Check if object value exists within a Javascript array of objects and if not add a new object to array

I've assumed that ids are meant to be unique here. some is a great function for checking the existence of things in arrays:

const arr = [{ id: 1, username: 'fred' }, { id: 2, username: 'bill' }, { id: 3, username: 'ted' }];

function add(arr, name) {
  const { length } = arr;
  const id = length + 1;
  const found = arr.some(el => el.username === name);
  if (!found) arr.push({ id, username: name });
  return arr;
}

console.log(add(arr, 'ted'));

It's rather trivial to check for existing username:

var arr = [{ id: 1, username: 'fred' }, 
  { id: 2, username: 'bill'}, 
  { id: 3, username: 'ted' }];

function userExists(username) {
  return arr.some(function(el) {
    return el.username === username;
  }); 
}

console.log(userExists('fred')); // true
console.log(userExists('bred')); // false

But it's not so obvious what to do when you have to add a new user to this array. The easiest way out - just pushing a new element with id equal to array.length + 1:

function addUser(username) {
  if (userExists(username)) {
    return false; 
  }
  arr.push({ id: arr.length + 1, username: username });
  return true;
}

addUser('fred'); // false
addUser('bred'); // true, user `bred` added

It will guarantee the IDs uniqueness, but will make this array look a bit strange if some elements will be taken off its end.

This small snippets works for me..

const arrayOfObject = [{ id: 1, name: 'john' }, {id: 2, name: 'max'}];

const checkUsername = obj => obj.name === 'max';

console.log(arrayOfObject.some(checkUsername))