Http PUT deletes data that hasn’t been changed

I am trying to edit a JSON data base of shifts. I am writing in Javascript using react. This is my understanding of the PUT syntax:

const editShift = async (changed, id) => {

  const res = await fetch(`http://localhost:5000/shifts/${id}`, {
    method: 'PUT', 
    headers: {
    'Content-type': 'application/json' 
    },
    body: JSON.stringify(changed)
 })
 const data = await res.json()
 setShifts([...shifts, data])
}

data.json:

{
  "shifts": [
    {
      "title": "test",
      "startDate": "2018-06-25T07:30:00.000Z",
      "endDate": "2018-06-25T08:00:00.000Z",
      "allDay": false,
      "id": 1
    },
    {
      "title": "test2",
      "startDate": "2018-06-28T07:30:00.000Z",
      "endDate": "2018-06-28T08:00:00.000Z",
      "allDay": false,
      "id": 2
    }
  ]
}

The result is that the new shift will hold only the fields that have been changed and delete the rest. Any ideas why?

Answer

I had to send the whole updated shift into the changed field, with the fields that were not changed. I don’t know why but it works so okay.