A Node.js function to get the latest commit's authorship timestamp in UTC (millisecond precision)

getLastCommitDatetimeUtc.ts

/**
 * Retrieves the datetime of the last commit in UTC for the current Git branch.
 *
 * The author timestamp is used for its consistency across different
 * repositories and its inclusion in the Git commit hash calculation. This makes
 * it a stable choice for reproducible builds.
 *
 * @returns Millisecond precision timestamp in UTC of the last commit on the
 * current branch. If the branch is detached or has no commits, it will throw an
 * error.
 */
export function getLastCommitDatetimeUtc(gitDir = join(__dirname, "..", ".git")): number {
  // Note: this function is syncronous because it needs to be used in a syncronous
  // context

  const { unzipSync } = require('node:zlib');
  const { readFileSync } = require('node:fs');

  const headPath = join(gitDir, 'HEAD');

  // read .git/HEAD to get the current branch/commit
  const ref = readFileSync(headPath, 'utf-8').trim();

  // determine if we're in a detached HEAD state or on a branch
  const oid = ref.startsWith('ref: ')
    ? readFileSync(join(gitDir, ref.slice(5)), 'utf-8').trim() // pointer to a branch, find the commit hash
    : ref; // detached HEAD state, so use the commit hash directly

  // read the commit object from the file system
  const commitPath = join(gitDir, 'objects', oid.slice(0, 2), oid.slice(2));
  const rawCommit = readFileSync(commitPath);
  // it's compressed with zlib DEFLATE, so we need to decompress it
  const decompressed = unzipSync(rawCommit);
  // the commit object is a text file with a header and a body, we just want the
  // body, which is after the first null byte
  const firstNull = decompressed.indexOf(0);
  const commitBuffer = decompressed.subarray(firstNull + 1);
  const commitText = new TextDecoder().decode(commitBuffer);
  // git commits are strictly formatted, so we can use a regex to extract the
  // authorship time fields
  const [, timestamp, timezoneOffset] = commitText.match(
    /^author .* <.*> (.*) (.*)$/mu
  )!;
  // convert git timestamp from seconds to milliseconds
  const msSinceLocalEpoch = (parseInt(timestamp, 10) * 1000);
  const msTimezoneOffset = (parseInt(timezoneOffset, 10) * 60000);
  return msSinceLocalEpoch - msTimezoneOffset;
}