add support for package.json exports

lib/async.js

@@ -309,6 +309,12 @@ module.exports = function resolve(x, options, callback) {
                         return;
                     }
 
+                    if (pkg && pkg.exports) {
+                        var res = resolveExports(pkg.exports, ['require', 'node'], '.');
+                        var exportPath = path.resolve(x, res);
+                        return loadAsFile(exportPath, pkg, cb);
+                    }
+
                     loadAsFile(path.join(x, '/index'), pkg, cb);
                 });
             });

lib/resolve-exports.js

@@ -0,0 +1,40 @@
+// Just walks the exports object synchronously looking for a match.
+// Does not validate that the module it finds actually exists.
+
+// Returns undefined if no match was found, null if a match was explicitly
+// forbidden by setting the value to null in the exports object. Either
+// null or undefined at the caller value have the same effective meaning,
+// no match is available.
+var resolveExports = function resolveExports (exp, conditions, sub) {
+    if (!exp) return exp;
+    if (typeof exp === 'string' && (sub === '.' || sub === null)) return exp;
+    // TODO: check if this should throw?
+    if (typeof exp !== 'object') return null;
+    if (Array.isArray(exp) && (sub === '.' || sub === null)) {
+        for (var i = 0; i < exp.length; i++) {
+            var resolved = resolveExports(exp[i], conditions);
+            if (resolved || resolved === null) return resolved;
+        }
+    }
+    if (sub !== null) {
+        if (Object.prototype.hasOwnProperty.call(exp, sub)) {
+            return resolveExports(exp[sub], conditions, null);
+        } else if (sub !== '.') {
+            // sub=./x, exports={require:'./y'}, not a match
+            return undefined;
+        }
+    }
+    var keys = Object.keys(exp);
+    for (var i = 0; i < keys.length; i++) {
+        var key = keys[i];
+        if (key === 'default') return resolveExports(exp[key], conditions, null);
+        var k = conditions.indexOf(key);
+        if (k !== -1) {
+            const resolved = resolveExports(exp[key], conditions, null);
+            if (resolved || resolved === null) return resolved;
+        }
+    }
+    return undefined;
+}
+
+module.exports = resolveExports;

lib/sync.js

@@ -5,6 +5,7 @@ var getHomedir = require('./homedir');
 var caller = require('./caller');
 var nodeModulesPaths = require('./node-modules-paths');
 var normalizeOptions = require('./normalize-options');
+var resolveExports = require('./resolve-exports');
 
 var realpathFS = process.platform !== 'win32' && fs.realpathSync && typeof fs.realpathSync.native === 'function' ? fs.realpathSync.native : fs.realpathSync;
 
@@ -199,6 +200,12 @@ module.exports = function resolveSync(x, options) {
                 incorrectMainError.code = 'INCORRECT_PACKAGE_MAIN';
                 throw incorrectMainError;
             }
+
+            if (pkg && pkg.exports) {
+                var res = resolveExports(pkg.exports, ['require', 'node'], '.');
+                var exportPath = path.resolve(x, res);
+                return loadAsFileSync(exportPath);
+            }
         }
 
         return loadAsFileSync(path.join(x, '/index'));