Cannot get the output think I should be getting

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Javascript

1st piece of code allows me to use an alert to compile a list of grades:


Add a grade!

var grades = [10,12,13];

function loadGrades(){
document.getElementById("grades").innerHTML =grades;

}
function myFunction() {
var grade = prompt("what is the next grade?");
grades[grades.length]=grade;

document.getElementById("grades").innerHTML = grades;
}




2nd piece of code allows me to calculate the mean of a list of numbers but only when I manipulate the list from within the code:





var Grades = [10,20];

var sum=0;
if(Grades.length>0){
for(index=0;index<Grades.length;index++){

sum+=Grades[index];
}


document.write(sum/Grades.length);

document.write(" is the average of the following grades" + Grades);
}
else
document.write("Emptyy");




I cannot figure out how to make the new list (when I enter a new number in the prompt) to automatically update the mean of the grades. I would have thought it would have been as easy as cutting and pasting the two parts together but I couldn't get it to work.

Any thoughts would be appreciated!

Thanks.

What I have tried:

I tried pasting the two parts together in a number of different ways but nothing worked.