JavaScript Numbers Addition Fix [JavaScript]

<input id= "test1" value="test1" onchange="test(this.id)">
<input id= "test2" value="test2" onchange="test(this.id)">
<input id= "test3" value="test3" onchange="test(this.id)">
<!-- https://stackoverflow.com/questions/8976627/how-to-add-two-strings-as-if-they-were-numbers -->
<!-- parseInt and numbers addition possible fix below -->
<!-- +id + +1 -->
<script>
    function test(string) {
        var id = string.replace("test", "");
        alert('curid' + parseInt(id));
        alert('previd' + parseInt(id-1));
        alert('nextid' + parseInt(+id + +1));
        var curElement = document.getElementById("test" + parseInt(id));
        var prevElement = document.getElementById("test" + parseInt(id-1));
        var nextElement = document.getElementById("test" + parseInt(+id + +1));
        if (curElement) {
            alert('current element: ' + curElement.value);
        }
        if (prevElement) {
            alert('previous element: ' + prevElement.value);
        }
        if (nextElement) {
            alert('next element: ' + nextElement.value);
        }
    }
</script>