feat: ESM configuration file

docs-next/src/content/docs/running/configuring.mdx

@@ -9,7 +9,8 @@ title: Configuring Mocha (Node.js)
 Mocha supports configuration files, typical of modern command-line tools, in several formats:
 
 - **JavaScript**: Create a `.mocharc.js` (or `.mocharc.cjs` when using [`"type"="module"`](/explainers/nodejs-native-esm-support) in your `package.json`)
-  in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.
+  in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.  For native ESM and using `type="module"`
+  or using `.mjs`, use a default export (`default export  {/* ... */}`).
 - **YAML**: Create a `.mocharc.yaml` (or `.mocharc.yml`) in your project's root directory.
 - **JSON**: Create a `.mocharc.json` (or `.mocharc.jsonc`) in your project's root directory.
   Comments--while not valid JSON--are allowed in this file, and will be ignored by Mocha.

docs/index.md

@@ -2257,7 +2257,9 @@ The HTML reporter is the default reporter when running Mocha in the browser. It
 
 Mocha supports configuration files, typical of modern command-line tools, in several formats:
 
-- **JavaScript**: Create a `.mocharc.js` (or `.mocharc.cjs` when using [`"type"="module"`](#nodejs-native-esm-support) in your `package.json`)
+- **JavaScript**: Create a `.mocharc.js` (or `.mocharc.cjs` when using [`"type"="module"`](/explainers/nodejs-native-esm-support) in your `package.json`)
+  in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.  For native ESM and using `type="module"`
+  or using `.mjs`, use a default export (`default export  {/* ... */}`).
   in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.
 - **YAML**: Create a `.mocharc.yaml` (or `.mocharc.yml`) in your project's root directory.
 - **JSON**: Create a `.mocharc.json` (or `.mocharc.jsonc`) in your project's root directory. Comments — while not valid JSON — are allowed in this file, and will be ignored by Mocha.

lib/cli/config.js

@@ -24,6 +24,7 @@ const utils = require("../utils");
 exports.CONFIG_FILES = [
   ".mocharc.cjs",
   ".mocharc.js",
+  ".mocharc.mjs",
   ".mocharc.yaml",
   ".mocharc.yml",
   ".mocharc.jsonc",
@@ -72,8 +73,9 @@ exports.loadConfig = (filepath) => {
   try {
     if (ext === ".yml" || ext === ".yaml") {
       config = parsers.yaml(filepath);
-    } else if (ext === ".js" || ext === ".cjs") {
-      config = parsers.js(filepath);
+    } else if (ext === ".js" || ext === ".cjs" || ext === ".mjs") {
+      const parsedConfig = parsers.js(filepath);
+      config = parsedConfig.default ?? parsedConfig;
     } else {
       config = parsers.json(filepath);
     }

test/integration/config.spec.js

@@ -10,13 +10,15 @@ var loadConfig = require("../../lib/cli/config").loadConfig;
 describe("config", function () {
   it("should return the same values for all supported config types", function () {
     var configDir = path.join(__dirname, "fixtures", "config");
-    var js = loadConfig(path.join(configDir, "mocharc.js"));
+    var js = loadConfig(path.join(configDir, "mocharc.js")); // canonical form
     var cjs = loadConfig(path.join(configDir, "mocharc.cjs"));
     var json = loadConfig(path.join(configDir, "mocharc.json"));
+    var mjs = loadConfig(path.join(configDir, "mocharc.mjs"));
     var yaml = loadConfig(path.join(configDir, "mocharc.yaml"));
-    expect(js, "to equal", json);
-    expect(js, "to equal", cjs);
-    expect(json, "to equal", yaml);
+    expect(cjs, "to equal", js);
+    expect(json, "to equal", js);
+    expect(mjs, "to equal", js);
+    expect(yaml, "to equal", js);
   });
 
   describe('when configuring Mocha via a ".js" file', function () {

test/integration/fixtures/config/mocharc.mjs

@@ -0,0 +1,7 @@
+// a comment
+export default {
+  require: ['foo', 'bar'],
+  bail: true,
+  reporter: 'dot',
+  slow: 60
+};

test/node-unit/cli/config.spec.js

@@ -30,23 +30,12 @@ describe("cli/config", function () {
         sinon.stub(parsers, "js").returns(phonyConfigObject);
       });
 
-      describe('when supplied a filepath with ".yaml" extension', function () {
-        const filepath = "foo.yaml";
-
-        it("should use the YAML parser", function () {
-          loadConfig(filepath);
-          expect(parsers.yaml, "to have calls satisfying", [
-            { args: [filepath], returned: phonyConfigObject },
-          ]).and("was called once");
-        });
-      });
-
-      describe('when supplied a filepath with ".yml" extension', function () {
-        const filepath = "foo.yml";
+      describe('when supplied a filepath with ".cjs" extension', function () {
+        const filepath = "foo.cjs";
 
-        it("should use the YAML parser", function () {
+        it("should use the JS parser", function () {
           loadConfig(filepath);
-          expect(parsers.yaml, "to have calls satisfying", [
+          expect(parsers.js, "to have calls satisfying", [
             { args: [filepath], returned: phonyConfigObject },
           ]).and("was called once");
         });
@@ -63,12 +52,12 @@ describe("cli/config", function () {
         });
       });
 
-      describe('when supplied a filepath with ".cjs" extension', function () {
-        const filepath = "foo.cjs";
+      describe('when supplied a filepath with ".json" extension', function () {
+        const filepath = "foo.json";
 
-        it("should use the JS parser", function () {
-          loadConfig(filepath);
-          expect(parsers.js, "to have calls satisfying", [
+        it("should use the JSON parser", function () {
+          loadConfig("foo.json");
+          expect(parsers.json, "to have calls satisfying", [
             { args: [filepath], returned: phonyConfigObject },
           ]).and("was called once");
         });
@@ -85,12 +74,34 @@ describe("cli/config", function () {
         });
       });
 
-      describe('when supplied a filepath with ".json" extension', function () {
-        const filepath = "foo.json";
+      describe('when supplied a filepath with ".mjs" extension', function () {
+        const filepath = "foo.mjs";
 
-        it("should use the JSON parser", function () {
-          loadConfig("foo.json");
-          expect(parsers.json, "to have calls satisfying", [
+        it("should use the JS parser", function () {
+          loadConfig(filepath);
+          expect(parsers.js, "to have calls satisfying", [
+            { args: [filepath], returned: phonyConfigObject },
+          ]).and("was called once");
+        });
+      });
+
+      describe('when supplied a filepath with ".yaml" extension', function () {
+        const filepath = "foo.yaml";
+
+        it("should use the YAML parser", function () {
+          loadConfig(filepath);
+          expect(parsers.yaml, "to have calls satisfying", [
+            { args: [filepath], returned: phonyConfigObject },
+          ]).and("was called once");
+        });
+      });
+
+      describe('when supplied a filepath with ".yml" extension', function () {
+        const filepath = "foo.yml";
+
+        it("should use the YAML parser", function () {
+          loadConfig(filepath);
+          expect(parsers.yaml, "to have calls satisfying", [
             { args: [filepath], returned: phonyConfigObject },
           ]).and("was called once");
         });