From d61aad670e2dcab76cfde23de98f3881246b343d Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sat, 18 Mar 2023 13:43:53 -0700
Subject: [PATCH 1/9] feat: debug build

---
 chompfile.toml                    |  4 +---
 rollup.config.js                  | 11 ++++++-----
 src/es-module-shims.js            | 20 +++++++++++++++-----
 test/test-shim-map-overrides.html |  2 --
 test/test-shim.html               |  2 --
 5 files changed, 22 insertions(+), 17 deletions(-)

diff --git a/chompfile.toml b/chompfile.toml
index 70e642c0..6d4ad309 100644
--- a/chompfile.toml
+++ b/chompfile.toml
@@ -2,8 +2,6 @@ version = 0.1
 
 extensions = ['chomp@0.1:footprint', 'chomp@0.1:npm']
 
-default-task = 'build'
-
 [server]
 port = 8080
 root = "."
@@ -18,7 +16,7 @@ run = 'rm -rf bench/results'
 
 [[task]]
 name = 'build'
-targets = ['dist/es-module-shims.js', 'dist/es-module-shims.wasm.js']
+targets = ['dist/es-module-shims.js', 'dist/es-module-shims.wasm.js', 'dist/es-module-shims.debug.js']
 deps = ['src/*.js', 'npm:install', 'README.md']
 run = 'rollup -c'
 
diff --git a/rollup.config.js b/rollup.config.js
index 5229831f..8670ffcd 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -5,17 +5,18 @@ import path from 'path';
 const version = JSON.parse(fs.readFileSync('package.json')).version;
 
 export default [
-  config(true),
-  config(false),
+  config(true, false),
+  config(false, false),
+  config(false, true),
 ];
 
-function config (isWasm) {
+function config (isWasm, isDebug) {
   const name = 'es-module-shims'
 
   return {
     input: `src/${name}.js`,
     output: {
-      file: `dist/${name}${isWasm ? '.wasm' : ''}.js`,
+      file: `dist/${name}${isWasm ? '.wasm' : ''}${isDebug ? '.debug' : ''}.js`,
       format: 'iife',
       strict: false,
       sourcemap: false,
@@ -29,7 +30,7 @@ function config (isWasm) {
         }
       },
       replace({
-        'self.ESMS_DEBUG': 'false',
+        'self.ESMS_DEBUG': isDebug.toString(),
         preventAssignment: true
       }),
     ]
diff --git a/src/es-module-shims.js b/src/es-module-shims.js
index a2b54ccc..c649dd90 100755
--- a/src/es-module-shims.js
+++ b/src/es-module-shims.js
@@ -120,7 +120,8 @@ let importMap = { imports: {}, scopes: {} };
 let baselinePassthrough;
 
 const initPromise = featureDetectionPromise.then(() => {
-  baselinePassthrough = esmsInitOptions.polyfillEnable !== true && supportsDynamicImport && supportsImportMeta && supportsImportMaps && (!jsonModulesEnabled || supportsJsonAssertions) && (!cssModulesEnabled || supportsCssAssertions) && !importMapSrcOrLazy && !self.ESMS_DEBUG;
+  baselinePassthrough = esmsInitOptions.polyfillEnable !== true && supportsDynamicImport && supportsImportMeta && supportsImportMaps && (!jsonModulesEnabled || supportsJsonAssertions) && (!cssModulesEnabled || supportsCssAssertions) && !importMapSrcOrLazy;
+  if (self.ESMS_DEBUG) console.info(`es-module-shims: init ${shimMode ? 'shim mode' : 'polyfill mode'}, ${baselinePassthrough ? 'baseline passthrough' : 'polyfill engaged'}`);
   if (hasDocument) {
     if (!supportsImportMaps) {
       const supports = HTMLScriptElement.supports || (type => type === 'classic' || type === 'module');
@@ -174,6 +175,7 @@ async function topLevelLoad (url, fetchOpts, source, nativelyLoaded, lastStaticL
   if (importHook) await importHook(url, typeof fetchOpts !== 'string' ? fetchOpts : {}, '');
   // early analysis opt-out - no need to even fetch if we have feature support
   if (!shimMode && baselinePassthrough) {
+    if (self.ESMS_DEBUG) console.info(`es-module-shims: load skipping polyfill due to baseline passthrough applying: ${url}`);
     // for polyfill case, only dynamic import needs a return value here, and dynamic import will never pass nativelyLoaded
     if (nativelyLoaded)
       return null;
@@ -186,7 +188,7 @@ async function topLevelLoad (url, fetchOpts, source, nativelyLoaded, lastStaticL
   lastLoad = undefined;
   resolveDeps(load, seen);
   await lastStaticLoadPromise;
-  if (source && !shimMode && !load.n && !self.ESMS_DEBUG) {
+  if (source && !shimMode && !load.n) {
     if (nativelyLoaded) return;
     if (revokeBlobURLs) revokeObjectURLs(Object.keys(seen));
     return await dynamicImport(createBlob(source), { errUrl: source });
@@ -463,6 +465,7 @@ function getOrCreateLoad (url, fetchOpts, parent, source) {
 }
 
