Add "extensions" option

- It can be set in programmatic option, babel.config.json, .babelrc.json and in presets
- Files not matching any extension are ignored
- If no filename is given, the file is not ignored
- "*" is supported as a catch-all extension

For backward compatibility reasons, this option defaults to ["*"].

packages/babel-core/src/config/full.js

@@ -4,7 +4,7 @@ import gensync, { type Handler } from "gensync";
 import { forwardAsync } from "../gensync-utils/async";
 
 import { mergeOptions } from "./util";
-import * as context from "../index";
+import * as babelContext from "../index";
 import Plugin from "./plugin";
 import { getItemDescriptor } from "./item";
 import {
@@ -70,7 +70,7 @@ export default gensync<[any], ResolvedConfig | null>(function* loadFullConfig(
     return null;
   }
 
-  const optionDefaults = {};
+  const optionDefaults: ValidatedOptions = {};
 
   const { plugins, presets } = options;
 
@@ -161,6 +161,8 @@ export default gensync<[any], ResolvedConfig | null>(function* loadFullConfig(
   const opts: Object = optionDefaults;
   mergeOptions(opts, options);
 
+  if (!isCompilableFile(context.filename, opts.extensions)) return null;
+
   yield* enhanceError(context, function* loadPluginDescriptors() {
     pluginDescriptorsByPass[0].unshift(...initialPluginsDescriptors);
 
@@ -229,7 +231,7 @@ const loadDescriptor = makeWeakCache(function* (
   let item = value;
   if (typeof value === "function") {
     const api = {
-      ...context,
+      ...babelContext,
       ...makeAPI(cache),
     };
     try {
@@ -395,3 +397,8 @@ function chain(a, b) {
     }
   };
 }
+
+function isCompilableFile(filename, extensions) {
+  if (filename == null) return true;
+  return extensions.some(ext => ext === "*" || filename.endsWith(ext));
+}

packages/babel-core/src/config/partial.js

@@ -115,11 +115,17 @@ export default function* loadPrivatePartialConfig(
   const configChain = yield* buildRootChain(args, context);
   if (!configChain) return null;
 
-  const options = {};
+  const options: ValidatedOptions = {};
   configChain.options.forEach(opts => {
     mergeOptions(options, opts);
   });
 
+  // If the programmatic options or config files don't set the "extensions"
+  // option, default to ["*"] for backward compatibility reasons.
+  // TODO(Babel 8): The default should be babel.DEFAULT_EXTENSIONS
+  // TODO: Use ??= once flow supports it.
+  options.extensions = options.extensions ?? ["*"];
+
   // Tack the passes onto the object itself so that, if this object is
   // passed back to Babel a second time, it will be in the right structure
   // to not change behavior.

packages/babel-core/src/config/util.js

@@ -2,6 +2,10 @@
 
 import type { ValidatedOptions } from "./validation/options";
 
+function unique<T>(...args: T[]): T[] {
+  return Array.from(new Set(args));
+}
+
 export function mergeOptions(
   target: ValidatedOptions,
   source: ValidatedOptions,
@@ -15,6 +19,11 @@ export function mergeOptions(
       const generatorOpts = source.generatorOpts;
       const targetObj = (target.generatorOpts = target.generatorOpts || {});
       mergeDefaultFields(targetObj, generatorOpts);
+    } else if (k === "extensions" && target.extensions && source.extensions) {
+      target.extensions = unique(
+        ...(target.extensions ?? []),
+        ...source.extensions,
+      );
     } else {
       const val = source[k];
       if (val !== undefined) target[k] = (val: any);

packages/babel-core/src/config/validation/option-assertions.js

@@ -3,7 +3,7 @@
 import type {
   ConfigFileSearch,
   BabelrcSearch,
-  IgnoreList,
+  FileExtension,
   IgnoreItem,
   PluginList,
   PluginItem,
@@ -217,17 +217,20 @@ export function assertArray(
   return value;
 }
 
