diff --git a/Makefile b/Makefile
index f96edcca..ba101d56 100644
--- a/Makefile
+++ b/Makefile
@@ -8,6 +8,7 @@ test: ## Run the pure-PHP test suites
 	php tests/QuotesTest.php
 	php tests/RevisionOverridesTest.php
 	php tests/IgnoreHtmlClassTest.php
+	php tests/JsDefaultsTest.php
 
 phpcs: vendor/autoload.php ## parallel-lint + minus-x + phpcs against the MediaWiki coding standard
 	composer test
diff --git a/resources/ext.SimpleMathJax.js b/resources/ext.SimpleMathJax.js
index d772ce13..c949ff5f 100644
--- a/resources/ext.SimpleMathJax.js
+++ b/resources/ext.SimpleMathJax.js
@@ -3,6 +3,39 @@ mw.libs.smj = mw.libs.smj || {};
 var loaded = false;
 var scriptLoadPromise = null;
 
+// Defaults match extension.json. The page HTML that carries these settings
+// can be cached (CDN, reverse proxy) separately from this module, so after an
+// upgrade the two may come from different versions; fall back instead of
+// failing to render any formula.
+var defaults = {
+  wgSmjCdnEnabled: true,
+  wgSmjCdnVersion: '4',
+  wgSmjDelimitersEnabled: false,
+  wgSmjDelimitersInlineMath: [],
+  wgSmjDelimitersDisplayMath: [],
+  wgSmjIgnoreHtmlClass: 'mathjax_ignore|comment|diff-(context|addedline|deletedline)',
+  wgSmjScale: 1
+};
+
+// Pre-1.0 names of settings that were only renamed (see docs/mig-1.0.md),
+// consulted for pages cached before the upgrade. Settings whose meaning or
+// default changed in 1.0 (e.g. delimiters) are not mapped.
+var legacyNames = {
+  wgSmjCdnEnabled: 'wgSmjUseCdn'
+};
+
+function isSet(value) {
+  return value !== null && value !== undefined;
+}
+
+function config(name) {
+  var value = mw.config.get(name);
+  if (!isSet(value) && legacyNames[name]) {
+    value = mw.config.get(legacyNames[name]);
+  }
+  return isSet(value) ? value : defaults[name];
+}
+
 function ensureLoaded() {
   if (loaded) {
     return;
@@ -10,11 +43,11 @@ function ensureLoaded() {
   loaded = true;
   window.MathJax = {
     tex: {
-      inlineMath: mw.config.get('wgSmjDelimitersInlineMath').concat([['[math]','[/math]']]),
-      displayMath: mw.config.get('wgSmjDelimitersDisplayMath'),
+      inlineMath: config('wgSmjDelimitersInlineMath').concat([['[math]','[/math]']]),
+      displayMath: config('wgSmjDelimitersDisplayMath'),
       processEnvironments: true,
-      processRefs: mw.config.get('wgSmjDelimitersEnabled'),
-      processEscapes: mw.config.get('wgSmjDelimitersEnabled'),
+      processRefs: config('wgSmjDelimitersEnabled'),
+      processEscapes: config('wgSmjDelimitersEnabled'),
       packages: mw.config.exists('smjPreloadChem') ? {'[+]': ['autoload','mhchem']} : {'[+]': ['autoload']},
       macros: {
         AA: "{\u00c5}",
@@ -124,17 +157,17 @@ function ensureLoaded() {
       }
     },
     options: {
-      ignoreHtmlClass: mw.config.get('wgSmjIgnoreHtmlClass'),
+      ignoreHtmlClass: config('wgSmjIgnoreHtmlClass'),
       processHtmlClass: "mathjax_process|smj-container"
     },
     chtml: {
-      scale: mw.config.get('wgSmjScale'),
+      scale: config('wgSmjScale'),
     },
     loader: {
       load: ['ui/safe','[tex]/autoload'].concat(mw.config.exists('smjPreloadChem') ? ['[tex]/mhchem'] : [])
     },
     startup: {
-      elements: mw.config.get('wgSmjDelimitersEnabled') ? null : ["span.smj-container"],
+      elements: config('wgSmjDelimitersEnabled') ? null : ["span.smj-container"],
       pageReady: () => {
         return MathJax.startup.defaultPageReady().then(() => {
           document.querySelectorAll("span.smj-container > .MathJax").forEach((mjx) => {
@@ -146,8 +179,8 @@ function ensureLoaded() {
   };
 
