Add support for babel.config.json (#10501)

* Add support for babel.config.json root config

* Throw if multiple configuration files are found

* Add tests

packages/babel-core/src/config/files/configuration.js

@@ -19,6 +19,11 @@ import type { CallerMetadata } from "../validation/options";
 const debug = buildDebug("babel:config:loading:files:configuration");
 
 const BABEL_CONFIG_JS_FILENAME = "babel.config.js";
+const BABEL_CONFIG_JSON_FILENAME = "babel.config.json";
+const ROOT_CONFIG_FILENAMES = [
+  BABEL_CONFIG_JS_FILENAME,
+  BABEL_CONFIG_JSON_FILENAME,
+];
 
 const BABELRC_FILENAME = ".babelrc";
 const BABELRC_JS_FILENAME = ".babelrc.js";
@@ -27,7 +32,10 @@ const BABELIGNORE_FILENAME = ".babelignore";
 export function findConfigUpwards(rootDir: string): string | null {
   let dirname = rootDir;
   while (true) {
-    if (fs.existsSync(path.join(dirname, BABEL_CONFIG_JS_FILENAME))) {
+    if (
+      fs.existsSync(path.join(dirname, BABEL_CONFIG_JS_FILENAME)) ||
+      fs.existsSync(path.join(dirname, BABEL_CONFIG_JSON_FILENAME))
+    ) {
       return dirname;
     }
 
@@ -110,13 +118,29 @@ export function findRootConfig(
   envName: string,
   caller: CallerMetadata | void,
 ): ConfigFile | null {
-  const filepath = path.resolve(dirname, BABEL_CONFIG_JS_FILENAME);
+  const config = ROOT_CONFIG_FILENAMES.reduce(
+    (previousConfig: ConfigFile | null, name) => {
+      const filepath = path.resolve(dirname, name);
+      const config = readConfig(filepath, envName, caller);
+
+      if (config && previousConfig) {
+        throw new Error(
+          `Multiple configuration files found. Please remove one:\n` +
+            ` - ${path.basename(previousConfig.filepath)}\n` +
+            ` - ${name}\n` +
+            `from ${dirname}`,
+        );
+      }
 
-  const conf = readConfig(filepath, envName, caller);
-  if (conf) {
-    debug("Found root config %o in %o.", BABEL_CONFIG_JS_FILENAME, dirname);
+      return config || previousConfig;
+    },
+    null,
+  );
+
+  if (config) {
+    debug("Found configuration %o from %o.", config.filepath, dirname);
   }
-  return conf;
+  return config;
 }
 
 export function loadConfig(

packages/babel-core/test/config-chain.js

@@ -944,6 +944,52 @@ describe("buildConfigChain", function() {
       }
     });
 
+    it("should load babel.config.json", () => {
+      const filename = fixture("config-files", "babel-config-json", "src.js");
+
+      expect(
+        loadOptions({
+          filename,
+          cwd: path.dirname(filename),
+        }),
+      ).toEqual({
+        ...getDefaults(),
+        filename: filename,
+        cwd: path.dirname(filename),
+        root: path.dirname(filename),
+        comments: true,
+      });
+    });
+
+    it("should load babel.config.js", () => {
+      const filename = fixture("config-files", "babel-config-js", "src.js");
+
+      expect(
+        loadOptions({
+          filename,
+          cwd: path.dirname(filename),
+        }),
+      ).toEqual({
+        ...getDefaults(),
+        filename: filename,
+        cwd: path.dirname(filename),
+        root: path.dirname(filename),
+        comments: true,
+      });
+    });
+
+    it("should whtow if both babel.config.json and babel.config.js are used", () => {
+      const filename = fixture(
+        "config-files",
+        "babel-config-js-and-json",
+        "src.js",
+      );
+
+      expect(() =>
+        loadOptions({ filename, cwd: path.dirname(filename) }),
+      ).toThrow(/Multiple configuration files found/);
+    });
+
     it("should load .babelrc", () => {
       const filename = fixture("config-files", "babelrc", "src.js");
 

packages/babel-core/test/fixtures/config/config-files/babel-config-js-and-json/babel.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  comments: true
+};

packages/babel-core/test/fixtures/config/config-files/babel-config-js-and-json/babel.config.json

@@ -0,0 +1,3 @@
+{
+  "comments": true
+}

packages/babel-core/test/fixtures/config/config-files/babel-config-js/babel.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  comments: true
+};

packages/babel-core/test/fixtures/config/config-files/babel-config-json/babel.config.json

@@ -0,0 +1,3 @@
+{
+  "comments": true
+}