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Onerror Callback Support for Errors Fetching External Import Map #2324

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Apr 28, 2021
Merged

Onerror Callback Support for Errors Fetching External Import Map #2324

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19e66bd
Execute onerror callback fxn when fail to fetch import-map + update d…
Apr 28, 2021
b349c12
Updating callback error message and documentation
Apr 28, 2021
c321dbc
Add example in documentation and change callback err message
Apr 28, 2021
53061c9
Remove try/catch for onerror script
Apr 28, 2021
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19 changes: 19 additions & 0 deletions docs/import-maps.md
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Original file line number Diff line number Diff line change
Expand Up @@ -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).
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8 changes: 8 additions & 0 deletions src/features/import-maps.js
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Expand Up @@ -53,6 +53,14 @@ 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') {
try {
script.onerror();
} catch(callbackErr) {
callbackErr.message = 'Error during onerror script for systemjs-importmap: ' + script.src + '\n' + callbackErr.message;
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This error should be obtainable from the stack anyway, so let's remove this try-catch wrapper rather. Or did you specifically find the stack didn't have the information needed?

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The stack has the required info, I will remove the try-catch, thanks for the quick review 👍

console.error(callbackErr);
}
}
return '{}';
}) : script.innerHTML;
importMapPromise = importMapPromise.then(function () {
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7 changes: 7 additions & 0 deletions test/test.html
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Expand Up @@ -28,6 +28,13 @@

<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="systemjs-importmap" onerror="callbackDNE()" 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>
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