   var script = document.createElement('script');
-  script.src = mw.config.get('wgSmjCdnEnabled')
-    ? 'https://cdn.jsdelivr.net/npm/mathjax@' + mw.config.get('wgSmjCdnVersion') + '/tex-chtml.js'
+  script.src = config('wgSmjCdnEnabled')
+    ? 'https://cdn.jsdelivr.net/npm/mathjax@' + config('wgSmjCdnVersion') + '/tex-chtml.js'
     : mw.config.get('wgExtensionAssetsPath') + '/SimpleMathJax/resources/MathJax/tex-chtml.js';
   script.async = true;
   scriptLoadPromise = new Promise((resolve, reject) => {
@@ -183,7 +216,7 @@ mw.hook('wikipage.content').add(function ($content) {
     return;
   }
   var $containers = $content.filter('.smj-container').add($content.find('.smj-container'));
-  var delimitersEnabled = mw.config.get('wgSmjDelimitersEnabled');
+  var delimitersEnabled = config('wgSmjDelimitersEnabled');
   if (!delimitersEnabled && !$containers.length) {
     return;
   }
diff --git a/tests/JsDefaultsTest.php b/tests/JsDefaultsTest.php
new file mode 100644
index 00000000..0d61e515
--- /dev/null
+++ b/tests/JsDefaultsTest.php
@@ -0,0 +1,62 @@
+<?php
+// The client module falls back to its own defaults when the page HTML does not
+// carry a setting (e.g. HTML cached before an upgrade). Keep those defaults in
+// sync with extension.json.
+
+$failures = 0;
+
+function assert_same( $name, $expected, $actual ) {
+	global $failures;
+	if ( $expected === $actual ) {
+		echo "ok   $name\n";
+		return;
+	}
+	$failures++;
+	echo "FAIL $name\n";
+	echo '  expected: ' . var_export( $expected, true ) . "\n";
+	echo '  actual:   ' . var_export( $actual, true ) . "\n";
+}
+
+$extension = json_decode( file_get_contents( __DIR__ . '/../extension.json' ), true );
+$js = file_get_contents( __DIR__ . '/../resources/ext.SimpleMathJax.js' );
+
+if ( !preg_match( '/var defaults = (\{.*?\});/s', $js, $m ) ) {
+	echo "FAIL defaults object not found in ext.SimpleMathJax.js\n";
+	exit( 1 );
+}
+// The object literal uses unquoted keys and single-quoted strings.
+$json = preg_replace( '/^(\s*)(\w+):/m', '$1"$2":', $m[1] );
+$json = preg_replace_callback( "/'((?:[^'\\\\]|\\\\.)*)'/", static function ( $s ) {
+	return json_encode( stripcslashes( $s[1] ) );
+}, $json );
+$defaults = json_decode( $json, true );
+assert_same( 'defaults object parses', true, is_array( $defaults ) );
+
+// Every setting the module reads must have a default.
+preg_match_all( "/config\\('(wgSmj\\w+)'\\)/", $js, $reads );
+foreach ( array_unique( $reads[1] ) as $name ) {
+	assert_same( "$name has a client default", true, array_key_exists( $name, $defaults ?? [] ) );
+}
+
+foreach ( $defaults ?? [] as $name => $value ) {
+	$key = substr( $name, 2 );
+	$expected = $extension['config'][$key]['value'] ?? null;
+	assert_same( "$name default matches extension.json", $expected, $value );
+}
+
+// Legacy names are only for settings the migration guide lists as renamed.
+if ( preg_match( '/var legacyNames = (\{.*?\});/s', $js, $m ) ) {
+	preg_match_all( "/(\w+): '(\w+)'/", $m[1], $pairs, PREG_SET_ORDER );
+	$guide = file_get_contents( __DIR__ . '/../docs/mig-1.0.md' );
+	foreach ( $pairs as [ , $name, $legacy ] ) {
+		assert_same( "$name has a client default", true, array_key_exists( $name, $defaults ?? [] ) );
+		assert_same(
+			"$legacy -> $name is a rename in docs/mig-1.0.md",
+			1,
+			preg_match( '/\| `\$' . $legacy . '` \| `\$' . $name . '` \|/', $guide )
+		);
+	}
+}
+
+echo $failures === 0 ? "\nAll tests passed.\n" : "\n$failures test(s) FAILED.\n";
+exit( $failures === 0 ? 0 : 1 );