var users = [
   {
      role: 'admin',
      email: 'fakeadmin1@email.com'
   },
   {
      role: 'user',
      email: 'fake2@email.com'
   },
   {
      role: 'admin',
      email: 'fakeadmin2@email.com'
   },
];
function getUsers(master_user_list, wherefunc) {
   var user_list = [];
   for ( var i = 0; i < master_user_list.length; i++ ) {
       if ( wherefunc(master_user_list[i]) )
           user_list.push(master_user_list[i]);
   }
   return user_list;
}

var admin_emails = getUsers(users,function (u) { 
    return u.role == 'admin'; 
}).map(function (u) { 
    return u.email; 
});
console.log(admin_emails);