title: Express API only returns first key value pair of object
body: <p>I'm fetching nested object data. I can get that object data on console but when I try to get that data like <code>return res.json(imageObject)</code> I only get first key value pair of that object. This is the error to the console. <code>UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client</code>.
  Console data is like this <code>{ bigImage: 'https://url.com' }</code></p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>router.get("/", async(req, res) =&gt; {
    //...fetch data
    if (mediaData.type === "type1") {
      let allData = await mediaData.media;
  
      allData.map(async(Data) =&gt; {
        if (Data.imageType === "big") {
          let bigImage = await Data.url;
          let imageObject = {
            bigImage
          };
          console.log(imageObject);
          return res.json(imageObject);
        }
      });
    }
  });</code></pre>
  </div>
  </div>
  </p>