How to remove key and value undefined in object?

Question

how to remove key and value that are undefined from object?

for example I have

var x = {
 firstname: undefined,
 lastname: 'blabla'
}

how to get object without undefined? in the most efficient and quick way?

the result should be:

x = {
 lastname: 'blabla'
}

Answer 1

var x = {
 firstname: undefined,
 lastname: 'blabla'
}

for (let key in x) {
  if (x[key] === undefined) {
   delete x[key];
  }
}