feat: warn when there are multiple configs

e2e/Utils.ts

@@ -176,20 +176,23 @@ interface JestPackageJson extends PackageJson {
 }
 
 const DEFAULT_PACKAGE_JSON: JestPackageJson = {
-  description: 'THIS IS AN AUTOGENERATED FILE AND SHOULD NOT BE ADDED TO GIT',
   jest: {
     testEnvironment: 'node',
   },
 };
 
 export const createEmptyPackage = (
   directory: Config.Path,
-  packageJson = DEFAULT_PACKAGE_JSON,
+  packageJson: PackageJson = DEFAULT_PACKAGE_JSON,
 ) => {
+  const packageJsonWithDefaults = {
+    ...packageJson,
+    description: 'THIS IS AN AUTOGENERATED FILE AND SHOULD NOT BE ADDED TO GIT',
+  };
   fs.mkdirSync(directory, {recursive: true});
   fs.writeFileSync(
     path.resolve(directory, 'package.json'),
-    JSON.stringify(packageJson, null, 2),
+    JSON.stringify(packageJsonWithDefaults, null, 2),
   );
 };
 

e2e/__tests__/configOverride.test.ts

@@ -0,0 +1,24 @@
+/**
+ * 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 {getConfig} from '../runJest';
+
+test('reads config from cjs file', () => {
+  const {configs} = getConfig(
+    'config-override',
+    ['--config', 'different-config.json'],
+    {
+      skipPkgJsonCheck: true,
+    },
+  );
+
+  expect(configs).toHaveLength(1);
+  expect(configs[0].displayName).toEqual({
+    color: 'white',
+    name: 'Config from different-config.json file',
+  });
+});

e2e/__tests__/dependencyClash.test.ts

@@ -18,7 +18,7 @@ const hasteImplModulePath = path
 
 beforeEach(() => {
   cleanup(tempDir);
-  createEmptyPackage(tempDir);
+  createEmptyPackage(tempDir, {});
 });
 
 // This test case is checking that when having both

e2e/config-override/__tests__/test.js

@@ -0,0 +1,10 @@
+/**
+ * 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.
+ */
+
+test('dummy test', () => {
+  expect(1).toBe(1);
+});

e2e/config-override/different-config.json

@@ -0,0 +1,3 @@
+{
+  "displayName": "Config from different-config.json file"
+}

e2e/config-override/jest.config.json

@@ -0,0 +1,3 @@
+{
+  "displayName": "Config from json file"
+}

e2e/config-override/package.json

@@ -0,0 +1 @@
+{}

e2e/esm-config/cjs/package.json

@@ -1,3 +1 @@
-{
-  "jest": {}
-}
+{}

e2e/esm-config/mjs/package.json

@@ -1,3 +1 @@
-{
-  "jest": {}
-}
+{}

packages/jest-config/src/__tests__/resolveConfigPath.test.ts

@@ -14,6 +14,7 @@ import resolveConfigPath from '../resolveConfigPath';
 const DIR = path.resolve(tmpdir(), 'resolve_config_path_test');
 const ERROR_PATTERN = /Could not find a config file based on provided values/;
 const NO_ROOT_DIR_ERROR_PATTERN = /Can't find a root directory/;
+const MULTIPLE_CONFIGS_ERROR_PATTERN = /Multiple configurations found/;
 
 beforeEach(() => cleanup(DIR));
 afterEach(() => cleanup(DIR));
@@ -92,12 +93,49 @@ describe.each(JEST_CONFIG_EXT_ORDER.slice(0))(
         resolveConfigPath(path.dirname(relativePackageJsonPath), DIR),
       ).toBe(absoluteJestConfigPath);
 
