feat: support package.json exports field

related to #17

lib/index.js

@@ -338,8 +338,11 @@ module.exports = class Bundler {
         });
       }
 
+      const parsedRequiredBy = requiredBy.map(id => parse(mapId(id, this._paths)));
+      const isLocalRequire = parsedRequiredBy.findIndex(parsed => parsed.parts[0] === packageName) !== -1;
+
       return this.packageReaderFor(stub || {name: packageName})
-        .then(reader => resource ? reader.readResource(resource) : reader.readMain())
+        .then(reader => resource ? reader.readResource(resource, isLocalRequire) : reader.readMain())
         .then(unit => this.capture(unit))
         .catch(err => {
           error('Resolving failed for module ' + bareId);

lib/package-reader.js

@@ -36,6 +36,7 @@ module.exports = class PackageReader {
         this.name = metadata.name;
         this.version = metadata.version || 'N/A';
         this.browserReplacement = _browserReplacement(metadata.browser);
+        this.exportsReplacement = _exportsReplacement(metadata.exports);
 
         return this._main(metadata)
           // fallback to "index.js" even when it's missing.
@@ -65,7 +66,7 @@ module.exports = class PackageReader {
       .then(() => this._readFile(this.mainPath));
   }
 
-  readResource(resource) {
+  readResource(resource, isLocalRequire = false) {
     return this.ensureMainPath().then(() => {
       let parts = this.parsedMainId.parts;
       let len = parts.length;
@@ -81,8 +82,48 @@ module.exports = class PackageReader {
 
         let fullResource = resParts.join('/');
 
-        const replacement = this.browserReplacement['./' + fullResource] ||
+        let replacement;
+
+        // exports subpath is designed for outside require.
+        if (!isLocalRequire) {
+          if (('./' + fullResource) in this.exportsReplacement) {
+            replacement = this.exportsReplacement['./' + fullResource];
+          } else if (('./' + fullResource + '.js') in this.exportsReplacement) {
+            replacement = this.exportsReplacement['./' + fullResource + '.js'];
+          }
+
+          if (replacement === null) {
+            throw new Error(`Resource ${this.name + '/' + resource} is not allowed to be imported (${this.name} package.json exports definition ${JSON.stringify(this.exportsReplacement)}).`);
+          }
+
+          if (!replacement) {
+            // Try wildcard replacement
+            for (const key in this.exportsReplacement) {
+              const starIndex = key.indexOf('*');
+              if (starIndex !== -1) {
+                const prefix = key.slice(2, starIndex); // remove ./
+                const subfix = key.slice(starIndex + 1);
+                if (fullResource.startsWith(prefix) && fullResource.endsWith(subfix)) {
+
+                  const target = this.exportsReplacement[key];
+                  if (target && target.includes('*')) {
+                    const flexPart = fullResource.slice(prefix.length, fullResource.length - subfix.length);
+                    replacement = target.replace('*', flexPart);
+                  } else {
+                    replacement = target;
+                  }
+                  break;
+                }
+              }
+            }
+          }
+        }
+
+        if (!replacement) {
+          replacement = this.browserReplacement['./' + fullResource] ||
           this.browserReplacement['./' + fullResource + '.js'];
+        }
+
         if (replacement) {
            // replacement is always local, remove leading ./
           fullResource = replacement.slice(2);
@@ -254,13 +295,10 @@ module.exports = class PackageReader {
   }
 
   _main(metadata, dirPath = '') {
-    // try 1.browser > 2.module > 3.main
+    // try 1. exports > 2.browser > 3.module > 4.main
     // the order is to target browser.
-    // it probably should use different order for electron app
-    // for electron 1.module > 2.browser > 3.main
     // note path.join also cleans up leading './'.
     const mains = [];
-
     if (typeof metadata.dumberForcedMain === 'string') {
       // dumberForcedMain is not in package.json.
       // it is the forced main override in dumber config,
@@ -269,6 +307,12 @@ module.exports = class PackageReader {
       // note there is no fallback to other browser/module/main fields.
       mains.push({field: 'dumberForcedMain', path: path.join(dirPath, metadata.dumberForcedMain)});
     } else {
+
+      const exportsMain = _exportsMain(metadata.exports);
+      if (typeof exportsMain === 'string') {
+        mains.push({field: 'exports', path: path.join(dirPath, exportsMain)});
+      }
+
       if (typeof metadata.browser === 'string')  {
         // use package.json browser field if possible.
         mains.push({field: 'browser', path: path.join(dirPath, metadata.browser)});
@@ -330,7 +374,10 @@ function _browserReplacement(browser) {
         // replacement is always local
         targetModule = './' + targetModule;
       }
-      replacement[sourceModule] = targetModule;
+      // Only replace when sourceModule cannot be resolved to targetModule.
+      if (!nodejsIds(sourceModule).includes(targetModule)) {
+        replacement[sourceModule] = targetModule;
+      }
     } else {
       replacement[sourceModule] = false;
     }
@@ -339,6 +386,87 @@ function _browserReplacement(browser) {
   return replacement;
 }
 
+function isExportsConditions(obj) {
+  if (typeof obj !== 'object' || obj === null) return false;
+  const keys = Object.keys(obj);
+  return keys.length > 0 && keys[0][0] !== '.';
+}
+
+function pickCondition(obj) {
+  // string or null
+  if (typeof obj !== 'object' || obj === null) return obj;
+  let env = process.env.NODE_ENV || '';
+  if (env === 'undefined') env = '';
+
+  // use env (NODE_ENV) to support "development" and "production"
+  for (const condition of ['import', 'module', 'browser', 'require', env, 'default']) {
+    // Recursive call to support nested conditions.
+    if (condition && condition in obj) return pickCondition(obj[condition]);
+  }
+
+  return null;
+}
+
+function _exportsMain(exports) {
+  // string exports field is alternative main
+  if (!exports || typeof exports === 'string') return exports;
+
+  if (isExportsConditions(exports)) {
+    return pickCondition(exports);
+  }
+
+  if (typeof exports === 'object') {
+    for (const key in exports) {
+      if (key === '.' || key !== './') {
+        return pickCondition(exports[key]);
+      }
+    }
+  }
+}
+
+function _exportsReplacement(exports) {
+  // string exports field is alternative main,
+  // leave to the main field replacment
+  if (!exports || typeof exports === 'string') return {};
+
+  if (isExportsConditions(exports)) {
+    // leave to the main field replacment
+    return {};
+  }
+
+  let replacement = {};
+
+  Object.keys(exports).forEach(key => {
+    // leave {".": ...} to the main field replacment
+    if (key === '.') return;
+
+    if (key[0] !== '.') {
+      throw new Error("Unexpected exports subpath: " + key);
+    }
+
+    let target = pickCondition(exports[key]);
+
+    let sourceModule = filePathToModuleId(key);
+
+    if (typeof target === 'string') {
+      let targetModule = filePathToModuleId(target);
+      if (!targetModule.startsWith('.')) {
+        // replacement is always local
+        targetModule = './' + targetModule;
+      }
+
+      // Only replace when sourceModule cannot be resolved to targetModule.
+      if (!nodejsIds(sourceModule).includes(targetModule)) {
+        replacement[sourceModule] = targetModule;
+      }
+    } else {
+      replacement[sourceModule] = null;
+    }
+  });
+
+  return replacement;
+}
+
 function filePathToModuleId(filePath) {
   return parse(filePath.replace(/\\/g, '/')).bareId;
 }