-export function assertIgnoreList(
-  loc: OptionPath,
-  value: mixed,
-): IgnoreList | void {
-  const arr = assertArray(loc, value);
-  if (arr) {
-    arr.forEach((item, i) => assertIgnoreItem(access(loc, i), item));
-  }
-  return (arr: any);
+function createArrayAssertion<T>(
+  assertValid: (loc: GeneralPath, value: mixed) => T,
+): (loc: GeneralPath, value: mixed) => $ReadOnlyArray<T> | void {
+  return (loc, value) => {
+    const arr = assertArray(loc, value);
+    if (arr) arr.forEach((item, i) => assertValid(access(loc, i), item));
+    return (arr: any);
+  };
 }
-function assertIgnoreItem(loc: GeneralPath, value: mixed): IgnoreItem {
+
+export const assertIgnoreList = createArrayAssertion(function assertIgnoreItem(
+  loc: GeneralPath,
+  value: mixed,
+): IgnoreItem {
   if (
     typeof value !== "string" &&
     typeof value !== "function" &&
@@ -240,7 +243,26 @@ function assertIgnoreItem(loc: GeneralPath, value: mixed): IgnoreItem {
     );
   }
   return value;
-}
+});
+
+export const assertExtensionsList = createArrayAssertion(
+  function assertExtension(
+    loc: GeneralPath,
+    value: mixed,
+  ): FileExtension | "*" {
+    if (value === "*") return value;
+
+    if (typeof value !== "string") {
+      throw new Error(
+        `${msg(loc)} must be an array of string values, or undefined`,
+      );
+    }
+    if (!value.startsWith(".")) {
+      throw new Error(`${msg(loc)} must start with a '.' (dot)`);
+    }
+    return (value: any);
+  },
+);
 
 export function assertConfigApplicableTest(
   loc: OptionPath,

packages/babel-core/src/config/validation/options.js

@@ -14,6 +14,7 @@ import {
   assertCallerMetadata,
   assertInputSourceMap,
   assertIgnoreList,
+  assertExtensionsList,
   assertPluginList,
   assertConfigApplicableTest,
   assertConfigFileSearch,
@@ -114,6 +115,10 @@ const COMMON_VALIDATORS: ValidatorSet = {
     $PropertyType<ValidatedOptions, "exclude">,
   >),
 
+  extensions: (assertExtensionsList: Validator<
+    $PropertyType<ValidatedOptions, "extensions">,
+  >),
+
   retainLines: (assertBoolean: Validator<
     $PropertyType<ValidatedOptions, "retainLines">,
   >),
@@ -195,9 +200,12 @@ export type ValidatedOptions = {
 
   extends?: string,
   env?: EnvSet<ValidatedOptions>,
+  overrides?: OverridesList,
+
+  // Options to enable/disable processing of some files
   ignore?: IgnoreList,
   only?: IgnoreList,
-  overrides?: OverridesList,
+  extensions?: ExtensionsList,
 
   // Generally verify if a given config object should be applied to the given file.
   test?: ConfigApplicableTest,
@@ -252,6 +260,9 @@ export type EnvSet<T> = {
 export type IgnoreItem = string | Function | RegExp;
 export type IgnoreList = $ReadOnlyArray<IgnoreItem>;
 
+export opaque type FileExtension = string;
+export type ExtensionsList = $ReadOnlyArray<FileExtension | "*">;
+
 export type PluginOptions = {} | void | false;
 export type PluginTarget = string | {} | Function;
 export type PluginItem =

packages/babel-core/src/index.js

@@ -1,5 +1,7 @@
 // @flow
 
+import type { ExtensionsList } from "./config/validation/options";
+
 export { default as File } from "./transformation/file/file";
 export { default as buildExternalHelpers } from "./tools/build-external-helpers";
 export { resolvePlugin, resolvePreset } from "./config/files";
@@ -41,13 +43,9 @@ export { parse, parseSync, parseAsync } from "./parse";
  * Recommended set of compilable extensions. Not used in @babel/core directly, but meant as
  * as an easy source for tooling making use of @babel/core.
  */
-export const DEFAULT_EXTENSIONS = Object.freeze([
-  ".js",
-  ".jsx",
-  ".es6",
-  ".es",
-  ".mjs",
-]);
+export const DEFAULT_EXTENSIONS: ExtensionsList = Object.freeze(
+  ([".js", ".jsx", ".es6", ".es", ".mjs"]: any),
+);
 
