httpGet: fallback to load-via-server; option 'forceServerLoad'

httpGet.php

@@ -0,0 +1,2 @@
+<?php
+readfile($_REQUEST['url']);

src/httpGet.js

@@ -1,9 +1,14 @@
 function httpGet (url, options, callback) {
-  var xhr = new XMLHttpRequest()
-  xhr.open('get', url, true)
-  xhr.responseType = 'text'
-  xhr.onreadystatechange = function () {
+  let corsRetry = true
+  var xhr
+
+  function readyStateChange () {
     if (xhr.readyState === 4) {
+      if (corsRetry && xhr.status === 0) {
+        corsRetry = false
+        return viaServer()
+      }
+
       if (xhr.status === 200) {
         callback(null, { body: xhr.responseText })
       } else {
@@ -11,7 +16,28 @@ function httpGet (url, options, callback) {
       }
     }
   }
-  xhr.send()
+
+  function direct () {
+    xhr = new XMLHttpRequest()
+    xhr.open('get', url, true)
+    xhr.responseType = 'text'
+    xhr.onreadystatechange = readyStateChange
+    xhr.send()
+  }
+
+  function viaServer () {
+    xhr = new XMLHttpRequest()
+    xhr.open('get', 'httpGet.php?url=' + encodeURIComponent(url), true)
+    xhr.responseType = 'text'
+    xhr.onreadystatechange = readyStateChange
+    xhr.send()
+  }
+
+  if (options.forceServerLoad) {
+    viaServer()
+  } else {
+    direct()
+  }
 }
 
 module.exports = httpGet