Simplified demo.html; show proxyError on error

demo.html

@@ -66,21 +66,19 @@ textarea {
 <script>
   var protocol = location.protocol === 'http:' ? 'http:' : 'https:';
   var cors_api_url = protocol + '//cors-anywhere.herokuapp.com/';
-  var cors_api_redirect_cache = {};
-  function doCORSRequest(options, redirectCount) {
+  function doCORSRequest(options, printResult) {
     var x = new XMLHttpRequest();
     x.open(options.method, cors_api_url + options.url);
-  
-    x.onload = function() {
-      if (x.status >= 200 && x.status < 300 || x.status === 304) {
-        options.success(options, x.status, x.statusText, x.responseText);
-      } else {
-        options.error(options, x.status, x.statusText);
-      }
-    };
-    x.onerror = function() {
-      options.error(options, x.status, x.statusText);
+    x.onload = x.onerror = function() {
+      printResult(
+        options.method + ' ' + options.url + '\n' +
+        x.status + ' ' + x.statusText + '\n\n' +
+        (x.responseText || '')
+      );
     };
+    if (/^POST/i.test(options.method)) {
+      x.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
+    }
     x.send(options.data);
   }
 
@@ -89,33 +87,22 @@ textarea {
     var urlField = document.getElementById('url');
     var dataField = document.getElementById('data');
     var outputField = document.getElementById('output');
-    document.getElementById('get').onclick = function(e) {
-      e.preventDefault();
-      doCORSRequest({
-        method: 'GET',
-        url: urlField.value,
-        success: onSuccess,
-        error: onError
-      });
-    };
+    document.getElementById('get').onclick =
     document.getElementById('post').onclick = function(e) {
       e.preventDefault();
       doCORSRequest({
-        method: 'POST',
+        method: this.id === 'post' ? 'POST' : 'GET',
         url: urlField.value,
-        data: dataField.value,
-        success: onSuccess,
-        error: onError
+        data: dataField.value
+      }, function printResult(result) {
+        outputField.value = result;
       });
     };
-    function onSuccess(options, statusCode, statusText, responseText) {
-      outputField.value = options.method + ' ' + options.url + '\n' + statusCode + ' ' + statusText + '\n\n' + responseText;
-
-    }
-    function onError(options, statusCode, statusText) {
-      outputField.value = options.method + ' ' + options.url + '\n' + statusCode + ' ' + statusText;
-    }
   })();
+  if (typeof console === 'object') {
+    console.log('// To test a local CORS Anywhere server, set cors_api_url. For example:');
+    console.log('cors_api_url = "http://localhost:8080/"');
+  }
 </script>
 </body>
 </html>