feat(node-resolve): support package entry points

.eslintignore

@@ -1 +1 @@
-.eslintrc.js
+.eslintrc.js

packages/node-resolve/README.md

@@ -34,16 +34,23 @@ export default {
   input: 'src/index.js',
   output: {
     dir: 'output',
-    format: 'cjs'
+    format: 'cjs',
   },
-  plugins: [nodeResolve()]
+  plugins: [nodeResolve()],
 };
 ```
 
 Then call `rollup` either via the [CLI](https://www.rollupjs.org/guide/en/#command-line-reference) or the [API](https://www.rollupjs.org/guide/en/#javascript-api).
 
 ## Options
 
+### `exportConditions`
+
+Type: `Array[...String]`<br>
+Default: `['module', 'default']`
+
+Conditions used to select exports defined using node js [package exports](https://nodejs.org/api/esm.html#esm_packages). Conditions are evaluated left to right, returning the first match that is found. Setting this property overwrites the default values.
+
 ### `browser`
 
 Type: `Boolean`<br>
@@ -164,9 +171,9 @@ export default {
   output: {
     file: 'bundle.js',
     format: 'iife',
-    name: 'MyModule'
+    name: 'MyModule',
   },
-  plugins: [resolve(), commonjs()]
+  plugins: [resolve(), commonjs()],
 };
 ```
 

packages/node-resolve/src/index.js

@@ -18,7 +18,7 @@ import {
 const builtins = new Set(builtinList);
 const ES6_BROWSER_EMPTY = '\0node-resolve:empty.js';
 const nullFn = () => null;
-const deepFreeze = object => {
+const deepFreeze = (object) => {
   Object.freeze(object);
 
   for (const value of Object.values(object)) {
@@ -30,6 +30,7 @@ const deepFreeze = object => {
   return object;
 };
 const defaults = {
+  exportConditions: ['module', 'default'],
   customResolveOptions: {},
   dedupe: [],
   // It's important that .mjs is listed before .js so that Rollup will interpret npm modules
@@ -41,7 +42,7 @@ export const DEFAULTS = deepFreeze(deepMerge({}, defaults));
 
 export function nodeResolve(opts = {}) {
   const options = Object.assign({}, defaults, opts);
-  const { customResolveOptions, extensions, jail } = options;
+  const { exportConditions, customResolveOptions, extensions, jail } = options;
   const warnings = [];
   const packageInfoCache = new Map();
   const idToPackageInfo = new Map();
@@ -220,7 +221,13 @@ export function nodeResolve(opts = {}) {
       resolveOptions = Object.assign(resolveOptions, customResolveOptions);
 
       try {
-        let resolved = await resolveImportSpecifiers(importSpecifierList, resolveOptions);
+        const warn = (...args) => this.warn(...args);
+        let resolved = await resolveImportSpecifiers(
+          importSpecifierList,
+          resolveOptions,
+          exportConditions,
+          warn
+        );
 
         if (resolved && packageBrowserField) {
           if (Object.prototype.hasOwnProperty.call(packageBrowserField, resolved)) {

packages/node-resolve/src/resolveId.js

@@ -0,0 +1,121 @@
+import fs from 'fs';
+import path from 'path';
+import { promisify } from 'util';
+
+import resolve from 'resolve';
+
+import { getPackageName } from './util';
+
+const resolveImportPath = promisify(resolve);
+const readFile = promisify(fs.readFile);
+
+const pathNotFoundError = (subPath, pkgPath) =>
+  new Error(`Package subpath '${subPath}' is not defined by "exports" in ${pkgPath}`);
+
+function mapSubPath(pkgJsonPath, subPath, value) {
+  if (typeof value === 'string') {
+    // mapping is a string, for example { "./foo": "./dist/foo.js" }
+    return value;
+  }
+
+  if (Array.isArray(value)) {
+    // mapping is an array with fallbacks, for example { "./foo": ["foo:bar", "./dist/foo.js"] }
+    return value.find((v) => v.startsWith('./'));
+  }
+
+  throw pathNotFoundError(subPath, pkgJsonPath);
+}
+
+function findEntrypoint(pkgJsonPath, subPath, exportMap, conditions) {
+  if (typeof exportMap !== 'object') {
+    return mapSubPath(pkgJsonPath, subPath, exportMap);
+  }
+
+  // iterate conditions recursively, find the first that matches all conditions
+  for (const [condition, subExportMap] of Object.entries(exportMap)) {
+    if (conditions.includes(condition)) {
+      const mappedSubPath = findEntrypoint(pkgJsonPath, subPath, subExportMap, conditions);
+      if (mappedSubPath) {
+        return mappedSubPath;
+      }
+    }
+  }
+  throw pathNotFoundError(subPath, pkgJsonPath);
+}
+
+export function findEntrypointTopLevel(pkgJsonPath, subPath, exportMap, conditions) {
+  if (typeof exportMap !== 'object') {
+    // the export map shorthand, for example { exports: "./index.js" }
+    if (subPath !== '.') {
+      // shorthand only supports a main entrypoint
+      throw pathNotFoundError(subPath, pkgJsonPath);
+    }
+    return mapSubPath(pkgJsonPath, subPath, exportMap);
+  }
+
+  // export map is an object, the top level can be either conditions or sub path mappings
+  const keys = Object.keys(exportMap);
+  const isConditions = keys.every((k) => !k.startsWith('.'));
+  const isMappings = keys.every((k) => k.startsWith('.'));
+
+  if (!isConditions && !isMappings) {
+    throw new Error(
+      `Invalid package config ${pkgJsonPath}, "exports" cannot contain some keys starting with '.'` +
+        ' and some not. The exports object must either be an object of package subpath keys or an object of main entry' +
+        ' condition name keys only.'
+    );
+  }
+
+  let exportMapForSubPath;
+
+  if (isConditions) {
+    // top level is conditions, for example { "import": ..., "require": ..., "module": ... }
+    if (subPath !== '.') {
+      // package with top level conditions means it only supports a main entrypoint
+      throw pathNotFoundError(subPath, pkgJsonPath);
+    }
+    exportMapForSubPath = exportMap;
+  } else {
+    // top level is sub path mappings, for example { ".": ..., "./foo": ..., "./bar": ... }
+    if (!(subPath in exportMap)) {
+      throw pathNotFoundError(subPath, pkgJsonPath);
+    }
+    exportMapForSubPath = exportMap[subPath];
+  }
+
+  return findEntrypoint(pkgJsonPath, subPath, exportMapForSubPath, conditions);
+}
+
+export default async function resolveId(importPath, options, exportConditions, warn) {
+  const pkgName = getPackageName(importPath);
+  if (pkgName) {
+    let pkgJsonPath;
+    let pkgJson;
+    try {
+      pkgJsonPath = await resolveImportPath(`${pkgName}/package.json`, options);
+      pkgJson = JSON.parse(await readFile(pkgJsonPath, 'utf-8'));
+    } catch (_) {
+      // if there is no package.json we defer to regular resolve behavior
+    }
+
+    if (pkgJsonPath && pkgJson && pkgJson.exports) {
+      try {
+        const packageSubPath =
+          pkgName === importPath ? '.' : `.${importPath.substring(pkgName.length)}`;
+        const mappedSubPath = findEntrypointTopLevel(
+          pkgJsonPath,
+          packageSubPath,
+          pkgJson.exports,
+          exportConditions
+        );
+        const pkgDir = path.dirname(pkgJsonPath);
+        return path.join(pkgDir, mappedSubPath);
+      } catch (error) {
+        warn(error);
+        return null;
+      }
+    }
+  }
+
+  return resolveImportPath(importPath, options);
+}