 function processScriptsAndPreloads (mapsOnly = false) {
+  if (self.ESMS_DEBUG) console.info(`es-module-shims: processing scripts`);
   if (!mapsOnly)
     for (const link of document.querySelectorAll(shimMode ? 'link[rel=modulepreload-shim]' : 'link[rel=modulepreload]'))
       processPreload(link);
@@ -492,8 +495,10 @@ let lastStaticLoadPromise = Promise.resolve();
 
 let domContentLoadedCnt = 1;
 function domContentLoadedCheck () {
-  if (--domContentLoadedCnt === 0 && !noLoadEventRetriggers && (shimMode || !baselinePassthrough))
+  if (--domContentLoadedCnt === 0 && !noLoadEventRetriggers && (shimMode || !baselinePassthrough)) {
+    if (self.ESMS_DEBUG) console.info(`es-module-shims: DOMContentLoaded refire`);
     document.dispatchEvent(new Event('DOMContentLoaded'));
+  }
 }
 // this should always trigger because we assume es-module-shims is itself a domcontentloaded requirement
 if (hasDocument) {
@@ -505,8 +510,10 @@ if (hasDocument) {
 
 let readyStateCompleteCnt = 1;
 function readyStateCompleteCheck () {
-  if (--readyStateCompleteCnt === 0 && !noLoadEventRetriggers && (shimMode || !baselinePassthrough))
+  if (--readyStateCompleteCnt === 0 && !noLoadEventRetriggers && (shimMode || !baselinePassthrough)) {
+    if (self.ESMS_DEBUG) console.info(`es-module-shims: readystatechange complete refire`);
     document.dispatchEvent(new Event('readystatechange'));
+  }
 }
 
 const hasNext = script => script.nextSibling || script.parentNode && hasNext(script.parentNode);
@@ -544,12 +551,15 @@ function processScript (script, ready = readyStateCompleteCnt > 0) {
   const isDomContentLoadedScript = domContentLoadedCnt > 0;
   if (isBlockingReadyScript) readyStateCompleteCnt++;
   if (isDomContentLoadedScript) domContentLoadedCnt++;
+  if (self.ESMS_DEBUG) console.info(`es-module-shims: processing ${script.src || '<inline>'}`);
   const loadPromise = topLevelLoad(script.src || pageBaseUrl, getFetchOpts(script), !script.src && script.innerHTML, !shimMode, isBlockingReadyScript && lastStaticLoadPromise)
     .then(() => {
       // if the type of the script tag "module-shim", browser does not dispatch a "load" event
       // see https://github.com/guybedford/es-module-shims/issues/346
-      if (shimMode)
+      if (shimMode) {
+        if (self.ESMS_DEBUG) console.info(`es-module-shims: load even refire ${script.src || '<inline>'}`);
         script.dispatchEvent(new Event('load'));
+      }
     })
     .catch(throwError);
   if (isBlockingReadyScript)
diff --git a/test/test-shim-map-overrides.html b/test/test-shim-map-overrides.html
index b0b7e56e..83408bd1 100644
--- a/test/test-shim-map-overrides.html
+++ b/test/test-shim-map-overrides.html
@@ -25,8 +25,6 @@
     shimMode: true,
     mapOverrides: true,
   };
-
-  window.ESMS_DEBUG = true;
 </script>
 <script type="module" src="../src/es-module-shims.js"></script>
 <script type="module" noshim>
diff --git a/test/test-shim.html b/test/test-shim.html
index 82cb6a1c..9d2e5736 100644
--- a/test/test-shim.html
+++ b/test/test-shim.html
@@ -77,8 +77,6 @@
     },
     onerror: e => window.e = e,
   };
-
-  window.ESMS_DEBUG = true;
 </script>
 <script type="module" src="../src/es-module-shims.js"></script>
 <script type="module" noshim>