 // For easier backward-compatibility, provide an API like the one we exposed in Babel 6.
 import { loadOptions } from "./config";

packages/babel-core/test/api.js

@@ -688,6 +688,115 @@ describe("api", function () {
     ]);
   });
 
+  it("extensions option", function () {
+    return Promise.all([
+      transformAsync("", {
+        filename: "bar.js",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        filename: "bar.mjs",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        filename: "bar.jsx",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        filename: undefined,
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        filename: "bar.ts",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: [".js", ".mjs"],
+        filename: "bar.ts",
+      }).then(assertIgnored),
+
+      transformAsync("", {
+        extensions: [".ts"],
+        filename: "bar.ts",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: [".ts"],
+        filename: "bar.js",
+      }).then(assertIgnored),
+
+      transformAsync("", {
+        extensions: ["*"],
+        filename: "bar.ts",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: ["*"],
+        filename: "bar.js",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: ["*"],
+        filename: undefined,
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: ["*"],
+        filename: "bar",
+      }).then(assertNotIgnored),
+
+      // Presets can add supported extensions but they don't prevent the
+      // default value of ["*"] from being set.
+
+      transformAsync("", {
+        extensions: [".js"],
+        filename: "bar.js",
+        presets: [() => ({ extensions: [".ts"], plugins: [] })],
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: [".js"],
+        filename: "bar.ts",
+        presets: [() => ({ extensions: [".ts"], plugins: [] })],
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: [".js"],
+        filename: "bar.tsx",
+        presets: [() => ({ extensions: [".ts"], plugins: [] })],
+      }).then(assertIgnored),
+
+      transformAsync("", {
+        filename: "bar.tsx",
+        presets: [() => ({ extensions: [".ts"], plugins: [] })],
+      }).then(assertNotIgnored),
+
+      // Test with 'extensions' option in config file
+
+      transformAsync("", {
+        configFile: `${__dirname}/fixtures/api/config-with-ts-extension/babel.config.json`,
+        filename: "foo.ts",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        configFile: `${__dirname}/fixtures/api/config-with-ts-extension/babel.config.json`,
+        filename: "foo.tsx",
+      }).then(assertIgnored),
+
+      transformAsync("", {
+        extensions: [".tsx"],
+        configFile: `${__dirname}/fixtures/api/config-with-ts-extension/babel.config.json`,
+        filename: "foo.tsx",
+      }).then(assertNotIgnored),
+
+      transformAsync("", {
+        extensions: ["*"],
+        configFile: `${__dirname}/fixtures/api/config-with-ts-extension/babel.config.json`,
+        filename: "foo.tsx",
+      }).then(assertNotIgnored),
+    ]);
+  });
+
   describe("env option", function () {
     const oldBabelEnv = process.env.BABEL_ENV;
     const oldNodeEnv = process.env.NODE_ENV;

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

@@ -1011,6 +1011,7 @@ describe("buildConfigChain", function () {
       plugins: [],
       presets: [],
       cloneInputAst: true,
+      extensions: ["*"],
     });
     const realEnv = process.env.NODE_ENV;
     const realBabelEnv = process.env.BABEL_ENV;

packages/babel-core/test/fixtures/api/config-with-ts-extension/babel.config.json

@@ -0,0 +1,3 @@
+{
+  "extensions": [".ts"]
+}

packages/babel-node/src/_babel-node.js

@@ -125,6 +125,7 @@ const _eval = function (code, filename) {
   if (!code) return undefined;
 
   code = babel.transform(code, {
+    extensions: ["*"],
     filename: filename,
     presets: program.presets,
     plugins: (program.plugins || []).concat([replPlugin]),