fix: polling usage in watchFiles option

lib/Server.js

@@ -973,9 +973,12 @@ class Server {
     // https://github.com/webpack/watchpack/blob/master/lib/DirectoryWatcher.js#L49
     // this isn't an elegant solution, but we'll improve it in the future
     // eslint-disable-next-line no-undefined
-    const usePolling = watchOptions.poll ? true : undefined;
+    const usePolling = watchOptions.usePolling || Boolean(watchOptions.poll);
     const interval =
-      typeof watchOptions.poll === 'number'
+      // eslint-disable-next-line no-nested-ternary
+      typeof watchOptions.interval !== 'undefined'
+        ? watchOptions.interval
+        : typeof watchOptions.poll === 'number'
         ? watchOptions.poll
         : // eslint-disable-next-line no-undefined
           undefined;

test/server/watchFiles-option.test.js

@@ -2,6 +2,7 @@
 
 const path = require('path');
 const fs = require('graceful-fs');
+const chokidar = require('chokidar');
 const testServer = require('../helpers/test-server');
 const config = require('../fixtures/contentbase-config/webpack.config');
 const port = require('../ports-map')['watchFiles-option'];
@@ -277,6 +278,8 @@ describe("'watchFiles' option", () => {
   describe('should work with options', () => {
     const file = path.join(watchDir, 'assets/example.txt');
 
+    const chokidarMock = jest.spyOn(chokidar, 'watch');
+
     beforeAll((done) => {
       server = testServer.start(
         config,
@@ -285,6 +288,7 @@ describe("'watchFiles' option", () => {
             paths: file,
             options: {
               usePolling: true,
+              interval: 400,
             },
           },
           port,
@@ -298,6 +302,21 @@ describe("'watchFiles' option", () => {
       fs.truncateSync(file);
     });
 
+    it('should pass correct options to chokidar config', () => {
+      expect(chokidarMock).toHaveBeenCalledWith(file, {
+        ignoreInitial: true,
+        persistent: true,
+        followSymlinks: false,
+        atomic: false,
+        alwaysStat: true,
+        ignorePermissionErrors: true,
+        // eslint-disable-next-line no-undefined
+        ignored: undefined,
+        usePolling: true,
+        interval: 400,
+      });
+    });
+
     it('should reload on file content changed', (done) => {
       server.staticWatchers[0].on('change', (changedPath) => {
         expect(changedPath).toBe(file);