From 8470bda66e3bdacf896f981ba88bf24158b2be09 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sun, 19 Mar 2023 10:50:40 -0700
Subject: [PATCH 2/9] debug build feature detection logging

---
 rollup.config.js       | 2 +-
 src/es-module-shims.js | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/rollup.config.js b/rollup.config.js
index 8670ffcd..37a8fba7 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -20,7 +20,7 @@ function config (isWasm, isDebug) {
       format: 'iife',
       strict: false,
       sourcemap: false,
-      banner: `/* ES Module Shims ${isWasm ? 'Wasm ' : ''}${version} */`
+      banner: `/* ES Module Shims ${isWasm ? 'Wasm ' : ''}${isDebug ? 'DEBUG BUILD ' : ''}${version} */`
     },
     plugins: [
       {
diff --git a/src/es-module-shims.js b/src/es-module-shims.js
index c649dd90..b670f67d 100755
--- a/src/es-module-shims.js
+++ b/src/es-module-shims.js
@@ -120,6 +120,7 @@ let importMap = { imports: {}, scopes: {} };
 let baselinePassthrough;
 
 const initPromise = featureDetectionPromise.then(() => {
+  if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMap ? '' : 'no '}import maps, ${supportsJsonAssertions ? '' : 'no '}JSON modules, ${supportsCssAssertions ? '' : 'no '}CSS modules`);
   baselinePassthrough = esmsInitOptions.polyfillEnable !== true && supportsDynamicImport && supportsImportMeta && supportsImportMaps && (!jsonModulesEnabled || supportsJsonAssertions) && (!cssModulesEnabled || supportsCssAssertions) && !importMapSrcOrLazy;
   if (self.ESMS_DEBUG) console.info(`es-module-shims: init ${shimMode ? 'shim mode' : 'polyfill mode'}, ${baselinePassthrough ? 'baseline passthrough' : 'polyfill engaged'}`);
   if (hasDocument) {

From 3eb8751cb5eecdc1e60917bef69308033565c0cd Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sun, 19 Mar 2023 10:56:24 -0700
Subject: [PATCH 3/9] feature detection tweaks

---
 src/es-module-shims.js | 2 +-
 src/features.js        | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/es-module-shims.js b/src/es-module-shims.js
index b670f67d..c7436037 100755
--- a/src/es-module-shims.js
+++ b/src/es-module-shims.js
@@ -120,7 +120,7 @@ let importMap = { imports: {}, scopes: {} };
 let baselinePassthrough;
 
 const initPromise = featureDetectionPromise.then(() => {
-  if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMap ? '' : 'no '}import maps, ${supportsJsonAssertions ? '' : 'no '}JSON modules, ${supportsCssAssertions ? '' : 'no '}CSS modules`);
+  if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMap ? '' : 'no '}import maps`);
   baselinePassthrough = esmsInitOptions.polyfillEnable !== true && supportsDynamicImport && supportsImportMeta && supportsImportMaps && (!jsonModulesEnabled || supportsJsonAssertions) && (!cssModulesEnabled || supportsCssAssertions) && !importMapSrcOrLazy;
   if (self.ESMS_DEBUG) console.info(`es-module-shims: init ${shimMode ? 'shim mode' : 'polyfill mode'}, ${baselinePassthrough ? 'baseline passthrough' : 'polyfill engaged'}`);
   if (hasDocument) {
diff --git a/src/features.js b/src/features.js
index f900a2d0..0aca67e1 100644
--- a/src/features.js
+++ b/src/features.js
@@ -5,7 +5,9 @@ import { createBlob, noop, nonce, cssModulesEnabled, jsonModulesEnabled, hasDocu
 export let supportsJsonAssertions = false;
 export let supportsCssAssertions = false;
 
