
      2,
    ),
    'utf8',
  );
  console.log(`saved: ${file}`);
}

main().catch((err: unknown) => {
  console.error(err instanceof Error ? err.message : String(err));
  process.exitCode = 1;
});

$ node egov-fetch.ts "労働基準法"
law_id=322AC0000000049  労働基準法  (昭和二十二年法律第四十九号)
law_revision_id=322AC0000000049_20260717_508AC0000000060  enforced=2026-07-17  status=CurrentEnforced
saved: ...\out\322AC0000000049_20260717_508AC0000000060.json

$ node egov-fetch.ts "労働基準法" 2025-06-01
law_id=322AC0000000049  労働基準法  (昭和二十二年法律第四十九号)
law_revision_id=322AC0000000049_20250601_504AC0000000068  enforced=2025-06-01  status=PreviousEnforced
saved: ...\out\322AC0000000049_20250601_504AC0000000068.json

$ node egov-fetch.ts "会社"
完全一致が1件に絞れません(38件)。ID を指定してください:
  321AC0000000007  会社経理応急措置法
  322AC0000000151  昭和二十二年法律第百五十一号（国際電気通信株式会社等の...）
  323CO0000000402  会社等臨時措置法等を廃止する政令
  ...
（終了コード 1）

$ node egov-fetch.ts "労働基準法" 2025-13-45
law_id=322AC0000000049  労働基準法  (昭和二十二年法律第四十九号)
HTTP 400 /api/2/law_data/322AC0000000049: 400004 日付（asof等）が誤っています。
（終了コード 1）