// INPUT
let people = [
    {
      "firstname": "John",
      "lastname": "Doe",
      "age": 20,
      "address": {
        "city": "San Francisco",
        "state": "CA"
      }
    },
    {
      "firstname": "Bob",
      "lastname": "Lee",
      "age": 30,
      "address": {
        "city": "New York",
        "state": "NY"
      }
    },
  ];

// COMBINE FIRST AND LAST NAME INTO NEW JSON PROPERTY
results = people.map(function (currentValue, index, arr) {
  // add a property to each object
  currentValue.fullname = currentValue.firstname + " " + currentValue.lastname;
  return currentValue;
});

// OUTPUT
/*[
    {
      "firstname": "John",
      "lastname": "Doe",
      "age": 20,
      "address": {
        "city": "San Francisco",
        "state": "CA"
      },
      "fullname": "John Doe"
    },
    {
      "firstname": "Bob",
      "lastname": "Lee",
      "age": 30,
      "address": {
        "city": "New York",
        "state": "NY"
      },
      "fullname": "Bob Lee"
    },
]*/