-export let supportsImportMaps = hasDocument && HTMLScriptElement.supports ? HTMLScriptElement.supports('importmap') : false;
+const supports = HTMLScriptElement.supports;
+
+export let supportsImportMaps = hasDocument && supports ? supports.name === 'supports' && supports('importmap') : false;
 export let supportsImportMeta = supportsImportMaps;
 
 const importMetaCheck = 'import.meta';

From acd118bb15496341225e4a86e99ddf21ef30e2a4 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sun, 19 Mar 2023 10:59:31 -0700
Subject: [PATCH 4/9] feature detection logging

---
 src/es-module-shims.js | 1 -
 src/features.js        | 2 ++
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/es-module-shims.js b/src/es-module-shims.js
index c7436037..c649dd90 100755
--- a/src/es-module-shims.js
+++ b/src/es-module-shims.js
@@ -120,7 +120,6 @@ let importMap = { imports: {}, scopes: {} };
 let baselinePassthrough;
 
 const initPromise = featureDetectionPromise.then(() => {
-  if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMap ? '' : 'no '}import maps`);
   baselinePassthrough = esmsInitOptions.polyfillEnable !== true && supportsDynamicImport && supportsImportMeta && supportsImportMaps && (!jsonModulesEnabled || supportsJsonAssertions) && (!cssModulesEnabled || supportsCssAssertions) && !importMapSrcOrLazy;
   if (self.ESMS_DEBUG) console.info(`es-module-shims: init ${shimMode ? 'shim mode' : 'polyfill mode'}, ${baselinePassthrough ? 'baseline passthrough' : 'polyfill engaged'}`);
   if (hasDocument) {
diff --git a/src/features.js b/src/features.js
index 0aca67e1..c5cc658c 100644
--- a/src/features.js
+++ b/src/features.js
@@ -26,6 +26,7 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
     ]);
 
   return new Promise(resolve => {
+    if (self.ESMS_DEBUG) console.info(`es-module-shims: performing feature detections for ${`${supportsImportMaps ? 'import maps, ' : ''}${cssModulesEnabled ? 'css modules, ' : ''}${jsonModulesEnabled ? 'json modules, ' : ''}`.slice(0, -2)}`);
     const iframe = document.createElement('iframe');
     iframe.style.display = 'none';
     iframe.setAttribute('nonce', nonce);
@@ -34,6 +35,7 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
       supportsImportMeta = b;
       supportsCssAssertions = c;
       supportsJsonAssertions = d;
+      if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMap ? '' : 'no '}import maps`);
       resolve();
       document.head.removeChild(iframe);
       window.removeEventListener('message', cb, false);

From a4b99463a20a98b4f8175a45c838c876b7e601e9 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Mon, 20 Mar 2023 08:36:01 -0700
Subject: [PATCH 5/9] fixup feature detection debugging

---
 src/features.js | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/features.js b/src/features.js
index c5cc658c..52bb5c1b 100644
--- a/src/features.js
+++ b/src/features.js
@@ -26,7 +26,8 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
     ]);
 
   return new Promise(resolve => {
-    if (self.ESMS_DEBUG) console.info(`es-module-shims: performing feature detections for ${`${supportsImportMaps ? 'import maps, ' : ''}${cssModulesEnabled ? 'css modules, ' : ''}${jsonModulesEnabled ? 'json modules, ' : ''}`.slice(0, -2)}`);
+    if (self.ESMS_DEBUG) if (supportsImportMaps) console.info(`es-module-shims: import maps supported from HTMLScriptElement check`);
+    if (self.ESMS_DEBUG) console.info(`es-module-shims: performing feature detections for ${`${supportsImportMaps ? '' : 'import maps, '}${cssModulesEnabled ? 'css modules, ' : ''}${jsonModulesEnabled ? 'json modules, ' : ''}`.slice(0, -2)}`);
     const iframe = document.createElement('iframe');
     iframe.style.display = 'none';
     iframe.setAttribute('nonce', nonce);
@@ -35,7 +36,7 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
       supportsImportMeta = b;
       supportsCssAssertions = c;
       supportsJsonAssertions = d;
-      if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMap ? '' : 'no '}import maps`);
+      if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMaps ? '' : 'no '}import maps`);
       resolve();
       document.head.removeChild(iframe);
       window.removeEventListener('message', cb, false);

