Создать функцию, которая будет возвращать массив уникальных имен из массива объектов people.

const people = [
  {firstName: 'Eileen', lastName: 'Graham'},
  {firstName: 'Peter', lastName: 'Graham'},
  {firstName: 'Jared', lastName: 'Miller'},
  {firstName: 'Robert', lastName: 'Williams'},
];
getUniqNames(people) === [
  'Eileen', 'Peter', 'Jared', 'Robert', 'David'

let getUniqNames = arr => [...new Set(arr.map(e => e.firstName))]
 
const people = [
    {firstName: 'Eileen', lastName: 'Graham'},
    {firstName: 'Peter', lastName: 'Graham'},
    {firstName: 'Jared', lastName: 'Miller'},
    {firstName: 'Robert', lastName: 'Williams'},
    {firstName: 'Peter', lastName: 'qwerty'}
]
 
console.log( getUniqNames(people) )  // ["Eileen", "Peter", "Jared", "Robert"]

let getUniqNames = arr => {
    let acc = []
    for (e of arr) {
        if (acc.indexOf(e.firstName) == -1)
            acc.push(e.firstName)
    }
    return acc
}
 
const people = [
    {firstName: 'Eileen', lastName: 'Graham'},
    {firstName: 'Peter', lastName: 'Graham'},
    {firstName: 'Jared', lastName: 'Miller'},
    {firstName: 'Robert', lastName: 'Williams'},
    {firstName: 'Peter', lastName: 'qwerty'}
]
 
console.log( getUniqNames(people) )