Read current import map

README.md

@@ -89,7 +89,7 @@ Shim mode is an alternative to polyfill mode and doesn't rely on native modules
 
 In shim mode, normal module scripts and import maps are entirely ignored and only the above shim tags will be parsed and executed by ES Module Shims instead.
 
-Shim mode also provides some additional features that aren't yet natively supported such as supporting multiple import maps, [external import maps](#external-import-maps) with a `"src"` attribute or [dynamicallly injecting import maps](#dynamic-import-maps), which can be useful in certain applications.
+Shim mode also provides some additional features that aren't yet natively supported such as supporting multiple import maps, [external import maps](#external-import-maps) with a `"src"` attribute, [dynamically injecting import maps](#dynamic-import-maps), and [reading current import map state](#reading-current-import-map-state), which can be useful in certain applications.
 
 ## Features
 
@@ -195,6 +195,16 @@ Both modes in ES Module Shims support dynamic injection using DOM Mutation Obser
 
 While in polyfill mode the same restrictions apply that multiple import maps, import maps with a `src` attribute, and import maps loaded after the first module load are not supported, in shim mode all of these behaviours are fully enabled for `"importmap-shim"`.
 
+#### Reading current import map state
+
+To make it easy to keep track of import map state, es-module-shims provides a `importShim.getImportMap` utility function, available only in shim mode.
+
+```js
+const importMap = importShim.getImportMap()
+
+// importMap will be an object in the same shape as the json in a importmap script
+```
+
 ### Dynamic Import
 
 > Stability: Stable browser standard

src/common.js

@@ -126,6 +126,9 @@ export function resolveUrl (relUrl, parentUrl) {
 function resolveAndComposePackages (packages, outPackages, baseUrl, parentMap) {
   for (let p in packages) {
     const resolvedLhs = resolveIfNotPlainOrUrl(p, baseUrl) || p;
+    if (outPackages[resolvedLhs]) {
+      throw new Error(`Dynamic import map rejected: Overrides entry "${resolvedLhs}" from ${outPackages[resolvedLhs]} to ${packages[resolvedLhs]}.`);
+    }
     let target = packages[p];
     if (typeof target !== 'string') 
       continue;

src/es-module-shims.js

@@ -174,6 +174,10 @@ async function importShim (id, parentUrl = pageBaseUrl, _assertion) {
 
 self.importShim = importShim;
 
+if (shimMode) {
+  importShim.getImportMap = () => JSON.parse(JSON.stringify(importMap));
+}
+
 const meta = {};
 
 async function importMetaResolve (id, parentUrl = this.url) {
@@ -460,9 +464,11 @@ function processImportMap (script) {
     importMapSrcOrLazy = true;
   }
   if (acceptingImportMaps) {
-    importMapPromise = importMapPromise.then(async () => {
-      importMap = resolveAndComposeImportMap(script.src ? await (await fetchHook(script.src)).json() : JSON.parse(script.innerHTML), script.src || pageBaseUrl, importMap);
-    });
+    importMapPromise = importMapPromise
+      .then(async () => {
+        importMap = resolveAndComposeImportMap(script.src ? await (await fetchHook(script.src)).json() : JSON.parse(script.innerHTML), script.src || pageBaseUrl, importMap);
+      })
+      .catch(error => setTimeout(() => { throw error }));
     if (!shimMode)
       acceptingImportMaps = false;
   }

test/shim.js

@@ -262,6 +262,41 @@ suite('Export variations', function () {
   });
 });
 
