async function maybeExtract(sourcePath, destDir) {
  return new Promise((resolve, reject) => {
    if (fs.existsSync(destDir)) {
      return resolve();
    }
    console.log(`Extracting: ${sourcePath} --> ${destDir}`);
    extract(sourcePath, {dir: destDir}, err => {
      if (err == null) {
        return resolve();
      } else {
        return reject(err);
      }
    });
  });
}
Get extracted files.

If the files are already extracted, this will be a no-op.

@param {string} sourcePath Source zip file path.
@param {string} destDir Extraction destination directory.