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__/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/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));
@@ -92,6 +93,20 @@ 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/'),

packages/jest-config/src/resolveConfigPath.ts

@@ -60,15 +60,17 @@ const resolveConfigPathByTraversing = (
   const jestConfig = JEST_CONFIG_EXT_ORDER.map(ext =>
     path.resolve(pathToResolve, getConfigFilename(ext)),
   ).find(isFile);
-  if (jestConfig) {
-    return jestConfig;
-  }
 
-  const packageJson = path.resolve(pathToResolve, PACKAGE_JSON);
-  if (isFile(packageJson)) {
-    return packageJson;
+  const packageJson = findPackageJson(pathToResolve);
+
+  // If user has jestConfig and 'jest' in package.json it's probably a mistake.
+  if (jestConfig && packageJson && hasPackageJsonJestKey(packageJson)) {
+    throw new Error(makeMultipleConfigsErrorMessage(jestConfig, packageJson));
   }
 
+  const configPath = jestConfig ?? packageJson;
+  if (configPath) return configPath;
+
   // This is the system root.
   // We tried everything, config is nowhere to be found ¯\_(ツ)_/¯
   if (pathToResolve === path.dirname(pathToResolve)) {
@@ -83,6 +85,25 @@ 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 +116,17 @@ 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 makeMultipleConfigsErrorMessage = (
+  jestConfigPath: Config.Path,
+  packagePath: Config.Path,
+) =>
+  '● Multiple configurations found:\n\n' +
+  `Jest will use \`${jestConfigPath}\` for configuration, but Jest also\n` +
+  `found configuration in \`${packagePath}\`. Delete the \`"jest"\` key\n` +
+  `in that file to silence this warning, or delete the \`${path.basename(
+    jestConfigPath,
+  )}\` file\n` +
+  `to use the configuration from \`${path.basename(packagePath)}\`.\n\n` +
+  'Configuration Documentation:\n' +
+  'https://jestjs.io/docs/configuration.html\n';