Onerror Callback Support for Errors Fetching External Import Map (#2324)

docs/import-maps.md

@@ -200,6 +200,25 @@ Any existing mappings are replaced, although in future this may be an error.
 
 Previous versions of the import maps spec had support for multiple import maps in a single web page ([explanation](https://github.com/WICG/import-maps/issues/199)). SystemJS added support for multiple import maps during that time and has decided to keep support for multiple import maps as an experimental feature ([discussion](https://github.com/systemjs/systemjs/issues/2095)). Note that the Chrome implementation of import maps does not yet allow for multiple maps, and use of multiple import maps within SystemJS should be considered experimental and subject to change.
 
+### Handling Import Map Errors
+
+For handling errors when fetching external import maps (specifically for [SystemJS Warning #W4](https://github.com/systemjs/systemjs/blob/master/docs/errors.md#w4)), we can use the `onerror` attribute in the `<script type="systemjs-importmap">` tag.
+
+In the `onerror` attribute, specify a script to execute when there is an error.
+
+```html
+<script type="systemjs-importmap" onerror="handleError()" src="unable-to-reach/importmap.json"></script>
+
+<script type="text/javascript">
+  function handleError() {
+    // Put error handling/retry logic here.
+  }
+</script>
+
+<!-- Can also have inline definition for error handling script -->
+<script type="systemjs-importmap" onerror='console.log("Running onerror script")' src="unable-to-reach/importmap.json"></script>
+```
+
 #### Spec and Implementation Feedback
 
 Part of the benefit of giving users a working version of an early spec is being able to get real user feedback on the [import maps specification](https://github.com/wicg/import-maps/blob/master/spec.md).

src/features/import-maps.js

@@ -53,6 +53,9 @@ function processScripts () {
       }).catch(function (err) {
         err.message = errMsg('W4', process.env.SYSTEM_PRODUCTION ? script.src : 'Error fetching systemjs-import map ' + script.src) + '\n' + err.message;
         console.warn(err);
+        if (typeof script.onerror === 'function') {
+            script.onerror();
+        }
         return '{}';
       }) : script.innerHTML;
       importMapPromise = importMapPromise.then(function () {

test/test.html

@@ -28,6 +28,12 @@
 
 	<script type="systemjs-importmap" src="https://hostname.invalid/importmap.json"></script>
 	<script type="systemjs-importmap" src="fixtures/browser/importmap-invalid.json"></script>
+	<script type="systemjs-importmap" onerror="retryImportMap()" src="fixtures/browser/importmap-invalid.json"></script>
+	<script type="text/javascript">
+		function retryImportMap() {
+			document.write('<scr'+'ipt type="systemjs-importmap" src="fixtures/browser/importmap-invalid.json"></sc'+'ript>');
+		}
+	</script>
 	<script type="systemjs-importmap" src="fixtures/browser/importmap.json" integrity="sha384-+t/f9i0vQYwxmcczlIpgCCTQz2mA1sMGYGErJ4fZb/Lcx/yYrFSiedO0XpSIX8oX"></script>
 	<script type="systemjs-module" src="/test/fixtures/browser/systemjs-module-early.js"></script>
 	<script src="../dist/system.js"></script>