How do I make an array from specific keys in another array?

[
 - {
    - 0: {
          firstname: "John",
          lastname: "Johnson",
          age: "34",
          etc: "",
          etc: "",
          etc: ""
      },
    - 1: {
          firstname: "David",
          lastname: "Davidson",
          age: "45",
          etc: "",
          etc: "",
          etc: ""
      },
    - 2: {
          firstname: "Peter",
          lastname: "Peterson",
          age: "56",
          etc: "",
          etc: "",
          etc: ""

      },
    - 3: {
          firstname: "Robert",
          lastname: "Robertson",
          age: "67",
          etc: "",
          etc: "",
          etc: ""
        }
    }
]