+// These tests are order dependent, as importMap state will change when dynamic import maps are injected in the suite.
+suite('Get import map', () => {
+  test("should get correct import map state", async () => {
+    const importMap = await importShim.getImportMap();
+
+    const sortEntriesByKey = (entries) => [...entries].sort(([key1], [key2]) => key1.localeCompare(key2));
+
+    assert.equal(JSON.stringify(Object.keys(importMap)), JSON.stringify(["imports", "scopes"]));
+    assert.equal(
+      JSON.stringify(sortEntriesByKey(Object.entries(importMap.imports))), 
+      JSON.stringify(sortEntriesByKey(Object.entries({
+        "test": "http://localhost:8080/test/fixtures/es-modules/es6-file.js",
+        "test/": "http://localhost:8080/test/fixtures/",
+        "global1": "http://localhost:8080/test/fixtures/es-modules/global1.js",
+        "bare-dynamic-import": "http://localhost:8080/test/fixtures/es-modules/bare-dynamic-import.js",
+        "react": "https://ga.jspm.io/npm:react@17.0.2/dev.index.js"
+      })))
+    );
+    assert.equal(
+      JSON.stringify(sortEntriesByKey(Object.entries(importMap.scopes))), 
+      JSON.stringify(sortEntriesByKey(Object.entries({
+        "http://localhost:8080/": {
+          "test-dep": "http://localhost:8080/test/fixtures/test-dep.js",
+        },
+        "http://localhost:8080/test/fixtures/es-modules/import-relative-path.js": {
+          "http://localhost:8080/test/fixtures/es-modules/relative-path": "http://localhost:8080/test/fixtures/es-modules/es6-dep.js",
+        },
+        "https://ga.jspm.io/": {
+          "object-assign": "https://ga.jspm.io/npm:object-assign@4.1.1/index.js",
+        }
+      })))
+    );
+  });
+});
+
 suite('Errors', function () {
   async function getImportError(module) {
     try {
@@ -331,11 +366,33 @@ suite('Errors', function () {
     assert.ok(lodash);
   })
 
+  test('Dynamic import map shim with override attempt', async function () {
+    const listeningForError = new Promise((resolve, reject) => {
+      window.addEventListener('error', (event) => resolve(event.error))
+      // ensure we don't wait forever in the test if the error never comes
+      setTimeout(reject, 5000)
+    })
+
+    const removeImportMap = insertDynamicImportMap({
+      "imports": {
+        "global1": "data:text/javascript,throw new Error('Shim should not allow dynamic import map to override existing entries');"
+      }
+    });
+
+    const error = await listeningForError;
+
+    removeImportMap();
+
+    assert(error.message === 'Dynamic import map rejected: Overrides entry "global1" from http://localhost:8080/test/fixtures/es-modules/global1.js to data:text/javascript,throw new Error(\'Shim should not allow dynamic import map to override existing entries\');.');
+  })
+
   function insertDynamicImportMap(importMap) {
-    document.body.appendChild(Object.assign(document.createElement('script'), {
+    const script = Object.assign(document.createElement('script'), {
       type: 'importmap-shim',
       innerHTML: JSON.stringify(importMap),
-    }));
+    });
+    document.body.appendChild(script);
+    return () => document.body.removeChild(script);
   }
 });
 

test/test-shim.html

@@ -8,17 +8,25 @@
   "imports": {
     "test": "/test/fixtures/es-modules/es6-file.js",
     "test/": "/test/fixtures/",
-    "global1": "/test/fixtures/es-modules/global1.js",
-    "bare-dynamic-import": "/test/fixtures/es-modules/bare-dynamic-import.js",
-    "react": "https://ga.jspm.io/npm:react@17.0.2/dev.index.js"
+    "global1": "/test/fixtures/es-modules/global1.js"
   },
   "scopes": {
     "/": {
       "test-dep": "/test/fixtures/test-dep.js"
     },
     "/test/fixtures/es-modules/import-relative-path.js": {
       "./fixtures/es-modules/relative-path": "./fixtures/es-modules/es6-dep.js"
-    },
+    }
+  }
+}
+</script>
+<script type="importmap-shim">
+{
+  "imports": {
+    "bare-dynamic-import": "/test/fixtures/es-modules/bare-dynamic-import.js",
+    "react": "https://ga.jspm.io/npm:react@17.0.2/dev.index.js"
+  },
+  "scopes": {
     "https://ga.jspm.io/": {
       "object-assign": "https://ga.jspm.io/npm:object-assign@4.1.1/index.js"
     }