fix: give CLI reporter-option precedence over config file

lib/cli/parse-args.js

@@ -150,6 +150,10 @@ const mergeConfigObjects = (defaultValues, configObjects) => {
           } else if (defaultArrayOptions.has(name)) {
             defaultArrayOptions.delete(name);
             merged[name] = value;
+          } else if (name === "reporter-option") {
+            // last-writer-wins downstream, so lower-priority sources come first
+            // https://github.com/mochajs/mocha/issues/5532
+            merged[name] = mergeValue(merged[name], value);
           } else {
             merged[name] = mergeValue(value, merged[name]);
           }

test/integration/options/reporter-option.spec.cjs

@@ -132,6 +132,39 @@ describe("--reporter-option", function () {
       );
     });
 
+    it("should allow the CLI to override clashing keys from the config file", function (done) {
+      var tmpdir = fs.mkdtempSync(
+        path.join(os.tmpdir(), "mocha-reporter-option-"),
+      );
+      fs.writeFileSync(
+        path.join(tmpdir, ".mocharc.yml"),
+        [
+          `reporter: ${customReporter}`,
+          "reporter-option:",
+          "  - foo=fromConfig",
+          "  - extra=keepMe",
+          `spec: ${path.join(__dirname, "..", "fixtures", "passing.fixture.cjs")}`,
+          "",
+        ].join("\n"),
+      );
+
+      invokeMocha(
+        ["--reporter-option", "foo=fromCli"],
+        function (err, res) {
+          rimraf.sync(tmpdir);
+          if (err) {
+            return done(err);
+          }
+          expect(res, "to have passed").and(
+            "to contain output",
+            /{"foo":"fromCli","extra":"keepMe"}/,
+          );
+          done();
+        },
+        { cwd: tmpdir, stdio: "pipe" },
+      );
+    });
+
     it("should preserve package config reporter options after respawning with a Node option", function (done) {
       var tmpdir = fs.mkdtempSync(
         path.join(os.tmpdir(), "mocha-reporter-option-"),

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

@@ -444,6 +444,37 @@ describe("options", function () {
       });
     });
 
+    describe("reporter-option", function () {
+      let result;
+
+      beforeEach(function () {
+        readFileSync = sinon.stub();
+        readFileSync.onFirstCall().returns("{}");
+        findConfig = sinon.stub().returns("/some/.mocharc.json");
+        loadConfig = sinon.stub().returns({
+          "reporter-option": ["output=config.xml", "extra=keepMe"],
+        });
+        findupSync = sinon.stub().returns("/some/package.json");
+
+        loadOptions = proxyLoadOptions({
+          readFileSync,
+          findConfig,
+          loadConfig,
+          findupSync,
+        });
+
+        result = loadOptions(["--reporter-option", "output=cli.xml"]);
+      });
+
+      it("should merge rc reporter options before CLI reporter options", function () {
+        expect(result, "to have property", "reporter-option", [
+          "output=config.xml",
+          "extra=keepMe",
+          "output=cli.xml",
+        ]);
+      });
+    });
+
     describe("env options", function () {
       it("should parse flags from MOCHA_OPTIONS", function () {
         readFileSync = sinon.stub().onFirstCall().returns("{}");

test/node-unit/cli/parse-args.spec.js

@@ -154,6 +154,29 @@ describe("parse-args", function () {
     );
   });
 
+  it("orders reporter-option values from lowest to highest priority source", function () {
+    expect(
+      parseMochaArgs(
+        [],
+        defaults,
+        { "reporter-option": ["output=cli.xml"] },
+        { "reporter-option": ["output=env.xml"] },
+        { "reporter-option": ["output=config.xml", "extra=keepMe"] },
+        { "reporter-option": ["output=pkg.xml"] },
+      ),
+      "to satisfy",
+      {
+        "reporter-option": [
+          "output=pkg.xml",
+          "output=config.xml",
+          "extra=keepMe",
+          "output=env.xml",
+          "output=cli.xml",
+        ],
+      },
+    );
+  });
+
   it("preserves boolean negation including long alias negation", function () {
     expect(
       parseMochaArgs(