fix: keep watching when the last test file is removed

lib/cli/collect-files.cjs

@@ -28,6 +28,7 @@ const { castArray } = require("../utils.cjs");
  * @private
  */
 module.exports = ({
+  allowEmpty,
   ignore,
   extension,
   file: fileArgs,
@@ -94,11 +95,17 @@ module.exports = ({
     // give full message details when only 1 file is missing
     const noneFoundMsg =
       unmatchedSpecFiles.length === 1
-        ? `Error: No test files found: ${JSON.stringify(
+        ? `No test files found: ${JSON.stringify(
             unmatchedSpecFiles[0].pattern,
           )}` // stringify to print escaped characters raw
-        : "Error: No test files found";
-    console.error(pc.red(noneFoundMsg));
+        : "No test files found";
+
+    if (allowEmpty) {
+      console.warn(pc.yellow(`Warning: ${noneFoundMsg}`));
+      return { files: [], unmatchedFiles: [] };
+    }
+
+    console.error(pc.red(`Error: ${noneFoundMsg}`));
     process.exit(1);
   } else {
     // print messages as a warning

lib/cli/run-helpers.cjs

@@ -229,6 +229,7 @@ exports.runMocha = async (mocha, options) => {
   } = options;
 
   const fileCollectParams = {
+    allowEmpty: watch,
     ignore,
     extension,
     file,

lib/nodejs/file-unloader.js

@@ -10,8 +10,18 @@ const require = createRequire(import.meta.url);
 
 /**
  * Deletes a file from the `require` cache.
+ * A file that no longer exists on disk has nothing left to unload.
  * @param {string} file - File
  */
 export function unloadFile(file) {
-  delete require.cache[require.resolve(file)];
+  let resolved;
+  try {
+    resolved = require.resolve(file);
+  } catch (err) {
+    if (err.code === "MODULE_NOT_FOUND") {
+      return;
+    }
+    throw err;
+  }
+  delete require.cache[resolved];
 }

lib/types.d.ts

@@ -194,6 +194,8 @@ export type PluginFinalizer = (
  * An object to configure how Mocha gathers test files
  */
 export interface FileCollectionOptions {
+  /** Return an empty list instead of exiting when no test files are found */
+  allowEmpty?: boolean;
   /** File extensions to use */
   extension?: string[];
   /** Files, dirs, globs to run */

test/integration/options/watch.spec.cjs

@@ -67,6 +67,26 @@ describe("--watch", function () {
       });
     });
 
+    it("keeps watching when the last test file is removed", function () {
+      const testFile = path.join(tempDir, "test/a.js");
+      copyFixture(DEFAULT_FIXTURE, testFile);
+
+      return runMochaWatchJSONAsync(
+        ["test/**/*.js", "--watch-files", "test/**/*.js"],
+        { cwd: tempDir, expectedRuns: 3 },
+        async (_mochaProcess, { waitForRuns }) => {
+          fs.rmSync(testFile, { force: true });
+          await waitForRuns(2);
+          copyFixture("passing.fixture.cjs", path.join(tempDir, "test/b.js"));
+        },
+      ).then((results) => {
+        expect(results, "to have length", 3);
+        expect(results[0].passes, "to have length", 1);
+        expect(results[1].passes, "to have length", 0);
+        expect(results[2].passes, "to have length", 2);
+      });
+    });
+
     describe("when in parallel mode", function () {
       it("reruns test when watched test file is touched", function () {
         const testFile = path.join(tempDir, "test.js");
@@ -78,6 +98,26 @@ describe("--watch", function () {
           expect(results, "to have length", 2);
         });
       });
+
+      it("keeps watching when the last test file is removed", function () {
+        const testFile = path.join(tempDir, "test/a.js");
+        copyFixture(DEFAULT_FIXTURE, testFile);
+
+        return runMochaWatchJSONAsync(
+          ["--parallel", "test/**/*.js", "--watch-files", "test/**/*.js"],
+          { cwd: tempDir, expectedRuns: 3 },
+          async (_mochaProcess, { waitForRuns }) => {
+            fs.rmSync(testFile, { force: true });
+            await waitForRuns(2);
+            copyFixture("passing.fixture.cjs", path.join(tempDir, "test/b.js"));
+          },
+        ).then((results) => {
+          expect(results, "to have length", 3);
+          expect(results[0].passes, "to have length", 1);
+          expect(results[1].passes, "to have length", 0);
+          expect(results[2].passes, "to have length", 2);
+        });
+      });
     });
 
     it("reruns test when file matching --watch-files changes", function () {