function updateRecords(records, id, prop, value) {
  if (value === '') {
    delete records[id][prop];
  } else if (prop !== 'tracks') {
    records[id][prop] = value;
  } else {
    records[id][prop] = records[id][prop] || [];    // <= Short-circuit evaluation
    records[id][prop].push(value);
  }
  return records;
}

短絡評価（Short-circuit evaluation）を利用すれば、下のif文が1行で書ける。

if (!records[id][prop]) {
    records[id][prop] = [];
}
records[id][prop].push(value);