feat(node-resolve): support package entry points

packages/node-resolve/src/resolveId.js

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+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 ${pkgPath}`);
+
+function findExportKeyMatch(keys, subPath) {
+  return keys.find((key) => (key.endsWith('/') ? subPath.startsWith(key) : key === subPath));
+}
+
+function mapSubPath(pkgJsonPath, subPath, key, value) {
+  if (typeof value === 'string') {
+    if (value.endsWith('/')) {
+      return `${value}${subPath.substring(key.length)}`;
+    }
+    // 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, key) {
+  if (typeof exportMap !== 'object') {
+    return mapSubPath(pkgJsonPath, subPath, key, 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, key);
+      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, null, 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 ${pkgJsonPath}, "exports" cannot contain some keys starting with '.'` +
+        ' and some not. The exports object must either be an object of package subpath keys or an object of main entry' +
+        ' condition name keys only.'
+    );
+  }
+
+  let key = null;
+  let exportMapForSubPath;
+
+  if (isConditions) {
+    // top level is conditions, for example { "import": ..., "require": ..., "module": ... }
+    if (subPath !== '.') {
+      // package with top level conditions means it only supports a main entrypoint
+      throw pathNotFoundError(subPath, pkgJsonPath);
+    }
+    exportMapForSubPath = exportMap;
+  } else {
+    // top level is sub path mappings, for example { ".": ..., "./foo": ..., "./bar": ... }
+    key = findExportKeyMatch(keys, subPath);
+    if (!key) {
+      throw pathNotFoundError(subPath, pkgJsonPath);
+    }
+    exportMapForSubPath = exportMap[key];
+  }
+
+  return findEntrypoint(pkgJsonPath, subPath, exportMapForSubPath, conditions, key);
+}
+
+export default async function resolveId(importPath, options, exportConditions, warn) {
+  const pkgName = getPackageName(importPath);
+  if (pkgName) {
+    let pkgJsonPath;
+    let pkgJson;
+    try {
+      pkgJsonPath = await resolveImportPath(`${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/test/fixtures/exports-conditions-fallback.js

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+import main from 'exports-conditions-fallback';
+
+export default main;

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

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+import a from 'exports-directory/foo/a.js';
+import b from 'exports-directory/foo/b.js';
+import c from 'exports-directory/foo/nested/c.js';
+
+export default { a, b, c };

packages/node-resolve/test/fixtures/exports-main-directory.js

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+import a from 'exports-main-directory/a.js';
+import b from 'exports-main-directory/foo/b.js';
+import c from 'exports-main-directory/foo/nested/c.js';
+
+export default { a, b, c };

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

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+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

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+import main from 'exports-nested-conditions';
+
+export default main;

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

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+import bar from 'exports-non-existing-subpath/bar';
+
+export default bar;

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

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+import bar from 'exports-top-level-mappings/bar';
+
+export default bar;

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

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+import exportsMapEntry from 'exports-shorthand';
+
+export default exportsMapEntry;

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

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+import exportsMapEntry from 'exports-shorthand/foo';
+
+export default exportsMapEntry;

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

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+import exportsMapEntry from 'exports-shorthand';
+
+export default exportsMapEntry;

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

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+import main from 'exports-top-level-conditions';
+
+export default main;

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

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