Update changelog based off of version tags. Useful as the `version` script in an npm package.

version.js

#!/usr/bin/env node

const fs = require("fs");
const path = require("path");
const { promisify } = require("util");
const { spawn } = require("child_process");

const readFile = promisify(fs.readFile);
const writeFile = promisify(fs.writeFile);

const versionPattern = "\\d+\\.\\d+\\.\\d";
const versionRegEx = RegExp(versionPattern, "i");
const versionSectionRegEx = RegExp(`^## ${versionPattern}\\w?.*$`);

module.exports = updateChangelogWithNewVersionInformation;
module.exports.ensureNoExistingVersionSections = ensureNoExistingVersionSections;
module.exports.gitCommits = gitCommits;

if (require.main === module) {
  const changelogFilename = `${__dirname}/../docs/CHANGELOG.md`;
  const changelogFilePath = path.resolve(changelogFilename);
  const version = process.argv[2] || process.env["npm_package_version"];

  if (!version) {
    throw new Error("Version not specified. Set environment variable 'npm_package_version' or pass as argument.");
  }

  return updateChangelogWithNewVersionInformation(changelogFilePath, version)
    .then(() => process.exit(0))
    .catch((err) => {
      process.stderr.write(`${err.message}\n`);
      process.exit(1);
    });
}

function updateChangelogWithNewVersionInformation (filepath, version) {
  if (!version.match(versionRegEx)) {
    throw new Error("Invalid version specified.");
  }

  return readFile(filepath, "utf8")
    .then((contents) => contents.split("\n"))
    .then((lines) => ensureNoExistingVersionSections(filepath, version, lines))
    .then((lines) => gitCommits().then((commits) => ({ lines, commits })))
    .then(({ lines, commits }) => {
      const firstVersionSectionPosition = lines.findIndex((line) => line.match(versionSectionRegEx));
      let newVersionSection = `## ${version}\n`;
      if (firstVersionSectionPosition < 0) newVersionSection = `\n${newVersionSection}`;

      lines.splice(firstVersionSectionPosition, 0, newVersionSection, commits);

      return lines;
    })
    .then((lines) => lines.join("\n"))
    .then((lines) => writeFile(filepath, lines, "utf8"));

}

function ensureNoExistingVersionSections (filepath, version, lines) {
  lines.filter((line) => line.match(versionSectionRegEx))
    .forEach((sectionLine) => {
      const [sectionVersion] = sectionLine.match(versionRegEx);
      if (sectionVersion === version) throw new Error(`Version '${sectionVersion}' already exists in '${filepath}'`);
    });

  return lines;
}

function gitCommits () {
  return new Promise((resolve,reject) => {
    const gitProcess = spawn(
      "git",
      [
        "log",
        "--oneline",
        "$(git describe --tags --abbrev=0 @^)..@",
        "--oneline",
      ],
      {
        shell : true,
        stdio : [process.stdin, "pipe", "pipe"],
      },
    );

    const stdout = [];
    const stderr = [];

    gitProcess.stdout && gitProcess.stdout.on("data", (data) => stdout.push(data.toString()));
    gitProcess.stderr && gitProcess.stderr.on("data", (data) => stderr.push(data.toString()));
    gitProcess.on("close", (code) => {
      if (code === 0) resolve(`${stdout.join("").trim()}\n`);
      else reject(stderr);
    });
  });
}