+      // jest.config.js and package.json with 'jest' cannot be used together
+
+      writeFiles(DIR, {[relativePackageJsonPath]: JSON.stringify({jest: {}})});
+
+      // absolute
+      expect(() =>
+        resolveConfigPath(path.dirname(absolutePackageJsonPath), DIR),
+      ).toThrowError(MULTIPLE_CONFIGS_ERROR_PATTERN);
+
+      // relative
+      expect(() =>
+        resolveConfigPath(path.dirname(relativePackageJsonPath), DIR),
+      ).toThrowError(MULTIPLE_CONFIGS_ERROR_PATTERN);
+
       expect(() => {
         resolveConfigPath(
           path.join(path.dirname(relativePackageJsonPath), 'j/x/b/m/'),
           DIR,
         );
       }).toThrowError(NO_ROOT_DIR_ERROR_PATTERN);
     });
+
+    describe.each(JEST_CONFIG_EXT_ORDER.slice(0))(
+      `jest.config.%s and jest.config${extension}`,
+      extension2 => {
+        if (extension === extension2) return;
+
+        const relativeJestConfigPaths = [
+          `a/b/c/jest.config${extension}`,
+          `a/b/c/jest.config${extension2}`,
+        ];
+
+        writeFiles(DIR, {
+          [relativeJestConfigPaths[0]]: '',
+          [relativeJestConfigPaths[1]]: '',
+        });
+
+        // multiple configs here, should throw
+
+        expect(() =>
+          resolveConfigPath(path.dirname(relativeJestConfigPaths[0]), DIR),
+        ).toThrowError(MULTIPLE_CONFIGS_ERROR_PATTERN);
+      },
+    );
   },
 );

packages/jest-config/src/resolveConfigPath.ts

@@ -57,16 +57,21 @@ const resolveConfigPathByTraversing = (
   initialPath: Config.Path,
   cwd: Config.Path,
 ): Config.Path => {
-  const jestConfig = JEST_CONFIG_EXT_ORDER.map(ext =>
+  const configFiles = JEST_CONFIG_EXT_ORDER.map(ext =>
     path.resolve(pathToResolve, getConfigFilename(ext)),
-  ).find(isFile);
-  if (jestConfig) {
-    return jestConfig;
+  ).filter(isFile);
+
+  const packageJson = findPackageJson(pathToResolve);
+  if (packageJson && hasPackageJsonJestKey(packageJson)) {
+    configFiles.push(packageJson);
+  }
+
+  if (configFiles.length > 1) {
+    throw new Error(makeMultipleConfigsError(configFiles));
   }
 
-  const packageJson = path.resolve(pathToResolve, PACKAGE_JSON);
-  if (isFile(packageJson)) {
-    return packageJson;
+  if (configFiles.length || packageJson) {
+    return configFiles[0] ?? packageJson;
   }
 
   // This is the system root.
@@ -83,6 +88,25 @@ const resolveConfigPathByTraversing = (
   );
 };
 
+const findPackageJson = (pathToResolve: Config.Path) => {
+  const packagePath = path.resolve(pathToResolve, PACKAGE_JSON);
+  if (isFile(packagePath)) {
+    return packagePath;
+  }
+
+  return undefined;
+};
+
+const hasPackageJsonJestKey = (packagePath: Config.Path) => {
+  const content = fs.readFileSync(packagePath, 'utf8');
+  try {
+    return 'jest' in JSON.parse(content);
+  } catch {
+    // If package is not a valid JSON
+    return false;
+  }
+};
+
 const makeResolutionErrorMessage = (
   initialPath: Config.Path,
   cwd: Config.Path,
@@ -95,3 +119,27 @@ const makeResolutionErrorMessage = (
   `traverse directory tree up, until it finds one of those files in exact order: ${JEST_CONFIG_EXT_ORDER.map(
     ext => `"${getConfigFilename(ext)}"`,
   ).join(' or ')}.`;
+
+const makeMultipleConfigsError = (configPaths: Array<Config.Path>) =>
+  '● Multiple configurations found:\n\n' +
+  `Jest found configuration in ${concatenateWords(
+    configPaths.map(wrapInBackticks),
+  )}.\n` +
+  'This is not allowed because it is probably a mistake.\n\n' +
+  'Configuration Documentation:\n' +
+  'https://jestjs.io/docs/configuration.html\n';
+
+const wrapInBackticks = (word: string) => `\`${word}\``;
+
+const concatenateWords = (words: Array<string>): string => {
+  if (words.length === 0) {
+    throw new Error('Cannot concatenate an empty array.');
+  } else if (words.length <= 2) {
+    return words.join(' and ');
+  } else {
+    return concatenateWords([
+      words.slice(0, -1).join(', '),
+      words[words.length - 1],
+    ]);
+  }
+};