How to select a value in dropdown javascript?

Use the selectedIndex property:

document.getElementById("Mobility").selectedIndex = 12; //Option 10

Alternate method:

Loop through each value:

//Get select object
var objSelect = document.getElementById("Mobility");

//Set selected
setSelectedValue(objSelect, "10");

function setSelectedValue(selectObj, valueToSet) {
    for (var i = 0; i < selectObj.options.length; i++) {
        if (selectObj.options[i].text== valueToSet) {
            selectObj.options[i].selected = true;
            return;
        }
    }
}

easiest way is to just use this

document.getElementById("mySelect").value = "banana";

myselect is name of your dropdown banana is just one of items in your dropdown list

function setSelectedIndex(s, v) {
    for ( var i = 0; i < s.options.length; i++ ) {
        if ( s.options[i].value == v ) {
            s.options[i].selected = true;
            return;
        }
    }
}

Where s is the dropdown and v is the value