better fetch() argument detection

lib/imap.js

@@ -930,7 +930,8 @@ ImapConnection.prototype.fetch = function(uids, options, what, cb) {
 };
 
 ImapConnection.prototype._fetch = function(which, uids, options, what, cb) {
-  if (uids === undefined || uids === null
+  if (uids === undefined
+      || uids === null
       || (Array.isArray(uids) && uids.length === 0))
     throw new Error('Nothing to fetch');
 
@@ -939,19 +940,53 @@ ImapConnection.prototype._fetch = function(which, uids, options, what, cb) {
   utils.validateUIDList(uids);
 
   var toFetch = '', prefix = ' BODY[', extensions, self = this,
-      parse = true, headers, key, stream,
+      parse, headers, key, stream,
       fetchers = {};
 
-  if (typeof what === 'function') {
-    cb = what;
-    what = options;
-    options = {};
+  // argument detection!
+  if (cb === undefined) {
+    // fetch(uids, xxxx, yyyy)
+    if (what === undefined) {
+      // fetch(uids, xxxx)
+      if (options === undefined) {
+        // fetch(uids)
+        what = options = {};
+      } else if (typeof options === 'function') {
+        // fetch(uids, callback)
+        cb = options;
+        what = options = {};
+      } else if (options.struct !== undefined
+               || options.size !== undefined
+               || options.markSeen !== undefined) {
+        // fetch(uids, options)
+        what = {};
+      } else {
+        // fetch(uids, what)
+        what = options;
+        options = {};
+      }
+    } else if (typeof what === 'function') {
+      // fetch(uids, xxxx, callback)
+      cb = what;
+      if (options.struct !== undefined
+          || options.size !== undefined
+          || options.markSeen !== undefined) {
+        // fetch(uids, options, callback)
+        what = {};
+      } else {
+        // fetch(uids, what, callback)
+        what = options;
+        options = {};
+      }
+    }
   }
+
   if (!Array.isArray(what))
     what = [what];
 
   for (var i = 0, wp, pprefix, len = what.length; i < len; ++i) {
     wp = what[i];
+    parse = true;
     if (wp.id !== undefined && !/^(?:[\d]+[\.]{0,1})*[\d]+$/.test(''+wp.id))
       throw new Error('Invalid part id: ' + wp.id);
     if (( (wp.headers
@@ -1153,7 +1188,7 @@ ImapConnection.prototype._fetch = function(which, uids, options, what, cb) {
       for (f = 0, lenf = fetches.length; f < lenf; ++f)
         fetches[f].emit('end');
     }
-    cb(err);
+    cb&&cb(err);
   });
 
   this._state.requests[this._state.requests.length - 1].fetchers = fetchers;