How can I update an object array in sequence with an array for and argument with .map() in JavaScript?

I'm a beginner-level student and for practice in understanding .map(), I'm trying to write a callback function that returns an updated list of full names that would return Jim's last name as Smith etc. employees[0].familyName = lastNames[0], employees[1].familyName = lastNames[1] and so on....

const lastNames = ["Smith", "Anderson"];

const employees = [
  {
    name: "Jim",
    familyName:"",
  },
  {
    name: "Jill",
    familyName: "",
    
  }
];

This is what I've tried expecting it to return a new object array called fullNames with the following key/value pairs but am lost in the wrong direction possibly with matching the index numbers:

function lastNameUpdater(arr, objArr) {
  let matchNumber = arr.findIndex() === objArr.findIndex();
  const fullNames = objArr.map((element) => {
    return matchNumber;
    if (matchNumber) {
      return objArr.key = arr;
    }
    return fullNames;
  })
}

lastNameUpdater(lastNames, employees);

Just use the second index parameter that's passed to the callback, then you can use spread notation to overwrite the familyName attribute. Because the indices match up, this will pair each family name with the correct employee:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function update(employees, lastNames) {
  return employees.map(function(employee, index) {
    return {...employee, familyName: lastNames[index]}
  });
}

const lastNames = ["Smith", "Anderson"];
const employees = [
  {
    name: "Jim",
    familyName:"",
  },
  {
    name: "Jill",
    familyName: "",
    
  }
];

console.log(update(employees, lastNames))

<!-- end snippet -->

You can use the index argument given to the map callback to access the corresponding last name from the other array.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const lastNames = ["Smith", "Anderson"];
const employees = [
  {
    name: "Jim",
    familyName:"",
  },
  {
    name: "Jill",
    familyName: "",
  }
];
const res = employees.map((o, i) => ({...o, familyName: lastNames[i]}));
console.log(res);

<!-- end snippet -->

You are misusing findIndex; actually, you don't even need it. You can read more about it here

What you really need is to use the map's second argument, which is the index of the current element in the array, and pick the element under the same index in the lastNames array.

const lastNames = ['Smith', 'Anderson'];

const employees = [
  {
    name: 'Jim',
    familyName: '',
  },
  {
    name: 'Jill',
    familyName: '',
  },
];

function lastNameUpdater(arr, objArr) {
  const fullNames = objArr.map((element, index) => ({
    ...element,
    familyName: arr[index],
  }));
  return fullNames;
}

// [
//     { name: 'Jim', familyName: 'Smith' },
//     { name: 'Jill', familyName: 'Anderson' }
// ]
console.log(lastNameUpdater(lastNames, employees));