From 03ddcedd6f09dac176ede8aa0078da60a93930e5 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Tue, 21 Mar 2023 08:40:17 -0700
Subject: [PATCH 6/9] rely on HTMLScriptElement supports

---
 src/features.js | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/src/features.js b/src/features.js
index 52bb5c1b..fe2349e7 100644
--- a/src/features.js
+++ b/src/features.js
@@ -5,17 +5,17 @@ import { createBlob, noop, nonce, cssModulesEnabled, jsonModulesEnabled, hasDocu
 export let supportsJsonAssertions = false;
 export let supportsCssAssertions = false;
 
-const supports = HTMLScriptElement.supports;
+const supports = hasDocument && HTMLScriptElement.supports;
 
-export let supportsImportMaps = hasDocument && supports ? supports.name === 'supports' && supports('importmap') : false;
-export let supportsImportMeta = supportsImportMaps;
+export let supportsImportMaps = supports && supports.name === 'supports' && supports('importmap');
+export let supportsImportMeta = !!supports;
 
 const importMetaCheck = 'import.meta';
 const cssModulesCheck = `import"x"assert{type:"css"}`;
 const jsonModulesCheck = `import"x"assert{type:"json"}`;
 
-export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(() => {
-  if (!supportsDynamicImport || supportsImportMaps && !cssModulesEnabled && !jsonModulesEnabled)
+export let featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(() => {
+  if (!supportsDynamicImport)
     return;
 
   if (!hasDocument)
@@ -25,8 +25,10 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
       jsonModulesEnabled && dynamicImport(createBlob(jsonModulescheck.replace('x', createBlob('{}', 'text/json')))).then(() => supportsJsonAssertions = true, noop),
     ]);
 
+  if (supports || (!cssModulesEnabled && !jsonModulesEnabled))
+    return;
+
   return new Promise(resolve => {
-    if (self.ESMS_DEBUG) if (supportsImportMaps) console.info(`es-module-shims: import maps supported from HTMLScriptElement check`);
     if (self.ESMS_DEBUG) console.info(`es-module-shims: performing feature detections for ${`${supportsImportMaps ? '' : 'import maps, '}${cssModulesEnabled ? 'css modules, ' : ''}${jsonModulesEnabled ? 'json modules, ' : ''}`.slice(0, -2)}`);
     const iframe = document.createElement('iframe');
     iframe.style.display = 'none';
@@ -36,7 +38,6 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
       supportsImportMeta = b;
       supportsCssAssertions = c;
       supportsJsonAssertions = d;
-      if (self.ESMS_DEBUG) console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMaps ? '' : 'no '}import maps`);
       resolve();
       document.head.removeChild(iframe);
       window.removeEventListener('message', cb, false);
@@ -70,3 +71,8 @@ export const featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(
       iframe.contentDocument.write(importMapTest);
   });
 });
+
+if (self.ESMS_DEBUG)
+  featureDetectionPromise = featureDetectionPromise.then(() => {
+    console.info(`es-module-shims: detected native support - ${supportsDynamicImport ? '' : 'no '}dynamic import, ${supportsImportMeta ? '' : 'no '}import meta, ${supportsImportMaps ? '' : 'no '}import maps`);
+  });

From c079a07beb6b828ef58daabd0cd66832aa924bd1 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sat, 25 Mar 2023 11:12:14 -0700
Subject: [PATCH 7/9] check feature detection internals

---
 src/features.js | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/features.js b/src/features.js
index fe2349e7..a1ceae33 100644
--- a/src/features.js
+++ b/src/features.js
@@ -25,9 +25,6 @@ export let featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(()
       jsonModulesEnabled && dynamicImport(createBlob(jsonModulescheck.replace('x', createBlob('{}', 'text/json')))).then(() => supportsJsonAssertions = true, noop),
     ]);
 
