Take the last position of a split ()

Tip:

var partes = nome.split(' ');
var formatado = [partes.pop().toUpperCase(), partes.join(' ')].join(', ');

The idea is:

• separate by spaces (as you already had)
• create an array with 2 positions
• in the first position put partes.pop().toUpperCase() (the last part with large print)
• in the second position put the rest, re-positioning the spaces
• join the parts with comma and space

Example:

function formatar() {
  var nome = document.getElementById("nome").value;

  if (nome === "" || Number(nome)) {
    alert("Digite seu nome corretamente");
    document.getElementById("nome").value = "";
    document.getElementById("nome").focus();
    return;
  }

  var partes = nome.split(' ');
  var formatado = [partes.pop().toUpperCase(), partes.join(' ')].join(', ');

  document.getElementById("formatacao").innerHTML = formatado;

}

document.querySelector("button").addEventListener('click', formatar);

input {
  width: 100%;
  padding: 10px;
}

<input id="nome" type="text" value="Antonio Manuel Antunes" />
<button>Formatar</button>
<div id="formatacao"></div>

Approach using regex:

var nome = "Jose da Silva Sauro";

function replacer(match, m1, m2){
    return m2.toUpperCase() + ', ' + m1;
}

nome.replace(/^(.+)\b(\w+)$/g, replacer); // "SAURO, Jose da Silva"

With split divide, and then get the last one with length-1 and join with the array all joined except the last one, using slice to cut the last and join to merge all again with spaces:

var nome = "Luis Carlos Teixeira";
var nomes = nome.split(" ");
var nomeFinal = nomes[nomes.length-1].toUpperCase() + ", " + nomes.slice(0,-1).join(" ");

console.log(nomeFinal);

In your code ultimo.toUpperCase() gives error because last is an array and not a string. Remember that slice returns an array.

In the functions used we have:

• split(" ") part a string in an array according to the separator past
• slice(0, -1) gets the array all but the last element
• join (" ") joins the array in question with spaces forming a new text (string)

  

I improved its function to test numeric names because if I entered 123 456 789 or Fulano de Tal 123 would accept. Credits - Sergio

function formatar() {
  var nome = document.getElementById("nome").value;
  var partes = nome.split(' ');
  var tam = (partes.length-1);

  var ultimo = partes.slice(-1);
  console.log(ultimo);
  ultimo = ultimo.toString();
  
  var HasNumericStrings = partes.filter(function(i) {
    return !isNaN(i);
  }).length > 0;

  if (HasNumericStrings) {
    alert("Digite seu nome corretamente");
    document.getElementById("nome").value = "";
    document.getElementById("nome").focus();
  } else {
    document.getElementById("formatacao").innerHTML = ultimo.toUpperCase() + ", " + partes.slice(0,-1).join(" ");
  }

}
document.querySelector("button").addEventListener('click', formatar);

<input id="nome" type="text" value="Fulano de Tal" />
<button>Formatar</button>
<div id="formatacao"></div>

• The split() method divides a String object into an array of strings by separating the string into substrings
• The% cos_de% property defines the number of elements in the array. It is a number one unit larger than the highest element defined in an array.
• The length method creates a new array with all the elements that passed the test implemented by the function provided. Read more
• The filter() method returns a string where all alphabetic characters have been converted to uppercase.

• The toUpperCase method joins all array elements within a string. join(separador) is optional. Specifies a string to separate each element from the array. The separator is converted to a string if necessary. If omitted, the array elements are separated by a comma. If the separator is an empty string, all elements are joined without any character between them.

  In the case in question, we use a space separador Read more

Only one variant of the elegant solution @ J.Guilherme (+1):

var n = "Jose da Silva Sauro";
n = n.replace(/(.+) (\w+)$/, function(_,a,b){return b.toUpperCase()+ ", "+ a});