var personNames = []; 

​

var persons = [{"firstname": "john", "id": "111"}, {"firstname": "doug", "id": "777"}, {"firstname": "smith", "id": "888"}];

​

if(persons.length > 0){

  persons.forEach(function(person){

    //use personName locally

    var personName = {};

    personName.name = person.firstname;

​

    personNames.push(personName);

  });

}

​

console.log(personNames);

​

​