dav.js
function co(gen) {
  var ctx = this;
  var args = slice.call(arguments, 1)

  // we wrap everything in a promise to avoid promise chaining,
  // which leads to memory leak errors.
  // see https://github.com/tj/co/issues/180
  return new Promise(function(resolve, reject) {
    if (typeof gen === 'function') gen = gen.apply(ctx, args);
    if (!gen || typeof gen.next !== 'function') return resolve(gen);

    onFulfilled();

    /**
     * @param {Mixed} res
     * @return {Promise}
     * @api private
     */

    function onFulfilled(res) {
      var ret;
      try {
        ret = gen.next(res);
      } catch (e) {
        return reject(e);
      }
      next(ret);
    }

    /**
     * @param {Error} err
     * @return {Promise}
     * @api private
     */

    function onRejected(err) {
      var ret;
      try {
        ret = gen.throw(err);
      } catch (e) {
        return reject(e);
      }
      next(ret);
    }

    /**
     * Get the next value in the generator,
     * return a promise.
     *
     * @param {Object} ret
     * @return {Promise}
     * @api private
     */

    function next(ret) {
      if (ret.done) return resolve(ret.value);
      var value = toPromise.call(ctx, ret.value);
      if (value && isPromise(value)) return value.then(onFulfilled, onRejected);
      return onRejected(new TypeError('You may only yield a function, promise, generator, array, or object, '
        + 'but the following object was passed: "' + String(ret.value) + '"'));
    }
  });
}
function co(gen) {
  var ctx = this;
  var args = slice.call(arguments, 1)

  // we wrap everything in a promise to avoid promise chaining,
  // which leads to memory leak errors.
  // see https://github.com/tj/co/issues/180
  return new Promise(function(resolve, reject) {
    if (typeof gen === 'function') gen = gen.apply(ctx, args);
    if (!gen || typeof gen.next !== 'function') return resolve(gen);

    onFulfilled();

    /**
     * @param {Mixed} res
     * @return {Promise}
     * @api private
     */

    function onFulfilled(res) {
      var ret;
      try {
        ret = gen.next(res);
      } catch (e) {
        return reject(e);
      }
      next(ret);
    }

    /**
     * @param {Error} err
     * @return {Promise}
     * @api private
     */

    function onRejected(err) {
      var ret;
      try {
        ret = gen.throw(err);
      } catch (e) {
        return reject(e);
      }
      next(ret);
    }

    /**
     * Get the next value in the generator,
     * return a promise.
     *
     * @param {Object} ret
     * @return {Promise}
     * @api private
     */

    function next(ret) {
      if (ret.done) return resolve(ret.value);
      var value = toPromise.call(ctx, ret.value);
      if (value && isPromise(value)) return value.then(onFulfilled, onRejected);
      return onRejected(new TypeError('You may only yield a function, promise, generator, array, or object, '
        + 'but the following object was passed: "' + String(ret.value) + '"'));
    }
  });
}
function co ( gen ) { var ctx = this ; var args = slice . call ( arguments , 1 ) // we wrap everything in a promise to avoid promise chaining, // which leads to memory leak errors. // see https://github.com/tj/co/issues/180 return new Promise ( function ( resolve , reject ) { if ( typeof gen === 'function' ) gen = gen . apply ( ctx , args ) ; if ( ! gen || typeof gen . next !== 'function' ) return resolve ( gen ) ; onFulfilled ( ) ; /**
     * @param {Mixed} res
     * @return {Promise}
     * @api private
     */ function onFulfilled ( res ) { var ret ; try { ret = gen . next ( res ) ; } catch ( e ) { return reject ( e ) ; } next ( ret ) ; } /**
     * @param {Error} err
     * @return {Promise}
     * @api private
     */ function onRejected ( err ) { var ret ; try { ret = gen . throw ( err ) ; } catch ( e ) { return reject ( e ) ; } next ( ret ) ; } /**
     * Get the next value in the generator,
     * return a promise.
     *
     * @param {Object} ret
     * @return {Promise}
     * @api private
     */ function next ( ret ) { if ( ret . done ) return resolve ( ret . value ) ; var value = toPromise . call ( ctx , ret . value ) ; if ( value && isPromise ( value ) ) return value . then ( onFulfilled , onRejected ) ; return onRejected ( new TypeError ( 'You may only yield a function, promise, generator, array, or object, ' + 'but the following object was passed: "' + String ( ret . value ) + '"' ) ) ; } } ) ; }
Execute the generator function or a generator
and return a promise.

@param {Function} fn
@return {Promise}
@api public
Execute the generator function or a generator and return a promise .