var url = 'http://localhost:8000/employees/5';

fetch (url,{
    method: "put",
    body: JSON.stringify({
        id: 5,
        fullname: 'Parkin Ede',
        city: 'Kátrány Levente',
        salary: 2175000
    }),
    headers: {
        "Content-Type": "application/json"
    }
})
.then(response => response.json())
.then(result => {
    console.log(result);
}).catch(err => {
    console.log(err);
});