How to save prompt input into array

Just try to ask them to input their numbers or grade, separated by a comma and then you can split on it.

var arr = prompt("Enter your numbers").split(",")

Or, ask prompt ten times

var arr = [];
for(var i = 0; i < 10; i++)
   arr.push(prompt("Enter a number");

If you want them to be numbers, just prefix prompt with +, so it becomes a number(provided they're actual numbers) or just do

arr = arr.map(Number);

See the explanations in comments:

var arr = [];                               // define our array

for (var i = 0; i < 10; i++) {              // loop 10 times
  arr.push(prompt('Enter grade ' + (i+1))); // push the value into the array
}

alert('Full array: ' + arr.join(', '));     // alert the results

NUMBER_OF_INPUTS = 10;

var i = 0;     // Loop iterator
var userInput; // Input from user
sum = 0; //initialise sum

// Collect inputs
for(i=0; i<NUMBER_OF_INPUTS; i++)
{   userInput = parseInt(prompt('Enter input '+(i+1)+' of '+NUMBER_OF_INPUTS));
    sum += userInput;
    sum /= NUMBER_OF_INPUTS;
}

// Output the average
alert('Average grade: '+ sum.toFixed(2)); //the .toFixed sets it to 2 decimal places

<script>
    var grades = [];
    var i;
        for (i = 0; i < 10; i++) {
            grades.push(Number(prompt("Enter your grades:" + (i + 1), "0-100")));
        }
    document.write("Your grades: " + grades);
</script>

Ok so I made this one. The user can enter 10 different numbers to the array and I can display them. Now - I need to caculate the avarage of the numbers and get the highest number.

I would like to have some help withj it, how can I make it?

  function myfunction() {
    let arr = [];

    for (i = 0; i < 5; i++) {
      arr.push(prompt());
    }

    for (i = 0; i < arr.length; i++) {
      arr[i] = parseInt(arr[i]);
    }
    console.log(arr);
  }
  myfunction();