mjancarik · mjancarik · Oct 27, 2025 · Oct 27, 2025
diff --git a/docs/docs/merkur-config.md b/docs/docs/merkur-config.md
@@ -16,14 +16,14 @@ The full merkur config can be look like:
 export default function ({ cliConfig, emitter,  }) {
   return {
     extends: [ '@merkur/preact/cli' ], // Merkur predefined extender
-    task: { // defined tasks to building your widget, default are node, es13 and es9
+    task: { // defined tasks to building your widget, default are node, es15 and es9
       node: {
         name: 'node',
         build: ESBuildConfiguration, // https://esbuild.github.io/api/#build
       }
-      es13: {
-        name: 'es13',
-        folder: 'es13',
+      es15: {
+        name: 'es15',
+        folder: 'es15',
         build: ESBuildConfiguration, // https://esbuild.github.io/api/#build
       }
       es9: {
@@ -132,13 +132,26 @@ export default function () {
     onMerkurConfig({ cliConfig, merkurConfig }) {
       const { projectFolder, isProduction, staticFolder } = cliConfig;
 
+      merkurConfig.task.es15Asset = {
+        name: 'es15Asset',
+        folder: 'es15',
+        build: {
+          entryPoints: [`${projectFolder}/src/customAsset.js`],
+          entryNames: !isProduction ? 'customAsset' : 'customAsset.[hash]',
+          platform: 'browser',
+          outdir: `${staticFolder}/es15`,
+          plugins: merkurConfig.task['es15'].build.plugins,
+        },
+      };
+
       merkurConfig.task.es13Asset = {
         name: 'es13Asset',
         folder: 'es13',
         build: {
           entryPoints: [`${projectFolder}/src/customAsset.js`],
           entryNames: !isProduction ? 'customAsset' : 'customAsset.[hash]',
           platform: 'browser',
+          target: 'es2022',
           outdir: `${staticFolder}/es13`,
           plugins: merkurConfig.task['es13'].build.plugins,
         },
@@ -162,7 +175,7 @@ export default function () {
     onCliConfig({ cliConfig }) {
       // add es13Asset task to be run for `merkur dev`
       if (cliConfig.command === 'dev') {
-        cliConfig.runTasks.push('es13Asset');
+        cliConfig.runTasks.push('es15Asset');
       }
     },
   };

diff --git a/packages/cli/bin/merkur.mjs b/packages/cli/bin/merkur.mjs
@@ -58,7 +58,7 @@ program.command(COMMAND_NAME.DEV)
     ...{
       writeToDisk: false,
       watch: true,
-      runTasks: ['node', 'es13'],
+      runTasks: ['node', 'es15'],
       hasRunDevServer: true,
       hasRunWidgetServer: true,
     },

diff --git a/packages/cli/src/merkurConfig.mjs b/packages/cli/src/merkurConfig.mjs
@@ -104,13 +104,22 @@ emitter.on(
           metafile: false,
         },
       },
+      es15: {
+        name: 'es15',
+        folder: 'es15',
+        build: {
+          platform: 'browser',
+          outdir: path.resolve(`${staticFolder}/es15`),
+          plugins: [devPlugin],
+        },
+      },
       es13: {
         name: 'es13',
         folder: 'es13',
         build: {
           platform: 'browser',
           outdir: path.resolve(`${staticFolder}/es13`),
-          plugins: [devPlugin],
+          target: 'es2022',
         },
       },
       es9: {

diff --git a/packages/cli/src/runBuild.mjs b/packages/cli/src/runBuild.mjs
@@ -7,7 +7,7 @@ export async function runBuild({ cliConfig, build, config }) {
   const logger = createLogger(undefined, cliConfig);
   const { watch } = cliConfig;
 
-  //es6 === es2015, es9 === es2018, es11 === es2020 es13 ===es2022
+  //es6 === es2015, es9 === es2018, es11 === es2020, es13 ===es2022, es15 === es2024
   try {
     const result = await (watch
       ? esbuild.context(build)

diff --git a/packages/cli/src/templates/head.ejs b/packages/cli/src/templates/head.ejs
@@ -20,7 +20,7 @@
       <script src='<%= asset.source %>' <%- asset.module ? 'type="module"' : 'defer="true"' %> data-merkur-asset-name="<%= asset.name %>"></script>
     <% } %>
     <%if (typeof asset.source==='object' ) { %>
-      <script src='<%= asset.source.es13 %>' <%- asset.module ? 'type="module"' : 'defer="true"' %> data-merkur-asset-name="<%= asset.name %>"></script>
+      <script src='<%= asset.source.es15 %>' <%- asset.module ? 'type="module"' : 'defer="true"' %> data-merkur-asset-name="<%= asset.name %>"></script>
     <% } %>
   <% } %>
 

diff --git a/packages/cli/types.d.ts b/packages/cli/types.d.ts
@@ -30,8 +30,8 @@ export type Extend =
   | undefined;
 
 export interface Task {
-  name: 'es13' | 'es9' | 'node' | string;
-  folder: 'es13' | 'es9' | string;
+  name: 'es15' | 'es13' | 'es9' | 'node' | string;
+  folder: 'es15' | 'es13' | 'es9' | string;
   build: BuildOptions;
   [key: string]: any;
 }

diff --git a/packages/create-widget/views/preact/template.json b/packages/create-widget/views/preact/template.json
@@ -1,6 +1,6 @@
 {
   "dependencies": {
-    "@merkur/preact": "0.38.0"
+    "@merkur/preact": "0.39.0"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.5.0",

