That's true, I could perhaps inline the call to getDescriptorsAddedInConstructor in getDescriptors?
Would that be better, something like this.

function getDescriptors(mockObject) {
	var descriptors = getDescriptorsAddedInConstructor();

    for(var methodName in mockObject) {
      descriptors[methodName] = { enumerable: true, configurable: true, writable: true, value: null };
    }

    return descriptors;
 }

 Making the call this,

 var descriptors = getDescriptors( objectToExtend );

 Does that seems better?