var notas = [
  {
    num: 6115,
    flag: 2
  },
  {
    num: 6116,
    flag: 2
  },
  {
    num: 6117,
    flag: 1
  },
  {
    num: 6118,
    flag: 1
  },
  {
    num: 6119,
    flag: 2
  },
  {
    num: 6120,
    flag: 1
  }
];
var i = 0;
var flag = notas[0].flag;
var intervalo = [];
while(i < notas.length){
  if(flag !== notas[i].flag){
    console.log(intervalo[0] + "|-" + intervalo[intervalo.length-1]);
    intervalo.length = 0;
    intervalo.push(notas[i].num);
  } else {
    intervalo.push(notas[i].num);
  }
  flag = notas[i].flag;
  i++;
  if((i === notas.length) && (intervalo.length > 0)){
    console.log(intervalo[0] + "|-" + intervalo[intervalo.length-1]);
  }
}