-  if (supports || (!cssModulesEnabled && !jsonModulesEnabled))
-    return;
-
   return new Promise(resolve => {
     if (self.ESMS_DEBUG) console.info(`es-module-shims: performing feature detections for ${`${supportsImportMaps ? '' : 'import maps, '}${cssModulesEnabled ? 'css modules, ' : ''}${jsonModulesEnabled ? 'json modules, ' : ''}`.slice(0, -2)}`);
     const iframe = document.createElement('iframe');
@@ -47,6 +44,7 @@ export let featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(()
     const importMapTest = `<script nonce=${nonce || ''}>b=(s,type='text/javascript')=>URL.createObjectURL(new Blob([s],{type}));document.head.appendChild(Object.assign(document.createElement('script'),{type:'importmap',nonce:"${nonce}",innerText:\`{"imports":{"x":"\${b('')}"}}\`}));Promise.all([${
       supportsImportMaps ? 'true,true' : `'x',b('${importMetaCheck}')`}, ${cssModulesEnabled ? `b('${cssModulesCheck}'.replace('x',b('','text/css')))` : 'false'}, ${
       jsonModulesEnabled ? `b('${jsonModulesCheck}'.replace('x',b('{}','text/json')))` : 'false'}].map(x =>typeof x==='string'?import(x).then(x =>!!x,()=>false):x)).then(a=>parent.postMessage(a,'*'))<${''}/script>`;
+    if (self.ESMS_DEBUG) console.info(`RUNNING FEATURE DETECTION TEST SCRIPT:\n${importMapTest}\n\n`);
 
     iframe.onload = () => {
       // WeChat browser doesn't support setting srcdoc scripts

From c5a810faefb65e55c820f9d61a01b64a8f883435 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sun, 26 Mar 2023 15:07:30 -0700
Subject: [PATCH 8/9] fixup bad import maps detection

---
 src/features.js | 36 ++++++++++++++++++++++++++----------
 1 file changed, 26 insertions(+), 10 deletions(-)

diff --git a/src/features.js b/src/features.js
index a1ceae33..d7300cd0 100644
--- a/src/features.js
+++ b/src/features.js
@@ -8,7 +8,7 @@ export let supportsCssAssertions = false;
 const supports = hasDocument && HTMLScriptElement.supports;
 
 export let supportsImportMaps = supports && supports.name === 'supports' && supports('importmap');
-export let supportsImportMeta = !!supports;
+export let supportsImportMeta = supportsDynamicImport;
 
 const importMetaCheck = 'import.meta';
 const cssModulesCheck = `import"x"assert{type:"css"}`;
@@ -30,23 +30,33 @@ export let featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(()
     const iframe = document.createElement('iframe');
     iframe.style.display = 'none';
     iframe.setAttribute('nonce', nonce);
-    function cb ({ data: [a, b, c, d] }) {
-      supportsImportMaps = a;
-      supportsImportMeta = b;
-      supportsCssAssertions = c;
-      supportsJsonAssertions = d;
+    function cb ({ data }) {
+      // failed feature detection (security policy) -> revert to default assumptions
+      if (Array.isArray(data)) {
+        supportsImportMaps = data[0];
+        supportsImportMeta = data[1];
+        supportsCssAssertions = data[2];
+        supportsJsonAssertions = data[3];
+      }
+      else if (self.ESMS_DEBUG) console.info(`es-module-shims: feature detection failure, using defaults`);
       resolve();
       document.head.removeChild(iframe);
       window.removeEventListener('message', cb, false);
     }
     window.addEventListener('message', cb, false);
 
