How do I update a model value in JavaScript in a Razor view?

This should work

function updatePostID(val)
{
    document.getElementById('PostID').value = val;

    //and probably call document.forms[0].submit();
}

Then have a hidden field or other control for the PostID

@Html.Hidden("PostID", Model.addcomment.PostID)
//OR
@Html.HiddenFor(model => model.addcomment.PostID)