1var arr = [{ id: 1, name: 'JOHN' }, 
2  { id: 2, name: 'JENNIE'}, 
3  { id: 3, name: 'JENNAH' }];
4
5function userExists(name) {
6  return arr.some(function(el) {
7    return el.name === name;
8  }); 
9}
10
11console.log(userExists('JOHN')); // true
12console.log(userExists('JUMBO')); // false

1var arr = [{ id: 1, username: 'fred' }, 
2  { id: 2, username: 'bill'}, 
3  { id: 3, username: 'ted' }];
4
5function userExists(username) {
6  return arr.some(function(el) {
7    return el.username === username;
8  }); 
9}
10
11console.log(userExists('fred')); // true
12console.log(userExists('bred')); // false
13