Select dropdown menu option with javascript

I have a dropdown menu and I cannot figure out how to make a javascript function select a drop down menu option. I have tested the output of the variables and they are all correct, but it still will not select the option when clicked. Here is the function and drop down menu.

Function

function formFill(a, b, c){
        theform.from.value = a;
        theform.to.value = b;
        for(var i = 0;i < document.getElementById("stateSelect").length;i++){
            if(document.getElementById("s开发者_开发技巧tateSelect").options[i].value == c ){
                document.getElementById("stateSelect").selected = true;
            }
        }
    }

Menu item

<select id="stateSelect" name="stateSelect">
    <option value="none">(None)</option>
    <option value="AL">Alabama</option>
    <option value="AK">Alaska</option>

Change the line that reads:

document.getElementById("stateSelect").selected = true;

to:

document.getElementById("stateSelect").selectedIndex = i;

Alt. you can set selected to the actual option: select.options[i].selected = true;

...
        var select = document.getElementById("stateSelect");
        for(var i = 0;i < select.options.length;i++){
            if(select.options[i].value == c ){
                select.options[i].selected = true;
            }
        }
...

I used Anatoly's solution above (thanks!) until a coworker pointed out I could simplify it further with

var select = document.getElementById("stateSelect");
select.options.forEach(opt=>{
    if(opt.value == c)
        opt.selected = true;
    })

which he claims requires less overhead, though I could only speculate why.