Show warning in cli in case of multiple configs

e2e/__tests__/__snapshots__/console.test.ts.snap

@@ -145,6 +145,57 @@ Snapshots:   0 total
 Time:        <<REPLACED>>
 `;
 
+exports[`does not warn of multiple configs when --config is passed 1`] = `
+  console.log
+    This is a log message.
+
+      at Object.log (__tests__/console.test.js:10:11)
+
+  console.info
+    This is an info message.
+
+      at Object.info (__tests__/console.test.js:12:11)
+
+  console.warn
+    This is a warning message.
+
+      12 |   console.info('This is an info message.');
+      13 | 
+    > 14 |   console.warn('This is a warning message.');
+         |           ^
+      15 | 
+      16 |   console.error('This is an error message.');
+      17 | });
+
+      at Object.warn (__tests__/console.test.js:14:11)
+
+  console.error
+    This is an error message.
+
+      14 |   console.warn('This is a warning message.');
+      15 | 
+    > 16 |   console.error('This is an error message.');
+         |           ^
+      17 | });
+      18 | 
+
+      at Object.error (__tests__/console.test.js:16:11)
+
+`;
+
+exports[`does not warn of multiple configs when --config is passed 2`] = `
+PASS __tests__/console.test.js
+  ✓ works just fine
+`;
+
+exports[`does not warn of multiple configs when --config is passed 3`] = `
+Test Suites: 1 passed, 1 total
+Tests:       1 passed, 1 total
+Snapshots:   0 total
+Time:        <<REPLACED>>
+Ran all test suites.
+`;
+
 exports[`respects --noStackTrace 1`] = `
   console.log
     This is a log message.
@@ -231,3 +282,53 @@ Snapshots:   0 total
 Time:        <<REPLACED>>
 Ran all test suites.
 `;
+
+exports[`warns of multiple configs 1`] = ``;
+
+exports[`warns of multiple configs 2`] = `
+PASS __tests__/console.test.js
+  ● Console
+
+    console.log
+      This is a log message.
+
+      at Object.log (__tests__/console.test.js:10:11)
+
+    console.info
+      This is an info message.
+
+      at Object.info (__tests__/console.test.js:12:11)
+
+    console.warn
+      This is a warning message.
+
+      12 |   console.info('This is an info message.');
+      13 | 
+    > 14 |   console.warn('This is a warning message.');
+         |           ^
+      15 | 
+      16 |   console.error('This is an error message.');
+      17 | });
+
+      at Object.warn (__tests__/console.test.js:14:11)
+
+    console.error
+      This is an error message.
+
+      14 |   console.warn('This is a warning message.');
+      15 | 
+    > 16 |   console.error('This is an error message.');
+         |           ^
+      17 | });
+      18 | 
+
+      at Object.error (__tests__/console.test.js:16:11)
+`;
+
+exports[`warns of multiple configs 3`] = `
+Test Suites: 1 passed, 1 total
+Tests:       1 passed, 1 total
+Snapshots:   0 total
+Time:        <<REPLACED>>
+Ran all test suites.
+`;

e2e/__tests__/console.test.ts

@@ -108,3 +108,29 @@ test('does not error out when using winston', () => {
   expect(wrap(rest)).toMatchSnapshot();
   expect(wrap(summary)).toMatchSnapshot();
 });
+
+test('warns of multiple configs', () => {
+  const {stderr, stdout, exitCode} = runJest('console');
+  const {summary, rest} = extractSummary(stderr);
+  console.log('in last test');
+  console.log(summary);
+  console.log(rest);
+  expect(exitCode).toBe(0);
+  expect(wrap(stdout)).toMatchSnapshot();
+  expect(wrap(rest)).toMatchSnapshot();
+  expect(wrap(summary)).toMatchSnapshot();
+});
+
+test('does not warn of multiple configs when --config is passed', () => {
+  const {stderr, stdout, exitCode} = runJest('console', [
+    '--config=' + JSON.stringify({testEnvironment: 'node'}),
+  ]);
+  const {summary, rest} = extractSummary(stderr);
+  console.log('in last with --config test');
+  console.log(summary);
+  console.log(rest);
+  expect(exitCode).toBe(0);
+  expect(wrap(stdout)).toMatchSnapshot();
+  expect(wrap(rest)).toMatchSnapshot();
+  expect(wrap(summary)).toMatchSnapshot();
+});

e2e/jest-config.js

@@ -0,0 +1,11 @@
+/**
+ * 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.
+ *
+ */
+
+/* -------------------------------------------------------------------------- */
+/*                    Empty file just for testing purposes.                   */
+/* -------------------------------------------------------------------------- */

packages/jest-config/src/index.ts

@@ -18,6 +18,7 @@ export {getTestEnvironment, isJSONString} from './utils';
 export {default as normalize} from './normalize';
 export {default as deprecationEntries} from './Deprecated';
 export {replaceRootDirInPath} from './utils';
+export {checkMultipleConfigs} from './resolveConfigPath';
 export {default as defaults} from './Defaults';
 export {default as descriptions} from './Descriptions';
 import * as constants from './constants';

packages/jest-config/src/resolveConfigPath.ts

@@ -83,6 +83,30 @@ const resolveConfigPathByTraversing = (
   );
 };
 
+export const checkMultipleConfigs = (pathToResolve: Config.Path): boolean => {
+  const jestConfig = JEST_CONFIG_EXT_ORDER.map(ext =>
+    path.resolve(pathToResolve, getConfigFilename(ext)),
+  ).find(isFile);
+
+  if (!jestConfig) {
+    return false;
+  }
+
+  const packageJson = path.resolve(pathToResolve, PACKAGE_JSON);
+
+  if (!isFile(packageJson)) {
+    return false;
+  }
+
+  const configObject = require(packageJson);
+
+  if (!configObject.jest) {
+    return false;
+  }
+
+  return true;
+};
+
 const makeResolutionErrorMessage = (
   initialPath: Config.Path,
   cwd: Config.Path,

packages/jest-core/src/cli/index.ts

@@ -9,7 +9,7 @@ import type {Config} from '@jest/types';
 import type {AggregatedResult} from '@jest/test-result';
 import {CustomConsole} from '@jest/console';
 import {createDirectory, preRunMessage} from 'jest-util';
-import {readConfigs} from 'jest-config';
+import {checkMultipleConfigs, readConfigs} from 'jest-config';
 import Runtime = require('jest-runtime');
 import type {ChangedFilesPromise} from 'jest-changed-files';
 import HasteMap = require('jest-haste-map');
@@ -48,6 +48,27 @@ export async function runCLI(
   const outputStream =
     argv.json || argv.useStderr ? process.stderr : process.stdout;
 
+  if (!argv.config) {
+    projects.forEach(project => {
+      const hasMultipleConfigs = checkMultipleConfigs(project);
+      if (hasMultipleConfigs) {
+        console.warn(
+          chalk.yellow(
+            chalk.bold(`Multiple configurations found:
+
+              Jest will use 'jest.config.js' for configuration, but Jest also 
+              found a configuration in 'package.json'. Delete the 'jest' key 
+              in that file to silence this warning, or delete the 'jest.config.js' file 
+              to use the configuration from 'package.json'.
+
+              Configuration Documentation:
+              https://jestjs.io/docs/en/configuration.html`),
+          ),
+        );
+      }
+    });
+  }
+
   const {globalConfig, configs, hasDeprecationWarnings} = await readConfigs(
     argv,
     projects,