JSFiddle - React, Tailwind, and code Playground

HTML

<div class="input-resp">
    <span>
        <input  class="textbox" id="num" name="count" type="text" size="5" maxlength="3"  value="" />
    </span>
</div>
<div id="test"></div>

JavaScript

document.getElementById('num').onkeyup = function() {
    var a = 1000 + parseFloat(this.value);
    document.getElementById("test").innerHTML = a || 0;
}