title: fetch().then / fetch different?
body: <p>I am using fetch to check if my images exist then return true/false to replace the original image. </p>
  
  <p>However, it kind of confusing me between fetch and fetch.then, when I only put url into fetch() to check its status, this works fine for me by using if condition, then I am trying to use fetch(url).then(response => ...) to do the same thing, it outputs different result. </p>
  
  <p>What I expect is these two ways should output the same result, is there any blind spot?</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>  $('.src-load .rwd-1440').each(function(index, ele){
        var srcset =  $(ele).attr("srcset");                
           if(!urlexist(srcset)){
              var src = srcset.substring(0, srcset.indexOf('/1440')); 
              $(ele).attr("srcset", src);
           }
    }); 
      
    function urlexist(url){
        var myRequest = new Request(url);
  
        fetch(myRequest).then(function(response) {
            if(response.status != 404){
              return true;
            }
            else
            return false;
        });
    }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;picture class="src-load"&gt;
     &lt;source class="rwd-1440" media="(min-width: 801px) and (max-width: 1440px)" srcset="image.png/1440.png"&gt;
     &lt;img src="image.png"&gt;
  &lt;/picture&gt;</code></pre>
  </div>
  </div>
  </p>