How to merge properties of two JavaScript objects and prefer not null values?

function merge(obj1, obj2) {
  answer = {}
  for(key in obj1) {
    if(answer[key] === undefined || answer[key] === null)
      answer[key] = obj1[key];
  }
  for(key in obj2) {
    if(answer[key] === undefined || answer[key] === null)
      answer[key] = obj2[key];
  }
  return answer
}