obj = { "table":"customers", "limit":20 };
dbParam = JSON.stringify(obj);
xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
        myObj = JSON.parse(this.responseText);
        txt += "<table border='1'>"
        for (x in myObj) {
            txt += "<tr><td>" + myObj[x].name + "</td></tr>";
        }
        txt += "</table>"
        document.getElementById("demo").innerHTML = txt;
    }
}
xmlhttp.open("POST", "json_demo_db_post.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.send("x=" + dbParam); 

 <select id="myselect" onchange="change_myselect(this.value)">
    <option value="">Choose an option:</option>
    <option value="customers">Customers</option>
    <option value="products">Products</option>
    <option value="suppliers">Suppliers</option>
</select>

<script>
function change_myselect(sel) {
    var obj, dbParam, xmlhttp, myObj, x, txt = "";
    obj = { "table":sel, "limit":20 };
    dbParam = JSON.stringify(obj);
    xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            myObj = JSON.parse(this.responseText);
            txt += "<table border='1'>"
            for (x in myObj) {
                txt += "<tr><td>" + myObj[x].name + "</td></tr>";
            }
            txt += "</table>"
            document.getElementById("demo").innerHTML = txt;
        }
    };
    xmlhttp.open("POST", "json_demo_db_post.php", true);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xmlhttp.send("x=" + dbParam);
}
</script> 

obj = { "table":"customers", "limit":20 };
dbParam = JSON.stringify(obj);
xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
        myObj = JSON.parse(this.responseText);
        txt += "<select>"
        for (x in myObj) {
            txt += "<option>" + myObj[x].name;
        }
        txt += "</select>"
        document.getElementById("demo").innerHTML = txt;
    }
}
xmlhttp.open("POST", "json_demo_db_post.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.send("x=" + dbParam);