title: return true actually returns undefined using async
body: <p>I'm trying to do something if the value is not exist, so I can update it. but isExist function always return undefined. what can I do with this? </p>
  
  <p>reference: <code>Ero</code> is already defined.</p>
  
  <pre><code>async.forEachOf(idArray, function(value, key, cb) {
      rp(baseURL + value)
        .then(function(json) {
            if (!isExist(json)) {
                // do something
              } else {
                  console.log(isExist(json), "It's not video or exists");
              }
          })
      .catch(function(err) {
        console.error(err);
      })
      cb();
    }, function() {
      res.status(200)
    });
  });
  
  function isExist(data) {
    let parsedData = JSON.parse(data);
    if (parsedData.items[0].type === 'Video') {
      Ero.find({
        videoUri: parsedData.items[0].url_mp4
      }, function(err, docs) {
        if (docs.length) {
            return true;
        } else {
            return false;
        }
      })
    } else {
        return false;
    }
  }
  </code></pre>