Extract resolvePreset method to babel-core public API

This encapsulates the logic for turning an acceptable preset name into
the absolute path for that preset. It can be used to preprocess a
presets list to map each preset to its absolute path, which is necessary
if `babel.transform` is going to be executed on a file outside the
directory subtree where the presets are installed.

This adds a getPossiblePresetNames helper encapsulating the logic for
what preset names we should try to resolve, and the resolvePreset method
just calls this helper and actually resolves them.

packages/babel-core/src/api/node.js

@@ -6,6 +6,7 @@ export { default as options } from "../transformation/file/options/config";
 export { default as buildExternalHelpers } from "../tools/build-external-helpers";
 export { default as template } from "babel-template";
 export { default as resolvePlugin } from "../helpers/resolve-plugin";
+export { default as resolvePreset } from "../helpers/resolve-preset";
 export { version } from "../../package";
 
 import * as util from "../util";

packages/babel-core/src/helpers/get-possible-preset-names.js

@@ -0,0 +1,13 @@
+export default function getPossiblePresetNames(presetName: string): Array<string> {
+  let possibleNames = [`babel-preset-${presetName}`, presetName];
+
+  // trying to resolve @organization shortcat
+  // @foo/es2015 -> @foo/babel-preset-es2015
+  let matches = presetName.match(/^(@[^/]+)\/(.+)$/);
+  if (matches) {
+    let [, orgName, presetPath] = matches;
+    possibleNames.push(`${orgName}/babel-preset-${presetPath}`);
+  }
+
+  return possibleNames;
+}

packages/babel-core/src/helpers/resolve-preset.js

@@ -0,0 +1,6 @@
+import resolve from "./resolve";
+import getPossiblePresetNames from "./get-possible-preset-names";
+
+export default function resolvePreset(presetName: string, dirname: string = process.cwd()): ?string {
+  return getPossiblePresetNames(presetName).reduce((accum, curr) => accum || resolve(curr, dirname), null);
+}

packages/babel-core/src/transformation/file/options/option-manager.js

@@ -5,8 +5,8 @@ import type Logger from "../logger";
 import Plugin from "../../plugin";
 import * as messages from "babel-messages";
 import { normaliseOptions } from "./index";
-import resolve from "../../../helpers/resolve";
 import resolvePlugin from "../../../helpers/resolve-plugin";
+import resolvePreset from "../../../helpers/resolve-preset";
 import cloneDeepWith from "lodash/cloneDeepWith";
 import clone from "lodash/clone";
 import merge from "../../../helpers/merge";
@@ -262,18 +262,7 @@ export default class OptionManager {
       let presetLoc;
       try {
         if (typeof val === "string") {
-          presetLoc = resolve(`babel-preset-${val}`, dirname) || resolve(val, dirname);
-
-          // trying to resolve @organization shortcat
-          // @foo/es2015 -> @foo/babel-preset-es2015
-          if (!presetLoc) {
-            let matches = val.match(/^(@[^/]+)\/(.+)$/);
-            if (matches) {
-              let [, orgName, presetPath] = matches;
-              val = `${orgName}/babel-preset-${presetPath}`;
-              presetLoc = resolve(val, dirname);
-            }
-          }
+          presetLoc = resolvePreset(val, dirname);
 
           if (!presetLoc) {
             throw new Error(`Couldn't find preset ${JSON.stringify(val)} relative to directory ` +

packages/babel-core/test/api.js

@@ -118,6 +118,10 @@ describe("api", function () {
     assert.equal(babel.resolvePlugin("nonexistent-plugin"), null);
   });
 
+  it("exposes the resolvePreset method", function() {
+    assert.equal(babel.resolvePreset("nonexistent-preset"), null);
+  });
+
   it("transformFile", function (done) {
     babel.transformFile(__dirname + "/fixtures/api/file.js", {}, function (err, res) {
       if (err) return done(err);
@@ -266,7 +270,7 @@ describe("api", function () {
           presets: ["@babel/es2015"]
         });
       },
-      /Couldn\'t find preset \"\@babel\/babel\-preset\-es2015\" relative to directory/
+      /Couldn\'t find preset \"\@babel\/es2015\" relative to directory/
     );
   });
 
@@ -277,7 +281,7 @@ describe("api", function () {
           presets: ["@babel/react/optimizations"]
         });
       },
-      /Couldn\'t find preset \"\@babel\/babel\-preset\-react\/optimizations\" relative to directory/
+      /Couldn\'t find preset \"\@babel\/react\/optimizations\" relative to directory/
     );
   });
 

packages/babel-core/test/get-possible-preset-names.js

@@ -0,0 +1,22 @@
+let assert                 = require("assert");
+let getPossiblePresetNames = require("../lib/helpers/get-possible-preset-names");
+
+describe("getPossiblePresetNames", function () {
+  it("adds the babel-preset prefix", function() {
+    assert.deepEqual(getPossiblePresetNames("foobar"), ["babel-preset-foobar", "foobar"]);
+  });
+
+  it("inserts babel-preset after @org/", function() {
+    assert.deepEqual(getPossiblePresetNames("@babel/es2015"), [
+      "babel-preset-@babel/es2015",
+      "@babel/es2015",
+      "@babel/babel-preset-es2015"
+    ]);
+
+    assert.deepEqual(getPossiblePresetNames("@babel/react/optimizations"), [
+      "babel-preset-@babel/react/optimizations",
+      "@babel/react/optimizations",
+      "@babel/babel-preset-react/optimizations"
+    ]);
+  });
+});