My grade calculator with special weighting

My school has a special 65% formal 35% informal weighting, so I made something to find what your grade would be if you added or changed a grade.
http://retep-mathwizard.github.io/gradecalc/gradecalc.html

HTML:

  
     Grade Calculator
    
    
    
  
  
    Grade Calculator
    
    
    New
    Change
    
    
    Informal
    Formal
     Go 
    
    
  

CSS:

#fog, #ing {

  width: 300px;
}

JS:

```
function changeoldgradedisplay() {
if (document.getElementById('radio_old_grade').checked) {
document.getElementById('old_grade_input').style.display = 'block';
} else {
document.getElementById('old_grade_input').style.display = 'none';
}
}
function get_average(items) {
var sum = 0;
var len = items.length;
for (var i = 0; i < len; i++) {
sum += parseFloat(items[i]);
}
return sum/len;
}

function add_grades() {
var formal_grades = document.getElementById('fog').value.split(',');
var informal_grades = document.getElementById('ing').value.split(',');
var previous_grade = (get_average(formal_grades) .65) + (get_average(informal_grades) .35);

document.getElementById('previous_grade').innerHTML = 'Estimate of previous grade - ' + previous_grade.toFixed(2);

var is_formal = document.getElementById('formal_radio').checked;
var gradelist = null;

if (is_formal) {
new_grade = document.getElementById("new_grade_input").value;
if (document.getElementById('radio_old_grade').checked) {
old_grade = document.getElementById("old_grade_input").value;
formal_grades[formal_grades.indexOf(old_grade)] = new_grade
} else {
formal_grades.push(new_grade);
}
} else {
new_grade = document.getElementById("new_grade_input").value;
if (document.getElementById('radio_old_grade').checked) {
old_grade = document.getElementById("old_grade_input").value;
informal_grades[info

A couple of minor things:

-
fog and ing aren't very meaningful id names, they would be better as something like formal_grades and informal_grades instead.

-
This part of the code:

var is_formal = document.getElementById('formal_radio').checked;
var gradelist = null;

if (is_formal) {
new_grade = document.getElementById("new_grade_input").value;
if (document.getElementById('radio_old_grade').checked) {
old_grade = document.getElementById("old_grade_input").value;
formal_grades[formal_grades.indexOf(old_grade)] = new_grade
} else {
formal_grades.push(new_grade);
}
} else {
new_grade = document.getElementById("new_grade_input").value;
if (document.getElementById('radio_old_grade').checked) {
old_grade = document.getElementById("old_grade_input").value;
informal_grades[informal_grades.indexOf(old_grade)] = new_grade
} else {
informal_grades.push(new_grade);
}
}


repeats very similar code where the only difference is which array is used. Also, gradelist isn't used anywhere. You can avoid the repetition like this:

var is_formal = document.getElementById('formal_radio').checked;
var gradelist = is_formal ? formal_grades : informal_grades;

new_grade = document.getElementById("new_grade_input").value;
if (document.getElementById('radio_old_grade').checked) {
old_grade = document.getElementById("old_grade_input").value;
gradelist[gradelist.indexOf(old_grade)] = new_grade
} else {
gradelist.push(new_grade);
}


by assigning the array that needs to be used to gradelist.

This works because when you assign an array (or any object) to a variable in javascript, it assigns the reference to the array instead of copying the value. For example, if formal_grades is assigned to gradelist they will both refer to the same array.

StackExchange Code Review Q#153041, answer score: 4