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

…#4591)

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

In this commit we are adding a new validation in the check method of the
jest-cli package, so we raise an exception if the user tries to run Jest
with undefined maxWorkers, e.g.

jest --maxWorkers

jest --maxWorkers 2

Some users were confusing -w as an alias to --watch, but in fact it is
an alias to --maxWorkers. This change will also make this distinction
clearer.

Fixes issue #4577

* Add expected exception message in the check method tests

* Improve error message for when maxWorkers is undefined

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

@@ -0,0 +1,54 @@
+/**
+ * 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(
+      'Both --runInBand and --maxWorkers were specified',
+    );
+  });
+
+  it('raises an exception if onlyChanged and watchAll are both specified', () => {
+    const argv: Argv = {onlyChanged: true, watchAll: true};
+    expect(() => check(argv)).toThrow(
+      'Both --onlyChanged and --watchAll were specified',
+    );
+  });
+
+  it('raises an exception if findRelatedTests is specified with no file paths', () => {
+    const argv: Argv = {_: [], findRelatedTests: true};
+    expect(() => check(argv)).toThrow(
+      'The --findRelatedTests option requires file paths to be specified',
+    );
+  });
+
+  it('raises an exception if maxWorkers is specified with no number', () => {
+    const argv: Argv = {maxWorkers: undefined};
+    expect(() => check(argv)).toThrow(
+      'The --maxWorkers (-w) option requires a number to be specified',
+    );
+  });
+
+  it('raises an exception if config is not a valid JSON string', () => {
+    const argv: Argv = {config: 'x:1'};
+    expect(() => check(argv)).toThrow(
+      'The --config option requires a JSON string literal, or a file path with a .js or .json extension',
+    );
+  });
+});

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 (-w) 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) &&