fix: --reporter-option on CLI overrides values from config (#5532)

lib/cli/options.mjs

@@ -283,6 +283,16 @@ export const loadPkgRc = (args = {}) => {
  * @alias module:lib/cli.loadOptions
  * @returns {external:yargsParser.Arguments} Parsed args from everything
  */
+// Reads `reporter-option` from a config source under any of its known
+// aliases. Keeps `loadOptions` agnostic of how each source was authored.
+const readReporterOption = (source) =>
+  source &&
+  (source["reporter-option"] ??
+    source["reporter-options"] ??
+    source.reporterOption ??
+    source.reporterOptions ??
+    source.O);
+
 export const loadOptions = (argv = []) => {
   let args = parse(argv);
   // short-circuit: look for a flag that would abort loading of options
@@ -299,6 +309,15 @@ export const loadOptions = (argv = []) => {
   const rcConfig = loadRc(args);
   const pkgConfig = loadPkgRc(args);
 
+  // Capture per-source `reporter-option` before yargs-parser's `combine-arrays`
+  // flattens them and loses provenance. See the re-merge step below for why.
+  const reporterOptionSources = {
+    pkg: readReporterOption(pkgConfig),
+    rc: readReporterOption(rcConfig),
+    env: readReporterOption(envConfig),
+    cli: readReporterOption(args),
+  };
+
   if (rcConfig) {
     args.config = false;
     args._ = args._.concat(rcConfig._ || []);
@@ -317,6 +336,20 @@ export const loadOptions = (argv = []) => {
     pkgConfig || {},
   );
 
+  // `combine-arrays` concatenates `reporter-option` from every source in
+  // high→low priority order. The downstream coerce in `lib/cli/run.js` reduces
+  // it left-to-right with last-writer-wins, which inverts the intended
+  // precedence. Re-emit lowest→highest priority so CLI overrides config.
+  const orderedReporterOption = [
+    reporterOptionSources.pkg,
+    reporterOptionSources.rc,
+    reporterOptionSources.env,
+    reporterOptionSources.cli,
+  ].reduce((acc, src) => (src ? acc.concat(list(src)) : acc), []);
+  if (orderedReporterOption.length) {
+    args["reporter-option"] = orderedReporterOption;
+  }
+
   // recombine positional arguments and "spec"
   if (args.spec) {
     args._ = args._.concat(args.spec);

test/integration/fixtures/options/reporter-option-mocharc.yml

@@ -0,0 +1,3 @@
+reporter-options:
+  - foo=fromConfig
+  - extra=keepMe

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

@@ -83,4 +83,46 @@ describe("--reporter-option", function () {
       );
     });
   });
+
+  describe("when given options on the CLI and in a config file", function () {
+    var reporterFixture = path.join(
+      __dirname,
+      "..",
+      "fixtures",
+      "options",
+      "reporter-with-options.fixture.js",
+    );
+    var configFixture = path.join(
+      __dirname,
+      "..",
+      "fixtures",
+      "options",
+      "reporter-option-mocharc.yml",
+    );
+
+    it("should allow the CLI to override clashing keys from the config", function (done) {
+      runMocha(
+        "passing.fixture.js",
+        [
+          "--config",
+          configFixture,
+          "--reporter",
+          reporterFixture,
+          "--reporter-option",
+          "foo=fromCli",
+        ],
+        function (err, res) {
+          if (err) {
+            return done(err);
+          }
+          expect(res, "to have passed").and(
+            "to contain output",
+            /{"foo":"fromCli","extra":"keepMe"}/,
+          );
+          done();
+        },
+        "pipe",
+      );
+    });
+  });
 });