-    const importMapTest = `<script nonce=${nonce || ''}>b=(s,type='text/javascript')=>URL.createObjectURL(new Blob([s],{type}));document.head.appendChild(Object.assign(document.createElement('script'),{type:'importmap',nonce:"${nonce}",innerText:\`{"imports":{"x":"\${b('')}"}}\`}));Promise.all([${
+    const importMapTest = `<script nonce=${nonce || ''}>self.b=(s,type='text/javascript')=>URL.createObjectURL(new Blob([s],{type}));document.head.appendChild(Object.assign(document.createElement('script'),{type:'importmap',nonce:"${nonce}",innerText:\`{"imports":{"x":"\${b('')}"}}\`}));Promise.all([${
       supportsImportMaps ? 'true,true' : `'x',b('${importMetaCheck}')`}, ${cssModulesEnabled ? `b('${cssModulesCheck}'.replace('x',b('','text/css')))` : 'false'}, ${
       jsonModulesEnabled ? `b('${jsonModulesCheck}'.replace('x',b('{}','text/json')))` : 'false'}].map(x =>typeof x==='string'?import(x).then(x =>!!x,()=>false):x)).then(a=>parent.postMessage(a,'*'))<${''}/script>`;
-    if (self.ESMS_DEBUG) console.info(`RUNNING FEATURE DETECTION TEST SCRIPT:\n${importMapTest}\n\n`);
 
-    iframe.onload = () => {
+    // Safari will call onload eagerly on head injection, but we don't want the Wechat
+    // path to trigger before setting srcdoc, therefore we track the timing
+    let readyForOnload = false, onloadCalledWhileNotReady = false;
+    function doOnload () {
+      if (!readyForOnload) {
+        onloadCalledWhileNotReady = true;
+        return;
+      }
       // WeChat browser doesn't support setting srcdoc scripts
       // But iframe sandboxes don't support contentDocument so we do this as a fallback
       const doc = iframe.contentDocument;
@@ -57,16 +67,22 @@ export let featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(()
         s.innerHTML = importMapTest.slice(15 + (nonce ? nonce.length : 0), -9);
         doc.head.appendChild(s);
       }
-    };
+    }
+
+    iframe.onload = doOnload;
     // WeChat browser requires append before setting srcdoc
     document.head.appendChild(iframe);
+
     // setting srcdoc is not supported in React native webviews on iOS
     // setting src to a blob URL results in a navigation event in webviews
     // document.write gives usability warnings
+    readyForOnload = true;
     if ('srcdoc' in iframe)
       iframe.srcdoc = importMapTest;
     else
       iframe.contentDocument.write(importMapTest);
+    // retrigger onload for Safari only if necessary
+    if (onloadCalledWhileNotReady) doOnload();
   });
 });
 

From 2a7421958d9bb4e2eca84958fcabe3feb2af605e Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Sun, 26 Mar 2023 15:42:28 -0700
Subject: [PATCH 9/9] remove self usage

---
 src/features.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/features.js b/src/features.js
index d7300cd0..ebf726af 100644
--- a/src/features.js
+++ b/src/features.js
@@ -45,7 +45,7 @@ export let featureDetectionPromise = Promise.resolve(dynamicImportCheck).then(()
     }
     window.addEventListener('message', cb, false);
 
-    const importMapTest = `<script nonce=${nonce || ''}>self.b=(s,type='text/javascript')=>URL.createObjectURL(new Blob([s],{type}));document.head.appendChild(Object.assign(document.createElement('script'),{type:'importmap',nonce:"${nonce}",innerText:\`{"imports":{"x":"\${b('')}"}}\`}));Promise.all([${
+    const importMapTest = `<script nonce=${nonce || ''}>b=(s,type='text/javascript')=>URL.createObjectURL(new Blob([s],{type}));document.head.appendChild(Object.assign(document.createElement('script'),{type:'importmap',nonce:"${nonce}",innerText:\`{"imports":{"x":"\${b('')}"}}\`}));Promise.all([${
       supportsImportMaps ? 'true,true' : `'x',b('${importMetaCheck}')`}, ${cssModulesEnabled ? `b('${cssModulesCheck}'.replace('x',b('','text/css')))` : 'false'}, ${
       jsonModulesEnabled ? `b('${jsonModulesCheck}'.replace('x',b('{}','text/json')))` : 'false'}].map(x =>typeof x==='string'?import(x).then(x =>!!x,()=>false):x)).then(a=>parent.postMessage(a,'*'))<${''}/script>`;