Fix module identity preservation with symlinks and browser field resolution

e2e/Utils.ts

@@ -90,6 +90,25 @@ export const writeFiles = (
   });
 };
 
+export const writeSymlinks = (
+  directory: string,
+  symlinks: {[existingFile: string]: string},
+) => {
+  createDirectory(directory);
+  Object.keys(symlinks).forEach(fileOrPath => {
+    const symLinkPath = symlinks[fileOrPath];
+    const dirname = path.dirname(symLinkPath);
+
+    if (dirname !== '/') {
+      createDirectory(path.join(directory, dirname));
+    }
+    fs.symlinkSync(
+      path.resolve(directory, ...fileOrPath.split('/')),
+      path.resolve(directory, ...symLinkPath.split('/')),
+    );
+  });
+};
+
 const NUMBER_OF_TESTS_TO_FORCE_USING_WORKERS = 25;
 /**
  * Forces Jest to use workers by generating many test files to run.

e2e/__tests__/resolveBrowserField.test.ts

@@ -0,0 +1,70 @@
+/**
+ * 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 {tmpdir} from 'os';
+import * as path from 'path';
+import {cleanup, writeFiles, writeSymlinks} from '../Utils';
+import runJest from '../runJest';
+
+const DIR = path.resolve(tmpdir(), 'resolve-browser-field-test');
+
+beforeEach(() => cleanup(DIR));
+afterAll(() => cleanup(DIR));
+
+test('preserves module identity for symlinks when using browser field resolution', () => {
+  /* eslint-disable sort-keys */
+  writeFiles(DIR, {
+    'packages/needs-preserved-id/index.js': `
+      console.log("needs-preserved-id executed");
+      module.exports = {};
+    `,
+    'packages/needs-preserved-id/package.json': JSON.stringify({
+      name: 'needs-preserved-id',
+    }),
+    'packages/has-browser-field/package.json': JSON.stringify({
+      name: 'has-browser-field',
+      browser: 'browser.js',
+    }),
+    'packages/has-browser-field/browser.js': `
+      module.exports = require("needs-preserved-id");
+    `,
+    'package.json': JSON.stringify({
+      jest: {
+        testMatch: ['<rootDir>/test-files/test.js'],
+        browser: true,
+      },
+    }),
+    'test-files/test.js': `
+      const id1 = require("needs-preserved-id");
+      const id2 = require("has-browser-field");
+
+      test("module should have reference equality", () => {
+        expect(id1).toBe(id2);
+      });
+    `,
+  });
+  /* eslint-enable sort-keys */
+
+  writeSymlinks(DIR, {
+    'packages/has-browser-field': 'node_modules/has-browser-field',
+    'packages/needs-preserved-id': 'node_modules/needs-preserved-id',
+  });
+
+  writeSymlinks(DIR, {
+    'packages/needs-preserved-id':
+      'packages/has-browser-field/node_modules/needs-preserved-id',
+  });
+
+  const {stdout, stderr, exitCode} = runJest(DIR, ['--no-watchman']);
+  expect(stderr).toContain('Test Suites: 1 passed, 1 total');
+  expect(stdout).toMatchInlineSnapshot(`
+    "  console.log packages/needs-preserved-id/index.js:2
+        needs-preserved-id executed
+    "
+  `);
+  expect(exitCode).toEqual(0);
+});

packages/jest-resolve/src/defaultResolver.ts

@@ -8,6 +8,7 @@
 import * as fs from 'fs';
 import * as path from 'path';
 import {sync as browserResolve} from 'browser-resolve';
+import {sync as realpath} from 'realpath-native';
 import pnpResolver from 'jest-pnp-resolver';
 import {Config} from '@jest/types';
 import isBuiltinModule from './isBuiltinModule';
@@ -35,14 +36,20 @@ export default function defaultResolver(
 
   const resolve = options.browser ? browserResolve : resolveSync;
 
-  return resolve(path, {
+  let result = resolve(path, {
     basedir: options.basedir,
     defaultResolver,
     extensions: options.extensions,
     moduleDirectory: options.moduleDirectory,
     paths: options.paths,
     rootDir: options.rootDir,
   });
+  if (options.browser && result) {
+    // Dereference symlinks to ensure we don't create a separate
+    // module instance depending on how it was referenced.
+    result = realpath(result);
+  }
+  return result;
 }
 
 export const clearDefaultResolverCache = () => {
@@ -101,7 +108,7 @@ function resolveSync(
     if (result) {
       // Dereference symlinks to ensure we don't create a separate
       // module instance depending on how it was referenced.
-      result = fs.realpathSync(result);
+      result = realpath(result);
     }
     return result;
   }