diff --git a/packages/create-widget/views/svelte/template.json b/packages/create-widget/views/svelte/template.json
@@ -1,6 +1,6 @@
 {
   "dependencies": {
-    "@merkur/svelte": "0.38.0"
+    "@merkur/svelte": "0.39.0"
   },
   "devDependencies": {}
 }
diff --git a/packages/create-widget/views/uhtml/template.json b/packages/create-widget/views/uhtml/template.json
@@ -1,6 +1,6 @@
 {
   "dependencies": {
-    "@merkur/uhtml": "0.38.0"
+    "@merkur/uhtml": "0.39.0"
   },
   "devDependencies": {}
 }
diff --git a/packages/integration-custom-element/cli/index.mjs b/packages/integration-custom-element/cli/index.mjs
@@ -109,7 +109,7 @@ export default function ({ emitter, EMITTER_EVENTS }) {
 
   emitter.on(EMITTER_EVENTS.CLI_CONFIG, function removeNodeTask({ cliConfig }) {
     if (cliConfig.runTasks.length === 0) {
-      cliConfig.runTasks = ['es13', 'es9'];
+      cliConfig.runTasks = ['es15', 'es9'];
     }
 
     cliConfig.runTasks = cliConfig.runTasks.filter((task) => task !== 'node');

diff --git a/packages/integration/src/__tests__/testScriptSpec.js b/packages/integration/src/__tests__/testScriptSpec.js
@@ -73,6 +73,30 @@ describe('Merkur component', () => {
     });
   });
 
+  describe('isES15Supported() function', () => {
+    beforeEach(() => {
+      jest.spyOn(testScript, 'test');
+    });
+
+    afterEach(() => {
+      jest.restoreAllMocks();
+    });
+
+    it('should return true', () => {
+      const isES15Supported = testScript.isES15Supported();
+
+      expect(isES15Supported).toEqual(true);
+      expect(testScript.test).toHaveBeenCalledTimes(2);
+    });
+
+    it('should return true but not test again', () => {
+      const isES15Supported = testScript.isES15Supported();
+
+      expect(isES15Supported).toEqual(true);
+      expect(testScript.test).toHaveBeenCalledTimes(0);
+    });
+  });
+
   describe('test() function', () => {
     it('should return true', () => {
       expect(testScript.test('return typeof window !== "undefined"')).toEqual(

diff --git a/packages/integration/src/testScript.js b/packages/integration/src/testScript.js
@@ -1,6 +1,7 @@
 let _isES9Supported;
 let _isES11Supported;
 let _isES13Supported;
+let _isES15Supported;
 
 function isES9Supported() {
   if (_isES9Supported === undefined) {
@@ -37,6 +38,16 @@ function isES13Supported() {
   return _isES13Supported;
 }
 
+function isES15Supported() {
+  if (_isES15Supported === undefined) {
+    _isES15Supported =
+      exported.test('return typeof Promise.withResolvers === "function"') &&
+      exported.test('return typeof Object.groupBy === "function"');
+  }
+
+  return _isES15Supported;
+}
+
 function test(snippet) {
   try {
     const fn = new Function(snippet);
@@ -52,6 +63,7 @@ const exported = {
   isES9Supported,
   isES11Supported,
   isES13Supported,
+  isES15Supported,
   test,
 };
 

diff --git a/website/docs/merkur-config.md b/website/docs/merkur-config.md
@@ -17,21 +17,26 @@ The full merkur config can be look like:
 export default function ({ cliConfig, emitter,  }) {
   return {
     extends: [ '@merkur/preact/cli' ], // Merkur predefined extender
-    task: { // defined tasks to building your widget, default are node, es13 and es9
+    task: { // defined tasks to building your widget, default are node, es15 and es9
       node: {
         name: 'node',
         build: ESBuildConfiguration, // https://esbuild.github.io/api/#build
-      }
+      },
+      es15: {
+        name: 'es15',
+        folder: 'es15',
+        build: ESBuildConfiguration, // https://esbuild.github.io/api/#build
+      },
       es13: {
         name: 'es13',
         folder: 'es13',
         build: ESBuildConfiguration, // https://esbuild.github.io/api/#build
-      }
+      },
       es9: {
         name: 'es9',
         folder: 'es9',
         build: ESBuildConfiguration, // https://esbuild.github.io/api/#build
-      }
+      },
     },
     devServer: { // configuration for Merkur dev server
       protocol: 'http:',
@@ -133,15 +138,15 @@ export default function () {
     onMerkurConfig({ cliConfig, merkurConfig }) {
       const { projectFolder, isProduction, staticFolder } = cliConfig;
 
-      merkurConfig.task.es13Asset = {
-        name: 'es13Asset',
-        folder: 'es13',
+      merkurConfig.task.es15Asset = {
+        name: 'es15Asset',
+        folder: 'es15',
         build: {
           entryPoints: [`${projectFolder}/src/customAsset.js`],
           entryNames: !isProduction ? 'customAsset' : 'customAsset.[hash]',
           platform: 'browser',
-          outdir: `${staticFolder}/es13`,
-          plugins: merkurConfig.task['es13'].build.plugins,
+          outdir: `${staticFolder}/es15`,
+          plugins: merkurConfig.task['es15'].build.plugins,
         },
       };
 
@@ -161,9 +166,9 @@ export default function () {
       return merkurConfig;
     },
     onCliConfig({ cliConfig }) {
-      // add es13Asset task to be run for `merkur dev`
+      // add es15Asset task to be run for `merkur dev`
       if (cliConfig.command === 'dev') {
-        cliConfig.runTasks.push('es13Asset');
+        cliConfig.runTasks.push('es15Asset');
       }
     },
   };