packages/node-resolve/src/util.js

@@ -1,14 +1,11 @@
 import { dirname, extname, resolve } from 'path';
-import { promisify } from 'util';
 
 import { createFilter } from '@rollup/pluginutils';
 
-import resolveModule from 'resolve';
+import resolveId from './resolveId';
 
 import { realpathSync } from './fs';
 
-const resolveId = promisify(resolveModule);
-
 // returns the imported package name for bare module imports
 export function getPackageName(id) {
   if (id.startsWith('.') || id.startsWith('/')) {
@@ -161,7 +158,12 @@ export function normalizeInput(input) {
 
 // Resolve module specifiers in order. Promise resolves to the first module that resolves
 // successfully, or the error that resulted from the last attempted module resolution.
-export function resolveImportSpecifiers(importSpecifierList, resolveOptions) {
+export function resolveImportSpecifiers(
+  importSpecifierList,
+  resolveOptions,
+  exportConditions,
+  warn
+) {
   let promise = Promise.resolve();
 
   for (let i = 0; i < importSpecifierList.length; i++) {
@@ -171,12 +173,14 @@ export function resolveImportSpecifiers(importSpecifierList, resolveOptions) {
         return value;
       }
 
-      return resolveId(importSpecifierList[i], resolveOptions).then((result) => {
-        if (!resolveOptions.preserveSymlinks) {
-          result = realpathSync(result);
+      return resolveId(importSpecifierList[i], resolveOptions, exportConditions, warn).then(
+        (result) => {
+          if (!resolveOptions.preserveSymlinks) {
+            result = realpathSync(result);
+          }
+          return result;
         }
-        return result;
-      });
+      );
     });
 
     if (i < importSpecifierList.length - 1) {

packages/node-resolve/test/fixtures/exports-mappings-and-conditions.js

@@ -0,0 +1,5 @@
+import main from 'exports-mappings-and-conditions';
+import foo from 'exports-mappings-and-conditions/foo';
+import bar from 'exports-mappings-and-conditions/bar';
+
+export default { main, foo, bar };

packages/node-resolve/test/fixtures/exports-nested-conditions.js

@@ -0,0 +1,3 @@
+import main from 'exports-nested-conditions';
+
+export default main;

packages/node-resolve/test/fixtures/exports-non-existing-subpath.js

@@ -0,0 +1,3 @@
+import bar from 'exports-non-existing-subpath/bar';
+
+export default bar;

packages/node-resolve/test/fixtures/exports-prevent-unspecified-subpath.js

@@ -0,0 +1,3 @@
+import bar from 'exports-top-level-mappings/bar';
+
+export default bar;

packages/node-resolve/test/fixtures/exports-shorthand-fallback.js

@@ -0,0 +1,3 @@
+import exportsMapEntry from 'exports-shorthand';
+
+export default exportsMapEntry;

packages/node-resolve/test/fixtures/exports-shorthand-subpath.js

@@ -0,0 +1,3 @@
+import exportsMapEntry from 'exports-shorthand/foo';
+
+export default exportsMapEntry;

packages/node-resolve/test/fixtures/exports-shorthand.js

@@ -0,0 +1,3 @@
+import exportsMapEntry from 'exports-shorthand';
+
+export default exportsMapEntry;

packages/node-resolve/test/fixtures/exports-top-level-conditions.js

@@ -0,0 +1,3 @@
+import main from 'exports-top-level-conditions';
+
+export default main;

packages/node-resolve/test/fixtures/exports-top-level-mappings.js

@@ -0,0 +1,4 @@
+import main from 'exports-top-level-mappings';
+import foo from 'exports-top-level-mappings/foo';
+
+export default { main, foo };