diff --git a/playground/eslint.config.mjs b/playground/eslint.config.mjs
index d0cf5cde0..b6bd81180 100644
--- a/playground/eslint.config.mjs
+++ b/playground/eslint.config.mjs
@@ -6,7 +6,7 @@ import mocha from 'eslint-plugin-mocha';
 
 export default ts.config(
     eslint.configs.recommended,
-    ...ts.configs.recommendedTypeChecked,
+    ...ts.configs.strictTypeChecked,
     {
         files: ['*.ts'],
         languageOptions: {
@@ -21,10 +21,11 @@ export default ts.config(
         files: ['*.ts', '*.mjs'],
         rules: {
             eqeqeq: ['error', 'always'],
-            'no-constant-condition': ['error', { checkLoops: false }],
+            '@typescript-eslint/no-unnecessary-condition': ['error', { allowConstantLoopConditions: true }],
             '@typescript-eslint/no-unsafe-member-access': 'off',
             '@typescript-eslint/no-unsafe-argument': 'off',
             '@typescript-eslint/no-unsafe-assignment': 'off',
+            '@typescript-eslint/restrict-template-expressions': 'off',
         },
     },
     mocha.configs.flat.recommended,
diff --git a/playground/index.ts b/playground/index.ts
index 1c49e6b75..647d5e700 100644
--- a/playground/index.ts
+++ b/playground/index.ts
@@ -145,6 +145,7 @@ jobs:
             successMessage.style.display = 'none';
             invalidInputMessage.style.display = 'none';
             editor.clearGutter('error-marker');
+            // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
             window.runActionlint!(editor.getValue());
         }
 
@@ -153,7 +154,9 @@ jobs:
             return;
         }
 
-        debounceId = window.setTimeout(() => startActionlint(), debounceInterval);
+        debounceId = window.setTimeout(() => {
+            startActionlint();
+        }, debounceInterval);
     });
 
     function getSource(): string {
@@ -192,7 +195,7 @@ jobs:
         let rest = text;
         while (true) {
             const m = rest.match(reUrl);
-            if (m === null || m.index === undefined || m[0] === undefined) {
+            if (m === null || m.index === undefined) {
                 if (rest.length > 0) {
                     ret.push(span(rest));
                 }
@@ -212,7 +215,9 @@ jobs:
             a.rel = 'noopener';
             a.textContent = url;
             a.className = 'has-text-link-my-light is-underlined';
-            a.addEventListener('click', e => e.stopPropagation());
+            a.addEventListener('click', e => {
+                e.stopPropagation();
+            });
             ret.push(a);
 
             rest = rest.slice(idx + url.length);
@@ -269,12 +274,11 @@ jobs:
     window.addEventListener('beforeunload', e => {
         if (contentChanged) {
             e.preventDefault();
-            e.returnValue = '';
         }
     });
 
     checkUrlInput.addEventListener('keyup', e => {
-        if (e.key === 'Enter' || e.keyCode === 13) {
+        if (e.key === 'Enter') {
             e.preventDefault();
             checkUrlButton.click();
         }
@@ -310,7 +314,7 @@ jobs:
         window.location.hash = b64;
     });
 
-    preferDark.addListener(event => {
+    preferDark.addEventListener('change', event => {
         editor.setOption('theme', colorTheme(event.matches));
     });
 
@@ -327,7 +331,8 @@ jobs:
     }
 
     await go.run(result.instance);
-})().catch(err => {
+})().catch((err: unknown) => {
     console.error('ERROR!:', err);
-    alert(`${err.name}: ${err.message}\n\n${err.stack}`);
+    const msg = err instanceof Error ? `${err.name}: ${err.message}\n\n${err.stack}` : `Error: ${err}`;
+    alert(msg);
 });
diff --git a/playground/test.ts b/playground/test.ts
index 89097ebc7..adb869940 100644
--- a/playground/test.ts
+++ b/playground/test.ts
@@ -76,6 +76,7 @@ jobs:
         const json = JSON.stringify(errors);
         assert.equal(errors.length, 1, json);
 
+        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
         const err = errors[0]!;
         assert.equal(err.message, '"runs-on" section is missing in job "test"', `message is unexpected: ${json}`);
         assert.equal(err.line, 5, `line is unexpected: ${json}`);
@@ -101,6 +102,7 @@ jobs:
         const json = JSON.stringify(errors);
         assert.equal(errors.length, 1, json);
 
+        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
         const err = errors[0]!;
         assert.ok(err.message.includes('unknown Webhook event "foo"'), `message is unexpected: ${json}`);
         assert.equal(err.line, 2, `line is unexpected: ${json}`);