arr ids

const arrIds = [
  { ids: ["id1", "id2"] },
  { ids: ["id3", "id4"] },
  { ids: ["id5"] },
  { ids: ["id6", "id7"] }
] // id2_id4_id6

const f = arr =>
  arr
    .map({ids} => ids)
    .filter(ids => ids.length > 1)
    .reduce((acc, ids) => acc.concat(ids), [])
    .filter(id => Number(id.slice(2)) % 2 === 0)
    .join("_")