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const list = [
  {
    "a": "2",
    "ss": [
      {
        "name": "pourush",
        "age": "25"
      },
      {
        "name": "Mak",
        "age": "25"
      }
    ]
  },
  {
    "b": "3",
    "ss": [
      {
        "name": "James",
        "age": "19"
      },
      {
        "name": "shaun",
        "age": "20"
      }
    ]
  }
]

function scan(list, key) {
  return list.reduce((a, b) => {
    let obj = b[key] || []
    return a.concat(obj)
  }, [])
}

console.log(scan(list, 'ss'))

This is more dynamic. It also works with "a" or "b".