parcel-bundler/src/utils/promisify.js

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module.exports = function(fn) {
  return function(...args) {
    return new Promise(function(resolve, reject) {
      fn(...args, function(err, ...res) {
        if (err) return reject(err);

        if (res.length === 1) return resolve(res[0]);

        resolve(res);
      });
    });
  };
};