var users = [
{firstname : 'Jack',
lastname : 'Black',
age: 47},

{firstname : 'Ty',
lastname : 'Simpkins',
age: 15},

{firstname : 'Finn',
lastname : 'Wolfhard',
age: 14},

{firstname : 'David',
lastname : 'Mazouz',
age: 16},

{firstname : 'Tom',
lastname : 'Holland',
age: 21},

{firstname : 'Margot',
lastname : 'Robbie',
age: 27},

{firstname : 'Gal',
lastname : 'Gadot',
age: 32},

{firstname : 'Jannifer',
lastname : 'Lawrence',
age: 26},

{firstname : 'Millie',
lastname : 'Bobby Brown',
age: 13},

{firstname : 'Clare',
lastname : 'Foley',
age: 15}
];

//1
var teensAge = users.filter(function(users) {
	return users.age < 18;
});
console.log(teensAge);
//2

users.forEach(function(item, i, users) {
	item.fullname = item.firstname + ' ' + item.lastname;
	return item.fullname;
});
//3
var arrFullname = users.map(function(users) {
	return users.fullname;
});
console.log(arrFullname);