made checks explicit

Author: bekzod

lib/es6-promise/then.js

@@ -21,11 +21,11 @@ export default function then(onFulfillment, onRejection) {
 
   const { _state } = parent;
 
-  if (_state) {
-    const callback = arguments[_state - 1];
-    asap(() => invokeCallback(_state, child, callback, parent._result));
-  } else {
+  if (_state === PENDING) {
     subscribe(parent, child, onFulfillment, onRejection);
+  } else {
+    const callback = _state === FULFILLED ? onFulfillment : onRejection;
+    asap(() => invokeCallback(_state, child, callback, parent._result));
   }
 
   return child;