Require only when changed

require_only_when_changed.js

function getJSON(filename) {
  if (!IS_PROD) {
    getJSON.lastMtimes = getJSON.lastMtimes || {};
    const filePath = require.resolve(filename);
    const { mtimeMs } = fs.statSync(filePath);
    const lastMtime = getJSON.lastMtimes[filePath];

    if (!lastMtime || lastMtime !== mtimeMs) {
      getJSON.lastMtimes[filePath] = mtimeMs;
      delete require.cache[filePath];
    }
  }

  return require(filename);
}

require_only_when_changed_v2.js

function getJSON(filename) {
  if (!IS_PROD) {
    /**
     * With this optimization, we're making sure a large ~20MB file
     * isn't JSON.parse(d) unnecessarily. I've tried using fs.statSync,
     * but for some reason it lags behind the actual file-contents
     * when used in docker with nfs.
     */
    getJSON.fileCache = getJSON.fileCache || {};

    const filePath = require.resolve(filename);
    const cache = getJSON.fileCache[filePath];
    const buffer = fs.readFileSync(filePath);

    if (cache && cache.buffer.equals(buffer)) {
      return cache.file;
    }

    const file = JSON.parse(buffer);
    getJSON.fileCache[filePath] = { buffer, file };

    return file;
  }

  return require(filename);
}