Check if the maxWorkers has a value and raises an exception otherwise

packages/jest-cli/src/__tests__/cli/args.test.js

@@ -0,0 +1,44 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
+
+'use strict';
+
+import type {Argv} from 'types/Argv';
+import {check} from '../../cli/args';
+
+describe('check', () => {
+  it('returns true if the arguments are valid', () => {
+    const argv: Argv = {};
+    expect(check(argv)).toBe(true);
+  });
+
+  it('raises an exception if runInBand and maxWorkers are both specified', () => {
+    const argv: Argv = {maxWorkers: 2, runInBand: true};
+    expect(() => check(argv)).toThrow();
+  });
+
+  it('raises an exception if onlyChanged and watchAll are both specified', () => {
+    const argv: Argv = {onlyChanged: true, watchAll: true};
+    expect(() => check(argv)).toThrow();
+  });
+
+  it('raises an exception if findRelatedTests is specified with no file paths', () => {
+    const argv: Argv = {findRelatedTests: true};
+    expect(() => check(argv)).toThrow();
+  });
+
+  it('raises an exception if maxWorkers is specified with no number', () => {
+    const argv: Argv = {maxWorkers: undefined};
+    expect(() => check(argv)).toThrow();
+  });
+
+  it('raises an exception if config is not a valid JSON string', () => {
+    const argv: Argv = {config: 'x:1'};
+    expect(() => check(argv)).toThrow();
+  });
+});

packages/jest-cli/src/cli/args.js

@@ -36,6 +36,14 @@ export const check = (argv: Argv) => {
     );
   }
 
+  if (argv.hasOwnProperty('maxWorkers') && argv.maxWorkers === undefined) {
+    throw new Error(
+      'The --maxWorkers option requires a number to be specified.\n' +
+        'Example usage: jest --maxWorkers 2\n' +
+        'Or did you mean --watch ?',
+    );
+  }
+
   if (
     argv.config &&
     !isJSONString(argv.config) &&