Add + 1 to the current number (Jquery)

This happens when you mixes String and numbers with the + operator that is sum but also concatenation. JavaScript uses the operator as "sum" or as concatenation depending on the operands.

function novo_id(lista) {
  var id = 0;
  //console.log(id);
  if (lista.length > 0) {
    for (var i = 0; i < lista.length; i++) {
      var _id = Number(lista[i].id);
      if (_id > id) id = _id
    }
    id = id + 1;
  }
  return (id == 0 ? 1 : id);
}

console.log(novo_id([{id: '20'}]));

Another way to do this even simpler would be like this:

function novo_id(lista) {
  const ids = lista.map(el => Number(el.id));
  return Math.max.apply(Math, ids) + 1;
}

console.log(novo_id([{id: '230'}, {id: '10'}])); // 231

The parameter must be arriving at function novo_id(lista) as string . Use the parseInt function before the sum:

function novo_id(lista) {
    var id = 0;
    //console.log(id);
    if (lista.length > 0) {     
        for (var i = 0; i < lista.length; i++) {
            if (lista[i].id > id) {
                id = lista[i].id
            }
        }
        id = parseInt(id) + 1;
    }
    return (id == 0 ? 1 : id);
}