JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1" method="post" action="">
<select id="selectUsers" name="users" onChange='Choice();'><option> </option>
<option value="1">bsmith</option>
<option value="2">lsmith</option>
<option value="3">asmith</option>
</select>
<p>ids <input type="text" id="ids" name="id" ></p>
<p>use <input type="text" id="use" name="username" ></p>
<p>ful <input type="text" id="ful" name="full_name" ></p>
</form>

JavaScript

var ids = new Array();
var use = new Array();
var ful = new Array();

ids[0] = "";
use[0] = "";
ful[0] = "";

ids[1] = 6;
use[1] = "bsmith";
ful[1] = "Buddy Smith";

ids[2] = 2;
use[2] = "lsmith";
ful[2] = "Libbie Smith";

ids[3] = 4;
use[3] = "asmith";
ful[3] = "Andy Smith";


        function Choice() {
            //x = document.getElementById("users");
            y = document.getElementById("selectUsers");

              //x.value = y.options[y.selectedIndex].text;
              document.getElementById("ids").value = ids[y.selectedIndex];
              document.getElementById("use").value = use[y.selectedIndex];
              document.getElementById("ful").value = ful[y.selectedIndex];
         }