Loop is stuck for some reason and then finally returns undefined

This is the problem I am trying to solve: Given: an array containing hashes of names

Return: a string formatted as a list of names separated by commas except for the last two names, which should be separated by an ampersand.

Example:

list([ {name: 'Bart'}, {name: 'Lisa'}, {name: 'Maggie'} ])
// returns 'Bart, Lisa & Maggie'

list([ {name: 'Bart'}, {name: 'Lisa'} ])
// returns 'Bart & Lisa'

list([ {name: 'Bart'} ])
// returns 'Bart'

list([])
// returns ''

Note: all the hashes are pre-validated and will only contain A-Z, a-z, '-' and '.

Here is my code:

var finalName;
var notFinal;

function list(names){
  var finalNames = names.forEach(returnNames);
        console.log(typeof finalNames);

  function returnNames() {
    for (var i = 0; i<names.length; i++) {
      var nameValue = Object.keys(names[i]).map(key => names[i][key])
    }
  }

  for(var i = 0; i<finalNames.length; i++) {
    if (finalNames.length / i == 1) {
      finalName = "& " + finalNames[i]; 
    }
    else {
      notFinal = finalNames[i] + ", ";
    }
  }

  console.log(notFinal + finalName);
}

list([{name: 'Bart'},{name: 'Lisa'},{name: 'Maggie'},{name: 'Homer'},{name: 'Marge'}])

It is stuck in the loop and finally gives an error:

TypeError: Cannot read property 'length' of undefined
    at list
    at /home/codewarrior/index.js:30:1
    at Object.handleError
        <anonymous>

How do I fix it?

You can simplify your code like below.

function list(names){
  //Determine length of array (= number of names)
  const len = names.length;
  //Use a simple for loop to go through the array
  let newNames = "";
  for (i=0; i<len; i++) {
    newNames += names[i].name;
    if ( i<len-2 ) newNames += ", "
      else if ( i<len-1 ) newNames += " & "
  }
  console.log(newNames);
}

list([{name: 'Bart'},{name: 'Lisa'},{name: 'Maggie'},{name: 'Homer'},{name: 'Marge'}])
list([{name: 'Bart'}, {name: 'Lisa'}]);
list([{name: 'Bart'}]);