From 17904ecede8fa709c75461bcec35812702774ae3 Mon Sep 17 00:00:00 2001
From: Avi Vahl <avi@dazl.dev>
Date: Thu, 21 Aug 2025 12:11:30 +0300
Subject: [PATCH] feat: support format=esm in esbuild config will use
 type="module" for tests.js when it is specified, allowing esm-specific syntax

---
 src/run-tests.ts                                      | 11 +++++++----
 .../with-esm-esbuild-config/esbuild.config.js         |  4 ++++
 .../with-esm-esbuild-config/with-import-meta.ts       |  7 +++++++
 test/mocha-web.spec.ts                                | 11 +++++++++++
 4 files changed, 29 insertions(+), 4 deletions(-)
 create mode 100644 test/fixtures/with-esm-esbuild-config/esbuild.config.js
 create mode 100644 test/fixtures/with-esm-esbuild-config/with-import-meta.ts

diff --git a/src/run-tests.ts b/src/run-tests.ts
index 3d1ad20..cad3608 100644
--- a/src/run-tests.ts
+++ b/src/run-tests.ts
@@ -38,7 +38,7 @@ export async function runTests(testFiles: string[], options: IRunTestsOptions =
     const indexContents = testFiles.map((f) => `require(${JSON.stringify(f)});`).join("\n");
 
     const { onBuildEndPlugin, onBuildEnd } = captureBuildEnd();
-
+    const format = esbuildConfig?.format ?? "iife";
     const combinedConfig = {
       ...esbuildConfig,
       stdin: {
@@ -51,11 +51,11 @@ export async function runTests(testFiles: string[], options: IRunTestsOptions =
       write: false,
       sourcemap: true,
       bundle: true,
-      format: "iife",
+      format,
       logLevel: "info",
       plugins: [
         ...(esbuildConfig?.plugins ?? []),
-        createOutputCapturePlugin(workingDir, buildFilesMap, options),
+        createOutputCapturePlugin(workingDir, buildFilesMap, options, format),
         onBuildEndPlugin,
       ],
     } satisfies esbuild.BuildOptions;
@@ -121,6 +121,7 @@ function createOutputCapturePlugin(
   workingDir: string,
   buildFilesMap: Map<string, esbuild.OutputFile>,
   options: IRunTestsOptions,
+  format: esbuild.Format,
 ): esbuild.Plugin {
   return {
     name: "capture-output",
@@ -132,6 +133,7 @@ function createOutputCapturePlugin(
             buildFilesMap.set("/" + path.relative(workingDir, outFile.path).replace("/\\/g", "/"), outFile);
           }
           const testsHTML = createTestsHTML(
+            format,
             "mocha tests",
             options.ui ?? "bdd",
             options.colors ?? true,
@@ -195,6 +197,7 @@ async function waitForTestResults(page: playwright.Page): Promise<number> {
 }
 
 function createTestsHTML(
+  format: esbuild.Format,
   title: string,
   ui: string,
   color: boolean,
@@ -253,7 +256,7 @@ function createTestsHTML(
             .on('end', () => {(mochaStatus.finished = true)});
       });
     </script>
-    <script src="tests.js"></script>
+    ${format === "esm" ? `<script type="module" src="tests.js"></script>` : `<script src="tests.js"></script>`}
   </body>
 </html>
 `;
diff --git a/test/fixtures/with-esm-esbuild-config/esbuild.config.js b/test/fixtures/with-esm-esbuild-config/esbuild.config.js
new file mode 100644
index 0000000..d509fd6
--- /dev/null
+++ b/test/fixtures/with-esm-esbuild-config/esbuild.config.js
@@ -0,0 +1,4 @@
+/** @type {import("esbuild").BuildOptions} */
+export default {
+  format: "esm",
+};
diff --git a/test/fixtures/with-esm-esbuild-config/with-import-meta.ts b/test/fixtures/with-esm-esbuild-config/with-import-meta.ts
new file mode 100644
index 0000000..b0a7e2e
--- /dev/null
+++ b/test/fixtures/with-esm-esbuild-config/with-import-meta.ts
@@ -0,0 +1,7 @@
+describe("suite", () => {
+  it("passes when esm is supported", () => {
+    if (typeof import.meta.url !== "string") {
+      throw new Error("import.meta.url is not a string");
+    }
+  });
+});
diff --git a/test/mocha-web.spec.ts b/test/mocha-web.spec.ts
index 88d1b15..139cc52 100644
--- a/test/mocha-web.spec.ts
+++ b/test/mocha-web.spec.ts
@@ -153,4 +153,15 @@ describe("mocha-web", function () {
     expect(output, output).to.include("body background-color: rgb(0, 255, 0)");
     expect(status).to.equal(0);
   });
+
+  it("supports esm syntax when esbuild config specifies format=esm", () => {
+    const { output, status } = runMochaPlay({
+      args: ["./with-import-meta.ts"],
+      fixture: "with-esm-esbuild-config",
+    });
+
+    expect(output).to.include("Found 1 test files");
+    expect(output).to.include("1 passing");
+    expect(status).to.equal(0);
+  });
 });