Show warning in cli in case of multiple configs

[Mansehej]

Summary

Helps the user have correct and consistent configuration and helps when trying to debug why config is not applied.

As mentioned in #10124 and #10123.

Warns in case of multiple configs, however the warning is overriden in case of the usage of --config.

Test plan

Tested it locally by trying a development build of jest in another package.
However, I'm unsure of writing the tests with respect to this functionality.
If someone could point me in the right direction (in case tests are necessary for this use case), it would be really helpful.

[SimenB]

This looks great, thanks! Could you add an e2e test that verifies the error triggers?

[SimenB]

just keep it under master - I'll add a version here when releasing 👍

[Mansehej]

Hey @SimenB,

I made an empty jest-config.js in the tests directory and then added snapshot tests.
I am not quite sure if this is the correct way to do it.

Could you please let me know if this is fine, so that it can be either merged/ I can edit it accordingly.

Thanks,
Mansehej.

[Mansehej]

Hey @SimenB
Did you get a chance to take a look at this?

Thanks,
Mansehej.

[SimenB]

@Mansehej sorry about the slow response! I've been away on holiday. I'll take a look at this next week when I'm back at work 🙂

[Mansehej]

@SimenB Oh no issues, have fun!

[SimenB]

I completely forgot about this, sorry about that @Mansehej! Should have created a reminder for myself 😬

What I would do is create some temporary directories and write config into them, sorta like https://github.com/facebook/jest/blob/a94730c685a40055fe1694d7e4833061d0055f79/e2e/__tests__/dependencyClash.test.ts#L40-L45

So something like

/**
 * Copyright (c) Facebook, Inc. and its affiliates. 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.
 */

import * as path from 'path';
import {tmpdir} from 'os';
import {cleanup, createEmptyPackage, writeFiles} from '../Utils';
import runJest from '../runJest';

const tempDir = path.resolve(tmpdir(), 'multiple-jest-configs');

beforeEach(() => {
  cleanup(tempDir);
});

test('throws when jest.config.js and package.json config exist', () => {
  createEmptyPackage(tempDir);
  writeFiles(tempDir, {
    '__tests__/test.js': `
      test('dummy test', () => expect(1).toBe(1));
    `,
    'jest.config.js': `module.exports = {
    };`,
  });
  const {stderr, exitCode} = runJest(tempDir);

  expect(exitCode).toBe(1);
  expect(stderr); // something - probably a `toContain` on the error text
});

test('does not throw when jest.config.js exist, but no config in package.json', () => {
  createEmptyPackage(tempDir, {
    description: 'THIS IS AN AUTOGENERATED FILE AND SHOULD NOT BE ADDED TO GIT',
  });
  writeFiles(tempDir, {
    '__tests__/test.js': `
      test('dummy test', () => expect(1).toBe(1));
    `,
    'jest.config.js': `module.exports = {
    };`,
  });
  const {stderr, exitCode} = runJest(tempDir);

  expect(exitCode).toBe(0);
});

Then I would write tests for all the different config files we have.

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