for loop printing out last item in my array only

Your issue is that you are reassigning the list variable each time through the loop, so you are overwriting the previous value.

To avoid this, use the += operator instead of the = operator like so:

var patients = ["Julia", "Kelly", "Thomas", "Clare"];

function lineOfPatients(line) {
 if (!line.length) {
     return "Empty"
 }
 var list = "";
 for(var i = 0; i < line.length; i++) {
     list += `${i + 1}. ${line[i]}, `
 }
  return `The line is currently: ${list}`
}

console.log(lineOfPatients(patients))

The problem with your code is that with each iteration of your for-loop, you redeclare var list = `${i + 1}. ${line[i]},` so that by the time you return, list only equals the last element in the array.

You could do this:

function lineOfPatients(line) {
  if (!line.length) {
    return "Empty"
  }
  var returnString = "The line is currently: "
  for(let i = 0; i < line.length; i++) {
    let patient = ` ${i + 1}. ${line[i]},`;
    returnString += patient;
  }
  return returnString;
}

Here's what you want. (Got carried away with code golf)

const patients = ["Julia", "Kelly", "Thomas", "Clare"]

const lineOfPatients = (line) => "The line is currently: " + (!line || !line.length) ? "Empty" : line.map((patient, idx) => `${idx + 1}. ${patient}`).join(', ')

console.log(lineOfPatients(patients))

The reason it's not working is because your redeclaring the variable liston every loop. Even if you moved it out of the loop, your not appending the output from the for-loop, your assigning it. It will always be the output from the last loop.

var list is being declared inside your loop. This means that it is being recreated with a new value on each itteration. Declare this variable outside of the loop, as an array. E.g,

var list = new Array();

Then, in your loop, add to the array;

list[i] = .....