Tiny promisify function

promisify.js

/**
 * Promisify a node style async func.
 *
 * @param {function} f Function to with a node callback as its last argument.
 * @param [{object}] scope Scope to run `f` in, if necessary.
 * @return {Promise}
 *
 * @example ```
 *  const read = promisify(fs.readFile, fs);
 *  read('./file')
 *    .then(data => console.log(data.toString('utf8')))
 *    .catch(err => console.log(err));
 * ```
 */

const promisify = ((slice) => (f, scope) => function () {
    return new Promise(
      (res, rej) => f.apply(
        scope, slice.call(arguments).concat(
          (err, data) => err ? rej(err) : res(data)
        )
      )
    );
  }
)(Array.prototype.slice);