ids.js

const obj = [{
  pid: 'p1',
  bid: 'b1',
  qid: 'q1',
}, {
  pid: 'p1',
  bid: 'b2',
  qid: 'q2',
}, {
  pid: 'p1',
  bid: 'b2',
  qid: 'q3',
}, {
  pid: 'p4',
  bid: 'b4',
  qid: 'q4',
}];


let ret = [];
for (let e of obj) {
  let pi = ret.findIndex(t => t.pid == e.pid);
  if (pi == -1) {
    ret.push({pid: e.pid});
    pi = ret.length - 1;
  }

  const po = ret[pi];
  if (!po.bos) {
    po.bos = [];
  }
  let bi = po.bos.findIndex(t => t.bid == e.bid);
  if (bi == -1) {
    po.bos.push({bid: e.bid});
    bi = po.bos.length - 1;
  }

  const bo = po.bos[bi];
  if (!bo.qids) {
    bo.qids = [];
  }

  bo.qids.push(e);
}

console.log(`ret: ` + JSON.stringify(ret, null, 4) + '\n')